Saturday, February 24, 2018

Basic Electronics on the Go - Full Wave Rectifier


Power Diodes can be connected together to form a full wave rectifier that convert AC voltage into pulsating DC voltage for use in power supplies.

In the previous Power Diodes tutorial we discussed ways of reducing the ripple or voltage variations on a direct DC voltage by connecting smoothing capacitors across the load resistance.
While this method may be suitable for low power applications it is unsuitable to applications which need a “steady and smooth” DC supply voltage. One method to improve on this is to use every half-cycle of the input voltage instead of every other half-cycle. The circuit which allows us to do this is called a Full Wave Rectifier.

Like the half wave circuit, a full wave rectifier circuit produces an output voltage or current which is purely DC or has some specified DC component. Full wave rectifiers have some fundamental advantages over their half wave rectifier counterparts. The average (DC) output voltage is higher than for half wave, the output of the full wave rectifier has much less ripple than that of the half wave rectifier producing a smoother output waveform.

 In a Full Wave Rectifier circuit two diodes are now used, one for each half of the cycle. A multiple winding transformer is used whose secondary winding is split equally into two halves with a common centre tapped connection, (C). This configuration results in each diode conducting in turn when its anode terminal is positive with respect to the transformer centre point C producing an output during both half-cycles, twice that for the half wave rectifier so it is 100% efficient as shown below.

Full Wave Rectifier Circuit

 The smoothing capacitor converts the full-wave rippled output of the rectifier into a more smooth DC output voltage. If we now run the Partsim Simulator Circuit with different values of smoothing capacitor installed, we can see the effect it has on the rectified output waveform as shown.


Partsim Simulation Waveform



  As the spaces between each half-wave developed by each diode is now being filled in by the other diode the average DC output voltage across the load resistor is now double that of the single half-wave rectifier circuit and is about  0.637Vmax  of the peak voltage, assuming no losses.

Where: VMAX is the maximum peak value in one half of the secondary winding and VRMS is the rms value.

The peak voltage of the output waveform is the same as before for the half-wave rectifier provided each half of the transformer windings have the same rms voltage value. To obtain a different DC voltage output different transformer ratios can be used.

The main disadvantage of this type of full wave rectifier circuit is that a larger transformer for a given power output is required with two separate but identical secondary windings making this type of full wave rectifying circuit costly compared to the “Full Wave Bridge Rectifier” circuit equivalent.

The Full Wave Bridge Rectifier

Another type of circuit that produces the same output waveform as the full wave rectifier circuit above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop “bridge” configuration to produce the desired output.

The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below.

The Diode Bridge Rectifier


The four diodes labelled D1 to D4 are arranged in “series pairs” with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load as shown below.

The Positive Half-cycle


 During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch “OFF” as they are now reverse biased. The current flowing through the load is the same direction as before.

The Negative Half-cycle

As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across the load is 0.637Vmax.
However in reality, during each half cycle the current flows through two diodes instead of just one so the amplitude of the output voltage is two voltage drops ( 2*0.7 = 1.4V ) less than the input VMAX amplitude. The ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply or 120Hz for a 60Hz supply.)

Although we can use four individual power diodes to make a full wave bridge rectifier, pre-made bridge rectifier components are available “off-the-shelf” in a range of different voltage and current sizes that can be soldered directly into a PCB circuit board or be connected by spade connectors.

 The image above shows a typical single phase bridge rectifier with one corner cut off. This cut-off corner indicates that the terminal nearest to the corner is the positive or +ve output terminal or lead with the opposite (diagonal) lead being the negative or -ve output lead. The other two connecting leads are for the input alternating voltage from a transformer secondary winding.

The Smoothing Capacitor

We saw in the previous section that the single phase half-wave rectifier produces an output wave every half cycle and that it was not practical to use this type of circuit to produce a steady DC supply. The full-wave bridge rectifier however, gives us a greater mean DC value (0.637 Vmax) with less superimposed ripple while the output waveform is twice that of the frequency of the input supply frequency.
We can improve the average DC output of the rectifier while at the same time reducing the AC variation of the rectified output by using smoothing capacitors to filter the output waveform. Smoothing or reservoir capacitors connected in parallel with the load across the output of the full wave bridge rectifier circuit increases the average DC output level even higher as the capacitor acts like a storage device as shown below.

Full-wave Rectifier with Smoothing Capacitor


 The smoothing capacitor converts the full-wave rippled output of the rectifier into a more smooth DC output voltage. If we now run the Partsim Simulator Circuit with different values of smoothing capacitor installed, we can see the effect it has on the rectified output waveform as shown.

5uF Smoothing Capacitor


 The blue plot on the waveform shows the result of using a 5.0uF smoothing capacitor across the rectifiers output. Previously the load voltage followed the rectified output waveform down to zero volts. Here the 5uF capacitor is charged to the peak voltage of the output DC pulse, but when it drops from its peak voltage back down to zero volts, the capacitor can not discharge as quickly due to the RC time constant of the circuit.

