Class 2 (2nd Engineer) MET 📅 Jul 2018

Exam Question

(a) Describe an accurate method of comparing capacities of two condensers. (6)

(b) A diode valve, having the following characteristic

Ia (milliamperes) 0 5.5 13 22 32 42 52 59 63

Va (volts) 0 25 50 75 100 125 150 175 200

Is connected in series with a resistor of 10,000 Ohms to a 240 V d.c. supply. If a resistor of 40,000 Ohms is connected between the anode and cathode, determine the current through the diode.

Reference Answer

(a) An accurate method to compare the capacitances of two capacitors involves using a Wheatstone Bridge configured for capacitance measurement, commonly known as a De Sauty Bridge .
The De Sauty Bridge is an AC bridge circuit specifically designed to compare two capacitors. It consists of four arms: two containing the capacitors under comparison and two containing known resistors. The bridge is balanced by adjusting these resistors until no current flows through the detector, indicating that the ratio of the capacitances is equal to the ratio of the resistances.
At balance, the ratio of the capacitances is inversely proportional to the ratio of the resistances:
$$\frac{C_2}{C_3}=\frac{R_2}{R_1}$$

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