Basic Electronics On The Go -Capacitive Voltage Divider

From http://www.electronicshub.org/capacitive-voltage-divider/

Introduction

In a voltage divider circuit,the supply voltage or circuit voltage is distributed among all the components in the circuit equally,depending on the capacity of those components.

The construction of capacitive voltage divider circuit is the  same as the resistive voltage divider circuit. But like resistors, the capacitive voltage divider circuit is not affected by the changes in the frequency even though it uses reactive elements.

The capacitor is a passive component which stores electrical energy in the metal plates. A capacitor has two plates and these two are separated by non-conducting or insulating material, such as called as “dielectric”.

Here the positive charge is stored on one plate and negative charge is stored on another plate.

When DC current is applied to the capacitor, it charges fully. The dielectric material between the plates acts as insulator and also it opposes the current flow through the capacitor.

This opposition to supply current through the capacitor is called reactance (XC) of a capacitor. The capacitor reactance is also measured in ohms.

A fully charged capacitor acts as an energy source, because a capacitor stores energy and discharges it to the circuit components.

Voltage Distribution in Series Capacitors

If the capacitors are connected in series , the voltage distribution between the capacitors is calculated. Because the capacitors have different voltage values depending on the capacitance values in series connection.

The reactance of a capacitor which opposes the flow of current , depends on the value of capacitance and frequency of the applied current.

So now let us see how the reactance affects the capacitors , by calculating the frequency and capacitance values. Below circuit shows the capacitive voltage divider circuit in which 2 capacitors are connected in series.

Capacitive Voltage Divider

The two capacitors which are connected in series have the capacitance values of 10uF and 22uF respectively. Here the circuit voltage is 10V,this voltage is distributed between both capacitors.

In the series connection all the capacitors have same charge (Q) on it but the supply voltage (VS) is not same for all capacitors.

The circuit voltage is shared by the capacitors depending on the capacitance values of the capacitors.i.e. in the ratio of V = Q/C.

From these values we have to calculate the reactance (XC) of each capacitor by using frequency and capacitance values of capacitors.

Capacitive Voltage Divider Example No1

Now we will calculate the voltage distribution to the capacitors 10uF and 22uF which are given in the above figure which have 10V supply voltage with 40HZ frequency.

Reactance of 10uF capacitor,

XC1 = 1/2πfC1 = 1/(2*3.142*40*10*10-6) = 400Ω

Reactance of 22uF capacitor,

XC\2 = 1/2πfC2 = 1/(2*3.142*40*22*10-6) = 180Ω

Total capacitive reactance of a circuit is,

XC= XC1+ XC2= 400Ω + 180Ω = 580Ω

CT= C1C2/(C1+C2) = (10*22*10-12)/(32*10-6) = 6.88uF

XCT = 1/2πfCT = 1/(2*3.142*40*6.88*10-6) = 580Ω

The current in the circuit is,

I = V/XC = 10V/580Ω = 17.2mA

Now, the voltage drop across each capacitor is,

VC1 = I*XC1 = 17.2mA*400Ω = 6.9V

VC2 = I*XC2 =17.2mA*180Ω = 3.1V

Summary

• The opposition for the flow of current in the capacitor is known as reactance (XC) of a capacitor. This capacitive reactance is influenced by the parameters like capacitance value,frequency of supply voltage and also these values are inversely proportional to the reactance.
• The AC voltage divider circuit will distribute the supply voltage to all the capacitors depending on their capacitance value.
• These voltage drops for the capacitors are same for any frequency of supply voltage. i.e. the voltage drops across capacitors are independent on frequency.
• But the current flowing is depending on frequency and also these two are directly proportional to each other.
• But in DC voltage divider circuits, it is not an easy task to calculate the voltage drops across capacitors as it depends on reactance value, because the capacitors block DC current flow through it after fully charged.
• The capacitive voltage divider circuits are used in large electronics applications.Mainly used in capacitive sensitive screens those change their output voltage when it is touched by a person's finger.
• And also used in transformers to increase voltage drop where generally the mains transformer contains low voltage drop chips and components.
• Finally one thing to say is in voltage divider circuit the voltage drops across capacitors are same for all frequency values.