# Types of AC Bridges

By Carlos Mano

Bridges are circuits that are used with measuring devices. A bridge consists of four components and a meter. The circuit has two branches--two components in each branch--with the meter connected as a bridge between the two branches. This allows a much more accurate measurement than using just the meter alone--especially when measuring some AC components that are hard to measure with a meter. One of the four components will be the component you are measuring.

### Resistance Measuring Bridges

A resistance bridge (for AC or DC) would have four resistors and an ohmmeter. Two of the resistors (R1 and R2) are connected in series and so are the other two (R3 and R4). These pairs of resistors are then connected in parallel so the current into the circuit splits--some going through R1 then R2 and the rest going through R3 then R4. The meter is connected across these two paths--from the junction between R1 and R2 to the junction between R3 and R4. If all for resistors are the same the meter should read zero, but it can also read zero if the resistors are related by the formula R1/R2 = R3/R4. The value of two of the resistors is known precisely, and one of the resistors is variable with a clearly marked scale. The fourth resistor is the one being measured. If the variable resistor is turned until the meter reads zero, the value of the unknown resistor can be read off the scale.

### Capacitance Measuring Bridges

Capacitors are devices that are used with AC circuits--they do not work in DC circuits except briefly when the DC circuit is turned on or off. Capacitors do affect AC circuits by shifting the relationship between the voltage and current cycles. The value of a capacitor--how it will affect an AC current is difficult to measure unless you use a bridge. For a capacitance measuring bridge the setup is exactly the same as for a resistance bridge except the four components are capacitors.

### Inductance Measuring Bridges

Inductors (aka coils or chokes) are devices that are used with AC circuits--they do not work in DC circuits except briefly when the DC circuit is turned on or off. Inductors do affect AC circuits by shifting the relationship between the voltage and current cycles. The value of an inductor--how it will affect an AC current is difficult to measure unless you use a bridge. For an inductor measuring bridge the setup is exactly the same as for a resistance bridge except the four components are inductors. With inductor bridges you must take care that the inductors are physically separated because inductors work with magnetism and if they are physically close together the magnetic fields can interact.