(to be updated)

Thursday, February 8, 2018

Basic Electronics on the Go - Power Diodes and Rectifiers


In the previous tutorials we saw that a semiconductor signal diode will only conduct current in one direction from its anode to its cathode (forward direction), but not in the reverse direction acting a bit like an electrical one way valve.

A widely used application of this feature and diodes in general is in the conversion of an alternating voltage (AC) into a continuous voltage (DC). In other words, Rectification.

Small signal diodes can be used as rectifiers in low-power, low current (less than 1-amp) rectifiers or applications, but when larger forward bias currents or higher reverse bias blocking voltages are involved, the PN junction of a small signal diode would eventually overheat and melt -  so larger more robust Power Diodes are used instead.
The power semiconductor diode, known simply as the Power Diode, has a much larger PN junction area compared to its smaller signal diode cousin, resulting in a high forward current capability of up to several hundred amps (KA) and a reverse blocking voltage of up to several thousand volts (KV).
Since the power diode has a large PN junction, it is not suitable for high frequency applications above 1MHz, but special and expensive high frequency, high current diodes are available. For high frequency rectifier applications,  Schottky Diodes are generally used because of their short reverse recovery time and low voltage drop in their forward bias condition.

Power diodes provide uncontrolled rectification of power and are used in applications such as battery charging and DC power supplies as well as AC rectifiers and inverters. Due to their high current and voltage characteristics they can also be used as free-wheeling diodes and snubber networks.
Power diodes are designed to have a forward “ON” resistance of fractions of an Ohm while their reverse blocking resistance is in the mega-Ohms range. Some of the larger value power diodes are designed to be “stud mounted” onto heatsinks reducing their thermal resistance to between 0.1 to 1oC/Watt.

If an alternating voltage is applied across a power diode, during the positive half cycle the diode will conduct passing current and during the negative half cycle the diode will not conduct, blocking the flow of current. Then conduction through the power diode only occurs during the positive half cycle and is therefore unidirectional i.e. DC as shown.

Power Diode Rectifier



 Power diodes can be used individually as above or connected together to produce a variety of rectifier circuits such as “Half-Wave”, “Full-Wave” or as “Bridge Rectifiers”. Each type of rectifier circuit can be classed as either uncontrolled, half-controlled or fully controlled where an uncontrolled rectifier uses only power diodes, a fully controlled rectifier uses thyristors (SCRs) and a half controlled rectifier is a mixture of both diodes and thyristors.

The most commonly used individual power diode for basic electronics applications is the general purpose 1N400x Series Glass Passivated type rectifying diode with standard ratings of continuous forward rectified current of about 1.0 ampere and reverse blocking voltage ratings from 50v for the 1N4001 up to 1000v for the 1N4007, with the small 1N4007GP being the most popular for general purpose mains voltage rectification.

Half Wave Rectification

A rectifier is a circuit which converts the Alternating Current (AC) input power into a Direct Current (DC) output power. The input power supply may be either a single-phase or a multi-phase supply with the simplest of all the rectifier circuits being that of the Half Wave Rectifier.

 The power diode in a half wave rectifier circuit passes just one half of each complete sine wave of the AC supply in order to convert it into a DC supply.

Half Wave Rectifier Circuit



During each “positive” half cycle of the AC sine wave, the diode is forward biased as the anode is positive with respect to the cathode resulting in current flowing through the diode.
Since the DC load is resistive (resistor, R), the current flowing in the load resistor is therefore proportional to the voltage (Ohm´s Law), and the voltage across the load resistor will therefore be the same as the supply voltage, Vs (minus ), that is the “DC” voltage across the load is sinusoidal for the first half cycle only so Vout = Vs.

During each “negative” half cycle of the AC sinusoidal input waveform, the diode is reverse biased as the anode is negative with respect to the cathode. Therefore, NO current flows through the diode or circuit. Then in the negative half cycle of the supply, no current flows in the load resistor as no voltage appears across it so therefore, Vout = 0.

 The current on the DC side of the circuit flows in one direction only making the circuit Unidirectional. As the load resistor receives from the diode a positive half of the waveform, zero volts, a positive half of the waveform, zero volts, etc, the value of this irregular voltage would be equal in value to an equivalent DC voltage of 0.318*Vmax of the input sinusoidal waveform or 0.45*Vrms of the input sinusoidal waveform.

VDC and the current IDC, flowing through a 100Ω resistor connected to a 240 Vrms single phase half-wave rectifier as shown above. Also calculate the DC power consumed by the load.

During the rectification process the resultant output DC voltage and current are therefore both “ON” and “OFF” during every cycle. As the voltage across the load resistor is only present during the positive half of the cycle (50% of the input waveform), this results in a low average DC value being supplied to the load.

The variation of the rectified output waveform between this “ON” and “OFF” condition produces a waveform which has large amounts of “ripple” which is an undesirable feature. The resultant DC ripple has a frequency that is equal to that of the AC supply frequency.

