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Measurements of basic electrical quantities
Bridge circuits
Universal measuring instruments
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To measure resistance, a milliammeter is used, into which a power source (battery) is inserted, and the measured resistance is connected to external terminals in series with the meter. The scale of such a device is graduated in ohms. However, this scale will be uneven and the measurement accuracy is low. Therefore, such devices are mainly used by installers of electronic equipment as probes showing the presence of a contact, and bridge circuits are used to measure resistance.
The bridge circuit consists of two parallel branches of current flow, between which the diagonal of the bridge is located. The diagonal includes a microammeter or other highly sensitive balance indicator (Fig. 2).
Fig 2. Bridge measuring circuit: R1 - R3 - bridge circuit resistors, Rx - measured resistor, R0 - limiting resistor.
The sections of two parallel branches R1, R2 and RЗ, Rx are called the arms of the bridge.
The measurement process consists in the fact that, by adjusting the variable resistor R1, such a value of its resistance is selected, at which the arrow of the measuring device reaches 0 (balance of the bridge).
When the bridge is in equilibrium, the current in the diagonal is zero. If the current does not branch off into a diagonal, each of the branches can be considered as a simple voltage divider. In this case, the supply voltage is distributed in proportion to the resistance:
Ux / U2 = Rx / R2; U1 / U3 = R1 / R3.
Equilibrium condition:
Ux = U1 and U2 = U3;
means that the proportion is set:
Rx / R2 = R1 / R3,
where the formula for calculating the measured resistance is obtained:
Rx = R1 (R2 / R3) = R1N,
where N = R2 / R3 - 1, 10, 100 or 0.1; 0.01, etc.
Bridge circuits are used to measure capacitances and inductances on alternating current, since in this case, complex (impedance) resistances are actually measured. Bridge measurements are highly accurate because the result is independent of the current meter reading.
In mass checks of a large number of resistors or capacitors with the same nominal value of the measured parameter, not the value itself (Rx or Cx) should be measured, but its deviation from the specified value. In this case, as a rule, the speed of testing is important. In such cases, unbalanced bridges can be used; the measurement consists in taking readings from instruments included in the diagonals of the bridges. The current in the diagonal of the bridge is practically proportional to the deviation from equilibrium, while it is small, but with a large deviation, proportionality is violated.
Unbalanced bridges are often used as direct-reading devices for approximate measurements, as well as for observing changes in resistance, capacitance and other parameters under the influence of, for example, temperature, humidity, etc.
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