Wheatstone Bridge

Source: wikipedia.org

The wheatstone bridge is a very simple circuit that is also very useful. As you can see, it only has six components, actually there are only five as the component labelled as R sub x is the component we are trying to find the value.

The components labelled R are resistors, the V sub G is a voltage meter and the long and short lines with a '+' is a battery. R sub 2 is a variable resistor or a potentiometer.

HOW IT WORKS

CURRENT

Mathematically, R sub 2 divided by R sub 1 = R sub x divided by R sub 3. From here on I will call R sub 1, R1. As the battery is applied to point A and point C current will be divided between the branch R1, R2 and branch R3, Rx. The current through each branch will depend on the total resistance of each branch. The branch with the lower overall resistance will have more current.

VOLTAGE

The voltage is the same on each branch. R1 and R2 act as a voltage divider as well as R3 and Rx. Due to the voltage divider in parallel, we can mathematically deduce that the ratio between R1, R2 must equal R3, Rx.

USING THE METER (V sub G)

You can use a meter or you can also use a VOM or a DMM. All you are using the meter for is to make sure the meter reads zero. The potential difference between point B and point D should be zero.

FINDING THE VALUE OF Rx

Once the meter reads zero, enter the known values into the equation R2/R1 = Rx/R3. Solve for Rx. When you set up this circuit, you'll want R1 and R3 to be a known value. R2 needs to be variable so use can adjust the resistance to zero out the meter. You will need to measure the resistance of R2 once you zero out the meter because you will need this value to solve the equation.

I just viewed a tutorial where a wheatstone bridge was used to identify two transistors that have a matching beta or gain. When you are repairing or building a stereo amplifier, it is wise to use transistors which have matching betas so one channel will not amplify more than the other channel.

As far as the battery, you can also use a power supply. Just be sure that the supply is filtered so it doesn't have too much ripple.

I will explain this procedure in another post, as this post was meant to discuss a wheatstone bridge.