Very often when rectifying an alternating voltage we wish to produce a “steady” and continuous DC voltage free from any voltage variations or ripple. One way of doing this is to connect a large value Capacitor across the output voltage terminals in parallel with the load resistor as shown below. This type of capacitor is known commonly as a “Reservoir” or Smoothing Capacitor.

Half-wave Rectifier with Smoothing Capacitor



When rectification is used to provide a direct voltage (DC) power supply from an alternating (AC) source, the amount of ripple voltage can be further reduced by using larger value capacitors but there are limits both on cost and size to the types of smoothing capacitors used.

For a given capacitor value, a greater load current (smaller load resistance) will discharge the capacitor more quickly ( RC Time Constant ) and so increases the ripple obtained. Then for single phase, half-wave rectifier circuit using a power diode it is not very practical to try and reduce the ripple voltage by capacitor smoothing alone. In this instance it would be more practical to use “Full-wave Rectification” instead.

In practice, the half-wave rectifier is used most often in low-power applications because of their major disadvantages being that the output amplitude is less than the input amplitude, there is no output during the negative half cycle so half the power is wasted and the output is pulsed DC resulting in excessive ripple.

To overcome these disadvantages a number of Power Diode are connected together to produce a Full Wave Rectifier as discussed in the next tutorial.

Monday, January 15, 2018

Basic Electronics on the Go - The Signal Diode


 The semiconductor Signal Diode is a small non-linear semiconductor devices generally used in electronic circuits, where small currents or high frequencies are involved such as in radio, television and digital logic circuits.

The signal diode which is also sometimes known by its older name of the Point Contact Diode or the Glass Passivated Diode, are physically very small in size compared to their larger Power Diode cousins.

Generally, the PN junction of a small signal diode is encapsulated in glass to protect the PN junction, and usually have a red or black band at one end of their body to help identify which end is the cathode terminal. The most widely used of all the glass encapsulated signal diodes is the very common 1N4148 and its equivalent 1N914 signal diode.

Small signal and switching diodes have much lower power and current ratings, around 150mA, 500mW maximum compared to rectifier diodes, but they can function better in high frequency applications or in clipping and switching applications that deal with short-duration pulse waveforms.

The characteristics of a signal point contact diode are different for both germanium and silicon types and are given as:
  • 1. Germanium Signal Diodes – These have a low reverse resistance value giving a lower forward volt drop across the junction, typically only about 0.2 to 0.3v, but have a higher forward resistance value because of their small junction area.
  • 2. Silicon Signal Diodes – These have a very high value of reverse resistance and give a forward volt drop of about 0.6 to 0.7v across the junction. They have fairly low values of forward resistance giving them high peak values of forward current and reverse voltage.
The electronic symbol given for any type of diode is that of an arrow with a bar or line at its end and this is illustrated below along with the Steady State V-I Characteristics Curve.

Silicon Diode V-I Characteristic Curve



The arrow always points in the direction of conventional current flow through the diode meaning that the diode will only conduct if a positive supply is connected to the Anode, ( a ) terminal and a negative supply is connected to the Cathodek ) terminal thus only allowing current to flow through it in one direction only, acting more like a one way electrical valve, ( Forward Biased Condition ).
However, we know from the previous tutorial that if we connect the external energy source in the other direction the diode will block any current flowing through it and instead will act like an open switch, ( Reversed Biased Condition ) as shown below.

Forward and Reversed Biased Diode


Signal Diode Parameters

Signal Diodes are manufactured in a range of voltage and current ratings and care must be taken when choosing a diode for a certain application. There are a bewildering array of static characteristics associated with the humble signal diode but the more important ones are.

1. Maximum Forward Current

The Maximum Forward CurrentIF(max) ) is as its name implies the maximum forward current allowed to flow through the device. When the diode is conducting in the forward bias condition, it has a very small “ON” resistance across the PN junction and therefore, power is dissipated across this junction ( Ohm´s Law ) in the form of heat.

 hen, exceeding its ( IF(max) ) value will cause more heat to be generated across the junction and the diode will fail due to thermal overload, usually with destructive consequences. When operating diodes around their maximum current ratings it is always best to provide additional cooling to dissipate the heat produced by the diode.

For example, our small 1N4148 signal diode has a maximum current rating of about 150mA with a power dissipation of 500mW at 25oC. Then a resistor must be used in series with the diode to limit the forward current, ( IF(max) ) through it to below this value.

2. Peak Inverse Voltage

The Peak Inverse Voltage (PIV) or Maximum Reverse VoltageVR(max) ), is the maximum allowable Reverse operating voltage that can be applied across the diode without reverse breakdown and damage occurring to the device. This rating therefore, is usually less than the “avalanche breakdown” level on the reverse bias characteristic curve. Typical values of VR(max) range from a few volts to thousands of volts and must be considered when replacing a diode.

The peak inverse voltage is an important parameter and is mainly used for rectifying diodes in AC rectifier circuits with reference to the amplitude of the voltage where the sinusoidal waveform changes from a positive to a negative value on each and every cycle.

