Thursday, September 25, 2014
Accurate Capacitance Meter Circuit Diagram
- The charges on Cr and CX are now equal and the meter indicates by how much the voltage across CX differs from that across Cy. Buffer IC3 presents a very high load impedance to CX.
- 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.
- Each time the output of gate N2 rises, the charges of capacitors C2 and C3 are transferred to Cr and CX by trensistors Tl and T3 respectively.
- For instance, to enable a capacitor of 470 pF to be measured, C2 and C3 have to be 10. . . 20 pF.
- The circuit is reasonably accurate for values of CX up to 100 uF:
- Two circuits ensure that reference capacitor Cr and the capacitor to be measured, CX, are charged equally. The circuit for Cr consists of C2, D1 and T1 and that for CX of C3, D2 and T3.
- ln this capacitance meter, the value of a capacitor is determined by giving it the same charge as a refer- ence capacitance and then comparing the voltages across them.
- The voltage across Cr is compared by lC2 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.
- When the output of N2 drops, C2 and C3 recharge via diodes D1 and D2. Gate N2 is controlled by astable multivibrator N 1 which operates at a frequency of about 2 kl-lz: Cr and CX are therefore charged at that frequency.
- Above that value the y measurement will be affected by leakage currents. To measure capacitors of up to 100 pF, the values of C2 and C3 should be increased to 1 AF. c l Current consumption is minimal so that a 9 V battery is an adequate power supply.
- 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 l smaller values, capacitors C2 and C3 will have to be reduced.
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