Duo-range Potentiometer - Construction, Working & Advantages

Construction of Duo-range Potentiometer :

The duo-range potentiometer is the modified version of Crompton's dc potentiometer with an extended range selector. The construction and connection of a Duo-range Potentiometer is shown below.

The duo-range potentiometer has two different ranges and the range can be selected by simply throwing the SPDT switch to position X 1 or X 0.1. To vary the range, a potential divider arrangement is provided by resistors R₁ and R₂. The main dial can be varied in steps and is used for the calibration of the meter while the DPDT switch is thrown to position ab.

The slide wire dial is used to balance the galvanometer when the DPDT switch is thrown to position a'b', which connects the circuit to the unknown emf to be measured. The separate dial for the slide wire circuit makes it possible to check the calibration at any time during the operation, without disturbing the slide wire contact position.

The series resistor R protects the galvanometer from damages due to over currents when the deflection is far away from the balance or null position. When the deflection comes nearer to the null position, switch S is closed to short circuit R_s.

Principle of Operation :

When the SPDT switch is at the X 1 position, the equivalent circuit diagram for the dial circuit is shown in the figure below. In the circuit, R_m is the total resistance of the main dial and slide wire. Hence, the range of the meter corresponds to this working current I_m through R_m.

When the SPDT switch is at X 0.1 position, the equivalent circuit diagram is shown below. It is obvious that the current through R_m decreases from I_m = V_m/R_m to I_m' = V_m / (R_m + R₁).

To make the current drawn from the source equal in both the case i.e., I_m + I_r = I_m' + I_r',

     Therefore, by selecting the value of R₂ equal to R_m, the working current in both cases can be made equal. To make the range half of the original, I_m' has to be half of I_m i.e., I_m' = 0.5 I_m.

     Similarly, to have the range of 1/10th of original I_m' = 0.1I_m and R_m = 9R₁. In this way by selecting the appropriate value of R₁, I_m can be adjusted to measure the required range of potential.

Advantages of Duo-range Potentiometer :

• For small voltages, the precision of measurement is increased by one decimal place.
• Accuracy of measurement is also increased since the major part of reading is made on the main dial resistors which have greater accuracy than slide wire.
• The meter can be easily converted into a triple range potentiometer by adding another potential divider circuit with appropriate resistances. This increases the precision to two decimal places.