3. Total Power Dissipation

Signal diodes have a Total Power Dissipation, ( PD(max) ) rating. This rating is the maximum possible power dissipation of the diode when it is forward biased (conducting). When current flows through the signal diode the biasing of the PN junction is not perfect and offers some resistance to the flow of current resulting in power being dissipated (lost) in the diode in the form of heat.

As small signal diodes are non-linear devices, the resistance of the PN junction is not constant - it is a dynamic property so we cannot use Ohms Law to define the power in terms of current and resistance or voltage and resistance as we can for resistors. Then to find the power that will be dissipated by the diode we must multiply the voltage drop across it times the current flowing through it: PD = VxI

4. Maximum Operating Temperature

The Maximum Operating Temperature actually relates to the Junction TemperatureTJ ) of the diode and is related to maximum power dissipation. It is the maximum temperature allowable before the structure of the diode deteriorates and is expressed in units of degrees centigrade per Watt, ( oC/W ).
This value is linked closely to the maximum forward current of the device so that at this value the temperature of the junction is not exceeded. However, the maximum forward current will also depend upon the ambient temperature in which the device is operating so the maximum forward current is usually quoted for two or more ambient temperature values such as 25oC or 70oC.
Then there are three main parameters that must be considered when either selecting or replacing a signal diode and these are:
  • The Reverse Voltage Rating
  • The Forward Current Rating
  • The Forward Power Dissipation Rating

Signal Diode Arrays

When space is limited, or matching pairs of switching signal diodes are required, diode arrays can be very useful. They generally consist of low capacitance high speed silicon diodes such as the 1N4148 connected together in multiple diode packages called an array for use in switching and clamping in digital circuits. They are encased in single inline packages (SIP) containing 4 or more diodes connected internally to give either an individual isolated array, common cathode, (CC), or a common anode, (CA) configuration as shown.

Signal diode arrays can also be used in digital and computer circuits to protect high speed data lines or other input/output parallel ports against electrostatic discharge, (ESD) and voltage transients.
By connecting two diodes in series across the supply rails with the data line connected to their junction as shown, any unwanted transients are quickly dissipated and as the signal diodes are available in 8-fold arrays they can protect eight data lines in a single package.

Signal diode arrays can also be used to connect together diodes in either series or parallel combinations to form voltage regulator or voltage reducing type circuits or even to produce a known fixed reference voltage.

We know that the forward volt drop across a silicon diode is about 0.7v and by connecting together a number of diodes in series the total voltage drop will be the sum of the individual voltage drops of each diode.

However, when signal diodes are connected together in series, the current will be the same for each diode so the maximum forward current must not be exceeded.

Connecting Signal Diodes in Series

Another application for the small signal diode is to create a regulated voltage supply. Diodes are connected together in series to provide a constant DC voltage across the diode combination. The output voltage across the diodes remains constant in spite of changes in the load current drawn from the series combination or changes in the DC power supply voltage that feeds them. Consider the circuit below.

As the forward voltage drop across a silicon diode is almost constant at about 0.7v, while the current through it varies by relatively large amounts, a forward-biased signal diode can make a simple voltage regulating circuit. The individual voltage drops across each diode are subtracted from the supply voltage to leave a certain voltage potential across the load resistor, and in our simple example above this is given as 10v - ( 3*0.7V ) = 7.9V.

This is because each diode has a junction resistance relating to the small signal current flowing through it and the three signal diodes in series will have three times the value of this resistance, along with the load resistance R, forms a voltage divider across the supply.

By adding more diodes in series a greater voltage reduction will occur. Also series connected diodes can be placed in parallel with the load resistor to act as a voltage regulating circuit. Here the voltage applied to the load resistor will be 3*0.7v = 2.1V. We can of course produce the same constant voltage source using a single Zener Diode. Resistor, RD is used to prevent excessive current flowing through the diodes if the load is removed.

Freewheel Diodes

Signal diodes can also be used in a variety of clamping, protection and wave shaping circuits with the most common form of clamping diode circuit being one which uses a diode connected in parallel with a coil or inductive load to prevent damage to the delicate switching circuit by suppressing the voltage spikes and/or transients that are generated when the load is suddenly turned “OFF”. This type of diode is generally known as a “Free Wheeling Diode”, “Flywheel Diode” or simply Freewheel diode as it is more commonly called.

The Freewheel diode is used to protect solid state switches such as power transistors and MOSFET’s from damage by reverse battery protection as well as protection from highly inductive loads such as relay coils or motors, and an example of its connection is shown below.

Use of the Freewheel Diode



Modern fast switching, power semiconductor devices require fast switching diodes such as free wheeling diodes to protect them form inductive loads such as motor coils or relay windings. Every time the switching device above is turned “ON”, the freewheel diode changes from a conducting state to a blocking state as it becomes reversed biased.

However, when the device rapidly turns “OFF”, the diode becomes forward biased and the collapse of the energy stored in the coil causes a current to flow through the freewheel diode. Without the protection of the freewheel diode high di/dt currents would occur causing a high voltage spike or transient to flow around the circuit possibly damaging the switching device.

