Saturday, June 15, 2024

How do I use concept of Wheatstone bridge in solving circuit of capacitance?

 How do I use concept of Wheatstone bridge in solving circuit of capacitance?

A Wheatstone Bridge is used to measure small changes in a much larger signal, either a voltage or a resistance ratio. It can also be used to measure small changes in capacitance, such as for a sensor that changes capacitance to indicate the quantity it is measuring.

A garden variety Wheatstone Bridge from Electronics-Tutorials dot com:

Or R3 may be fixed, and R2 made a variable resistor.

To measure capacitance, you must use an AC voltage source. Then you use resistors on the left, and the other a reference capacitor and the capacitance you are measuring. On the left side, there is no phase shift because it is pure resistance, on the right, there is no phase shift because it is pure capacitance. Assuming a very low impedance AC driving source.

Thanks to Play-Hookey dot com for the image:

Or both along the top are resistors, and the bottom elements are the reference capacitor and the capacitance you are measuring.

Thanks again to Play-Hookey dot com:

Here, C1 is a variable, but it could be a fixed reference and make R2 a variable resistor. Don’t make R1 the variable. With fixedR1, R1C1 and R2Cx make a shift in phase, and by making R2 variable, you are making the phase shift equal when the voltages are equal.

If you make R1 variable, then except for the special case of R1 and R2 being equal and C1 and Cx being equal, you’ll never get both phase and voltage equal at the same time and so the null will be shallow and inaccurate.

Although to be pedantic, a Wheatstone Bridge is DC current and resistive only. A Shering Bridge is the name given for a particular configuration used to measure capacitance and the dissipation factor.

Schering Bridge Measurement of Capacitance using Schering Bridge | Electrical4U

C1 is the capacitor being measured. C4 is variable, and R4 is variable.

Heathkit sold several different varieties of RLC bridges, which are just Wheatstone Bridges using AC as the driving voltage. When measuring a capacitor, a reference capacitor is switched into the circuit. When measuring an inductor, a reference inductor is switched in the circuit. Here is a simplified diagram from the Heathkit IG-5281 solid state RLC bridge:

Different frequencies are used to suit the ranges:

In the case of Heathkit’s RLC bridges, they do not include any reference inductors, instead they must be supplied by the user.

Although it isn’t strictly correct to call all of these Wheatstone Bridges, we do anyway.

Another device that uses the same principle to measure antenna impedance is called a Noise Bridge. A wideband noise is used as the signal source. A receiver tuned to the frequency of interest will hear a null in the noise when the variable resistor and capacitor balance the bridge. It is a bit difficult to see how this is a modified Wheatstone Bridge. The secondary is centertapped, often wound as a trifilar transformer. When the reactance and resistance of VR1//VC1 = C2//Unknown, the centertap has zero signal and so the receiver has a null (the noise gets quiet).

C2 is an offset value to allow inductive impedance to be measured, too. When the unknown impedance is purely resistive, the bridge is balanced when VR1 is equal to the load, and VC1 = C2. If there is some capacitance in the load, then VC1 will be set to a higher value to match C2 + the load capacitance.

When the load is partly inductive, VC1 will be set to less than C2. The two dials are then calibrated in ohms on VR1, and reactance for VC1.

A series connection may also be used.

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