The Wheatstone Bridge
When extremely accurate resistance measurements are required, a Wheatstone bridge is used. A Wheatstone bridge consists of four resistors connected in a diamond-shaped array. One of the resistors is the unknown one to be measured. A current source is connected to two opposite junctions and a sensitive meter is connected between the other two junctions. The meter has a zero-center reading.
To understand how a Wheatstone bridge measures resistance, assume that resistors R I and R2 each equal 400 ohms, and resistor R3 is variable from 0 to 1000 ohms. Now connect resistor Rx into the bridge circuit and close the switch. You can see that R1 and R3 form one divider network, and R2 and Rx form another divider network. Therefore, since R1 equals R2, if R3 is made equal to Rx, the current and voltage drops in both dividers will be identical. Thus, the potentials at points C and D will be the same so that no current will flow through the meter. Therefore, when R3 is adjusted for a zero reading, you know its value equals that of Rx. The dial of variable resistor R3 is calibrated to show its exact resistance when adjusted. Therefore, its setting is also the value of unknown resistor Rx. Usually, the Wheatstone bridge contains many components so that different values of R 1, R2, and R3 can be switched in to test a wide range of resistances accurately.
Capacitors and Inductors
Prior to the development of the inexpensive digital meter, meters that measured capacitors and inductors were limited to expensive lab-type equipment. Today, though, many multimeters, as well as specialized meters, provide for routine testing of these components. Many digital volt-ohm-ammeters can check capacitors, with typical values from the low picofarads range to about 20 microfarads. Specialized test meters can test values up to 1 farad, as well as for capacitor leakage, equivalent series resistance, and dielectric absorption; some can also test inductors for values and for shorted turns.
The Wheatstone bridge can also be used to measure unknown values of capacitors and inductors in the same way as it does for resistors. However, since capacitors and inductors are reactive devices, an ac source must be used.
The same diamond-shaped array is used for the bridge, and a sensitive meter is connected between the same opposite junctions. The ac currents that are produced by the capacitive or inductive reactance’s in each leg will be the same when L3 or C3 equals Lx or Cx, as the case may be. This will cause points C and D to be at the same potential, and zero current will flow through the meter. The calibrated setting of L3 or C3 will show the value of Lx or Cx.
Diodes and Transistors
As explained for capacitors and inductors, the progress in digital meter design has allowed the meter to be used for a wide variety of sophisticated functions. In the past, analog multimeters were limited to testing resistive components; and then were able to handle capacitors and inductors. But these are all considered to be passive components with relatively fixed values. Active components, such as diodes and transistors, always required highly specialized test equipment. They still do, for a complete and reliable analysis.
A few simple static tests can be made with diodes and transistors to give an initial idea of their reliability. With simple battery circuits and the diode connected with forward or reverse bias, the forward (high) current or the reverse (low) current can be measured. These quantities and the ratio of these quantities give an indication of the reliability of the diode.
Bipolar transistors are more difficult to check because of the complex characteristics and interaction among its three elements- the base, emitter, and collector. Highly sophisticated equipment is needed to make complete dynamic tests. A typical digital multimeter, though, provides for testing the static forward current transfer ratio, which is the gain of the transistor in a circuit.