Previously, the operating speed of the semiconductor switching device, either transistor, MOSFET, IGBT or digital has been impaired by the addition of a freewheel diode across the inductive load with Schottky and Zener diodes being used instead in some applications. But during the past few years however, freewheel diodes had regained importance due mainly to their improved reverse-recovery characteristics and the use of super fast semiconductor materials capable at operating at high switching frequencies.

Other types of specialized diodes not included here are Photo-Diodes, PIN Diodes, Tunnel Diodes and Schottky Barrier Diodes. By adding more PN junctions to the basic two layer diode structure, other types of semiconductor devices can be made.

For example a three layer semiconductor device becomes a Transistor, a four layer semiconductor device becomes a Thyristor or Silicon Controlled Rectifier and five layer devices known as Triac’s are also available.

In the next tutorial about diodes, we will look at the large signal diode sometimes called the Power Diode. Power diodes are silicon diodes designed for use in high-voltage, high-current mains rectification circuits.


Saturday, January 6, 2018

Basic Electronics on the Go - PN Junction Diode


 The effect described in the previous tutorial is achieved without any external voltage being applied to the actual PN junction resulting in the junction being in a state of equilibrium.
However, if we were to make electrical connections at the ends of both the N-type and the P-type materials and then connect them to a battery source, an additional energy source now exists to overcome the potential barrier.

The effect of adding this additional energy source results in the free electrons being able to cross the depletion region from one side to the other. The behaviour of the PN junction with regards to the potential barrier’s width produces an asymmetrical conducting two terminal device, better known as the PN Junction Diode.

 A PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as the diode has an exponential current-voltage ( I-V ) relationship and therefore we can not described its operation by simply using an equation such as Ohm’s law.

If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.

Applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking current flow through the diode.

The depletion layer widens with an increase in the application of a reverse voltage and narrows with an increase in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction resulting in physical changes taking place. One of the results produces rectification as seen in the PN junction diodes static I-V (current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when the polarity of bias voltage is altered as shown below.

Junction Diode Symbol and Static I-V Characteristics.



But before we can use the PN junction as a practical device or as a rectifying device we need to firstly bias the junction, ie connect a voltage potential across it. On the voltage axis above, “Reverse Bias” refers to an external voltage potential which increases the potential barrier. An external voltage which decreases the potential barrier is said to act in the “Forward Bias” direction.
There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are:
  • 1. Zero Bias – No external voltage potential is applied to the PN junction diode.
  • 2. Reverse Bias – The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN junction diode’s width.
  • 3. Forward Bias – The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN junction diodes width.

Reverse Biased PN Junction Diode

When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material.
The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode.
The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material.

Increase in the Depletion Layer due to Reverse Bias



This condition represents a high resistance value to the PN junction and practically zero current flows through the junction diode with an increase in bias voltage. However, a very small leakage current does flow through the junction which can be measured in micro-amperes, ( μA ).
One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough value, it will cause the diode’s PN junction to overheat and fail due to the avalanche effect around the junction. This may cause the diode to become shorted and will result in the flow of maximum circuit current, and this shown as a step downward slope in the reverse static characteristics curve below.

Reverse Characteristics Curve for a Junction Diode

Sometimes this avalanche effect has practical applications in voltage stabilising circuits where a series limiting resistor is used with the diode to limit this reverse breakdown current to a preset maximum value thereby producing a fixed voltage output across the diode. These types of diodes are commonly known as Zener Diodes and are discussed in a later tutorial.

Forward Biased PN Junction Diode

When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow.
This is because the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this voltage point, called the “knee” on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below.

Forward Characteristics Curve for a Junction Diode

  The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the “knee” point.

Reduction in the Depletion Layer due to Forward Bias


This condition represents the low resistance path through the PN junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes.

Since the diode can conduct “infinite” current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device.


Junction Diode Summary

The PN junction region of a Junction Diode has the following important characteristics:
  • Semiconductors contain two types of mobile charge carriers, Holes and Electrons.
  • The holes are positively charged while the electrons negatively charged.
  • A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so that it contains mobile charges which are primarily electrons.
  • A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so that it contains mobile charges which are mainly holes.
  • The junction region itself has no charge carriers and is known as the depletion region.
  • The junction (depletion) region has a physical thickness that varies with the applied voltage.
  • When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer which is approximately 0.5 to 0.7v for silicon diodes and approximately 0.3 of a volt for germanium diodes.
  • When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short circuit allowing full current to flow.
  • When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow, (only a very small leakage current).
We have also seen above that the diode is two terminal non-linear device whose I-V characteristic are polarity dependent as depending upon the polarity of the applied voltage, VD the diode is either Forward Biased, VD > 0 or Reverse Biased, VD < 0. Either way we can model these current-voltage characteristics for both an ideal diode and for a real diode.

