Voltase hobby

ln this capacitance meter, the value of a capacitor is determined by giving it the same charge as a reference capacitance and then comparing the voltages across them.

This relies on the formula C = O/V where C is the capacitance in Farads, O is the charge in Coulombs and V is the voltage in volts. lf therefore two capacitances have equal charges, their values can be calculated when the voltages across them are known. Two circuits ensure that reference capacitor Cr and the capacitor to be measured, CX, are charged equally. The circuit for Cr consists of C2, Di and T1 and that for CX of C3, D2 and T3. Each time the output of gate N2 rises, the charges of capacitors C2 and C3 are transferred to Cr and CX { by trer:cFstorsT1 and T3 respectively.

When the output of N2 drops, C2 and C3 recharge via diodes D1 and D2. Gate N2 is controlled by astable multivibrator N1 which operates at a frequency of about 2 kHz: Cr and CX are therefore charged at that frequency. The voltage across Cy is compared by IC2 with a reference voltage derived from the power supply via R3/R4. When the voltage across Cr exceeds the reference voltage, com- parator IC2 inverts which inhibits N2 and causes N3 to light LED D3. The charges on Cr and CX are now equal and the meter indicates by how much the voltage across CX differs from that across Cr. Buffer lC3 presents a very high load impedance to CX. Pressing reset button S1 causes both Cr and CX to discharge via T2 and T4 respectively, after which the charging process restarts and the circuit is ready for the next measurement. The meter is calibrated by using two identical 10 nF capacitors for Cr and CX. Press the reset button and, when the LED lights, adjust preset P1 to give a meter reading of exactly one tenth of full scale deflection (fsd).

That reading corresponds to 1 x Cr. lf, therefore, Cr = 100 nF and CX = 470 nF, the meter will read 0.47 of fsd. To ensure a sufficient number of charging cycles during a measure- ment, Cr and CX should not be smaller than 4.7nF. To measure smaller values, capacitors C2 and C3 will have to be reduced. For instance to enable a capacitor of 470 pF to be measured, C2 and C3 have to be T0. . . 20 pF. The circuit is reason- ably accurate for values of CX up to 100 pl:. Above that value the measurement will be affected by leakage currents. To measure capaci- tors of up to 100 pF, the values of C2 and C3 should be increased to 1 AF. Current consumption is minimal so that a 9 V battery is an adequate power supply.