Electrical Engineering ⇒ Topic : Volt Ratio Box

Volt Ratio Box

We have already seen that a potentiometer cannot measure a potential difference or e.m.f.greater than the e.m.f. of the supply battery B (about 2 volts). Therefore, the range of potentiometer is less than 2 volts. For measuring voltages above 2 volts, we use a volt ratio box (or volt-box) in conjunction with the potentiometer. The volt-box consists of a high resistance (30 Q to 50 Q per volt) having a number of tappings. Its function is that of a potential divider.

                                                              figure (a)

Fig. (a) shows the connections of a volt-box having resistance of 50 Ω per volt. The connections to the potentiometer are taken from two tapping points, which include between them (say) 75 Ω. If a voltage greater than 75 V but less than 150 V is to be measured, then this voltage is connected between the two terminals marked 150 V and "common". Thus if the measured value of voltage on the potentiometer is 1.25 V, then the value of unknown voltage connected across them terminals of volt box