Junction Diode Ideal and Real Characteristics


In the next tutorial about diodes, we will look at the small signal diode sometimes called a switching diode which is used in general electronic circuits. As its name implies, the signal diode is designed for low-voltage or high frequency signal applications such as in radio or digital switching circuits.
Signal diodes, such as the 1N4148 only pass very small electrical currents as opposed to the high-current mains rectification diodes in which silicon diodes are usually used. Also in the next tutorial we will examine the Signal Diode static current-voltage characteristics curve and parameters.

(to be updated)

Tuesday, December 19, 2017

Basic Electronics on the Go - PN Junction Theory


If p-type semiconductor is joined with n-type semiconductor, a p-n junction is formed. The region in which the p-type and n-type semiconductors are joined is called p-n junction. This p-n junction separates n-type semiconductor from p-type semiconductor.

In n-type semiconductors, large number of free electrons is present - due to this they get repelled from each other and try to move from a high concentration region (n-side) to a low concentration region (p-side). Moreover, near the junction free electrons and holes are close to each other. According to coulombs law there exist a force of attraction between opposite charges.

Hence, the free electrons from n-side attracted towards the holes at p-side. Thus, the free electrons move from n-side to p-side. Similarly, holes move from p-side to n-side.

Positive and negative charge at p-n junction

The free electrons that are crossing the junction from n-side provide extra electrons to the atoms on the p-side by filling holes in the p-side atoms. The atom that gains extra electron at p-side has more number of electrons than protons. We know that, when the atom gains an extra electron from the outside atom it will become a negative ion. 

Thus, each free electron that is crossing the junction from n-side to fill the hole in p-side atom creates a negative ion at p-side. Similarly, each free electron that left the parent atom at n-side to fill the hole in p-side atom creates a positive ion at n-side.

Negative ion has more number of electrons than protons. Hence, it is negatively charged. Thus, a net negative charge is build at the p-side of p-n junction. Similarly, positive ion has more number of protons than electrons. Hence, it is positively charged. Thus, a net positive charge is build at n-side of the p-n junction.

The net negative charge at p-side of the p-n junction prevents further flow of free electrons crossing from n-side to p-side because the negative charge present at the p-side of the p-n junction repels the free electrons. Similarly, the net positive charge at n-side of the p-n junction prevents further flow of holes from p-side to n-side.

Thus, immobile positive charge at n-side and immobile negative charge at p-side near the junction acts like a barrier or wall and prevent the further flow of free electrons and holes. The region near the junction where flow of charges carriers are decreased over a given time and finally results in empty charge carriers or full of immobile charge carriers is called depletion region.

The depletion region is also called as depletion zone, depletion layer, space charge region, or space charge layer. The depletion region acts like a wall between p-type and n-type semiconductor and prevents further flow of free electrons and holes.

Thursday, December 14, 2017

Basic Electronics on the Go - Semiconductor Basics


 If Resistors are the most basic passive component in electrical or electronic circuits, then we have to consider the Signal Diode as being the most basic “Active” component.

However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as it has an exponential I-V relationship and therefore can not be described simply by using Ohm’s law as we do for resistors.

Diodes are basic unidirectional semiconductor devices that will only allow current to flow through them in one direction only, acting more like a one way electrical valve, (Forward Biased Condition). But, before we have a look at how signal or power diodes work we first need to understand the semiconductors basic construction and concept.

Diodes are made from a single piece of Semiconductor material which has a positive “P-region” at one end and a negative “N-region” at the other, and which has a resistivity value somewhere between that of a conductor and an insulator. But what is a “Semiconductor” material?, firstly let’s look at what makes something either a Conductor or an Insulator.


The electrical Resistance of an electrical or electronic component or device is generally defined as being the ratio of the voltage difference across it to the current flowing through it, basic Ohm´s Law principals. The problem with using resistance as a measurement is that it depends very much on the physical size of the material being measured as well as the material out of which it is made. For example, if we were to increase the length of the material (making it longer) its resistance would also increase proportionally.

 Likewise, if we increased its diameter (making it fatter) its resistance value would decrease. So we want to be able to define the material in such a way as to indicate its ability to either conduct or oppose the flow of electrical current through it no matter what its size or shape happens to be.

The quantity that is used to indicate this specific resistance is called Resistivity and is given the Greek symbol of ρ, (Rho). Resistivity is measured in Ohm-metres, ( Ω-m ). Resistivity is the inverse of conductivity.

If the resistivity of various materials is compared, they can be classified into three main groups, Conductors, Insulators and Semi-conductors as shown below.

Resistivity Chart




From above we now know that Conductors are materials that have very low values of resistivity, usually in the micro-ohms per metre. This low value allows them to easily pass an electrical current due to many free electrons floating about,within their basic atom structure. But these electrons will only flow through a conductor if there is something to spur their movement, and that something is an electrical voltage.

 When a positive voltage potential is applied to the material these “free electrons” leave their parent atom and travel together through the material forming an electron drift, more commonly known as a current. How “freely” these electrons can move through a conductor depends on how easily they can break free from their constituent atoms when a voltage is applied. Then the amount of electrons that flow depends on the amount of resistivity the conductor has.

Examples of good conductors are generally metals such as Copper, Aluminium, Silver or non metals such as Carbon because these materials have very few electrons in their outer “Valence Shell” or ring, resulting in them being easily knocked out of the atom’s orbit and become "free".

This allows them to flow freely through the material until they join up with other atoms, producing a “Domino Effect” through the material thereby creating an electrical current. Copper and Aluminium is the main conductor used in electrical cables.

Generally speaking, most metals are good conductors of electricity, as they have very small resistance values, usually in the region of micro-ohms per metre. While metals such as copper and aluminium are very good conducts of electricity, they still have some resistance to the flow of electrons and consequently do not conduct perfectly. Also the conductivity of conductors increases with ambient temperature because metals are also generally good conductors of heat.


Insulators on the other hand are the exact opposite of conductors. They are made of materials, generally non-metals, that have very few or no “free electrons” floating about within their basic atom structure because the electrons in the outer valence shell are strongly attracted by the positively charged inner nucleus.

In other words, the electrons are stuck to the parent atom and can not move around freely so if a potential voltage is applied to the material, no current will flow as there are no “free electrons” available to move and which gives these materials their insulating properties.

Insulators also have very high resistances, millions of ohms per metre, and are generally not affected by normal temperature changes. Examples of good insulators are marble, fused quartz, p.v.c. plastics, rubber etc.

Insulators play a very important role within electrical and electronic circuits, because without them electrical circuits would short together and not work. For example, insulators made of glass or porcelain are used for insulating and supporting overhead transmission cables while epoxy-glass resin materials are used to make printed circuit boards, PCB’s etc. while PVC is used to insulate electrical cables.

Semiconductor Basics

Semiconductors materials such as silicon (Si), germanium (Ge) and gallium arsenide (GaAs), have electrical properties somewhere in the middle, between those of a “conductor” and an “insulator”. They are not good conductors nor good insulators (hence their name “semi”-conductors). They have very few “free electrons” because their atoms are closely grouped together in a crystalline pattern called a “crystal lattice” but electrons are still able to flow, but only under special conditions.

The ability of semiconductors to conduct electricity can be greatly improved by replacing or adding certain donor or acceptor atoms to this crystalline structure thereby, producing more free electrons than holes or vice versa. That is by adding a small percentage of another element to the base material, either silicon or germanium.

On their own Silicon and Germanium are classed as intrinsic semiconductors, that is they are chemically pure, containing nothing but semi-conductive material. But by controlling the amount of impurities added to this intrinsic semiconductor material it is possible to control its conductivity. Various impurities called donors or acceptors can be added to this intrinsic material to produce free electrons or holes respectively.

This process of adding donor or acceptor atoms to semiconductor atoms (the order of 1 impurity atom per 10 million (or more) atoms of the semiconductor) is called Doping. As the doped silicon is no longer pure, these donor and acceptor atoms are collectively referred to as “impurities”.

The most commonly used semiconductor basics material by far is silicon. Silicon has four valence electrons in its outermost shell which it shares with its neighbouring silicon atoms to form full orbital’s of eight electrons. The structure of the bond between the two silicon atoms is such that each atom shares one electron with its neighbour making the bond very stable.

As there are very few free electrons available to move around the silicon crystal, crystals of pure silicon (or germanium) are therefore good insulators, or at the very least very high value resistors.
Silicon atoms are arranged in a definite symmetrical pattern making them a crystalline solid structure. A crystal of pure silica (silicon dioxide or glass) is generally said to be an intrinsic crystal (it has no impurities) and therefore has no free electrons.

 But simply connecting a silicon crystal to a battery supply is not enough to extract an electric current from it. To do that we need to create a “positive” and a “negative” pole within the silicon allowing electrons and therefore electric current to flow out of the silicon. These poles are created by doping the silicon with certain impurities.

A Silicon Atom Structure



The diagram above shows the structure and lattice of a ‘normal’ pure crystal of Silicon.

N-type Semiconductor Basics

In order for our silicon crystal to conduct electricity, we need to introduce an impurity atom such as Arsenic, Antimony or Phosphorus into the crystalline structure making it extrinsic (impurities are added). These atoms have five outer electrons in their outermost orbital to share with neighbouring atoms and are commonly called “Pentavalent” impurities.

 This allows four out of the five orbital electrons to bond with its neighbouring silicon atoms leaving one “free electron” to become mobile when an electrical voltage is applied (electron flow). As each impurity atom “donates” one electron, pentavalent atoms are generally known as “donors”.

 Antimony (symbol Sb) as well as Phosphorus (symbol P), are frequently used as a pentavalent additive to silicon. Antimony has 51 electrons arranged in five shells around its nucleus with the outermost orbital having five electrons. The resulting semiconductor basics material has an excess of current-carrying electrons, each with a negative charge, and is therefore referred to as an N-type material with the electrons called “Majority Carriers” while the resulting holes are called “Minority Carriers”.

When stimulated by an external power source, the electrons freed from the silicon atoms by this stimulation are quickly replaced by the free electrons available from the doped Antimony atoms. But this action still leaves an extra electron (the freed electron) floating around the doped crystal making it negatively charged.
Then a semiconductor material is classed as N-type when its donor density is greater than its acceptor density, in other words, it has more electrons than holes thereby creating a negative pole as shown.

Antimony Atom and Doping



 The diagram above shows the structure and lattice of the donor impurity atom Antimony.


P-Type Semiconductor Basics

If we go the other way, and introduce a “Trivalent” (3-electron) impurity into the crystalline structure, such as Aluminium, Boron or Indium, which have only three valence electrons available in their outermost orbital, the fourth closed bond cannot be formed. Therefore, a complete connection is not possible, giving the semiconductor material an abundance of positively charged carriers known as holes in the structure of the crystal where electrons are effectively missing.

As there is now a hole in the silicon crystal, a neighbouring electron is attracted to it and will try to move into the hole to fill it. However, the electron filling the hole leaves another hole behind it as it moves. This in turn attracts another electron which in turn creates another hole behind it, and so forth giving the appearance that the holes are moving as a positive charge through the crystal structure (conventional current flow).

This movement of holes results in a shortage of electrons in the silicon turning the entire doped crystal into a positive pole. As each impurity atom generates a hole, trivalent impurities are generally known as “Acceptors” as they are continually “accepting” extra or free electrons.

Boron (symbol B) is commonly used as a trivalent additive as it has only five electrons arranged in three shells around its nucleus with the outermost orbital having only three electrons. The doping of Boron atoms causes conduction to consist mainly of positive charge carriers resulting in a P-type material with the positive holes being called “Majority Carriers” while the free electrons are called “Minority Carriers”.

Then a semiconductor basics material is classed as P-type when its acceptor density is greater than its donor density. Therefore, a P-type semiconductor has more holes than electrons.

Boron Atom and Doping


 The diagram above shows the structure and lattice of the acceptor impurity atom Boron.


Saturday, November 25, 2017

Basic Electronics on the Go - Star Delta Transformation


Star Delta Transformation

 We can now solve simple series, parallel or bridge type resistive networks using Kirchhoff´s Circuit Laws, mesh current analysis or nodal voltage analysis techniques but in a balanced 3-phase circuit we can use different mathematical techniques to simplify the analysis of the circuit and thereby reduce the amount of math’s involved which in itself is a good thing.

 Standard 3-phase circuits or networks take on two major forms with names that represent the way in which the resistances are connected, a Star connected network which has the symbol of the letter, Υ (wye) and a Delta connected network which has the symbol of a triangle, Δ (delta).

 If a 3-phase, 3-wire supply or even a 3-phase load is connected in one type of configuration, it can be easily transformed or changed it into an equivalent configuration of the other type by using either the Star Delta Transformation or Delta Star Transformation process.
A resistive network consisting of three impedances can be connected together to form a T or “Tee” configuration but the network can also be redrawn to form a Star or Υ type network as shown below.

T-connected and Equivalent Star Network


As we have already seen, we can redraw the T resistor network above to produce an electrically equivalent Star or Υ type network. But we can also convert a Pi or π type resistor network into an electrically equivalent Delta or Δ type network as shown below.

Pi-connected and Equivalent Delta Network.


  Having now defined exactly what is a Star and Delta connected network it is possible to transform the Υ into an equivalent Δ circuit and also to convert a Δ into an equivalent Υ circuit using a the transformation process. This process allows us to produce a mathematical relationship between the various resistors giving us a Star Delta Transformation as well as a Delta Star Transformation.


Delta Star Transformation

To convert a delta network to an equivalent star network we need to derive a transformation formula for equating the various resistors to each other between the various terminals. Consider the circuit below.

Delta to Star Network.


 Compare the resistances between terminals 1 and 2.

 Resistance between the terminals 2 and 3.



 Resistance between the terminals 1 and 3.

 This now gives us three equations and taking equation 3 from equation 2 gives:


Then, re-writing Equation 1 will give us:

Adding together equation 1 and the result above of equation 3 minus equation 2 gives:

 From which gives us the final equation for resistor P as:

Similarly, resistor Q and R  can be found  :-

When converting a delta network into a star network the denominators of all of the transformation formulas are the same: A + B + C, and which is the sum of ALL the delta resistances. 

Star Delta Transformation

Star Delta transformation is simply the reverse of above. We have seen that when converting from a delta network to an equivalent star network that the resistor connected to one terminal is the product of the two delta resistances connected to the same terminal, for example resistor P is the product of resistors A and B connected to terminal 1.

The value of the resistor on any one side of the delta, Δ network is the sum of all the two-product combinations of resistors in the star network divide by the star resistor located “directly opposite” the delta resistor being found. For example, resistor A is given as:

 Resistor B is given as:

Resistor C given as: