is more complex than the others described earlier. However, when
finished, you will have an instrument capable of measuring all but
the largest capacitors used in radio circuits. Unlike variable
resistors, most variable capacitors are not marked with their
values. As well, the markings of capacitors from salvaged
equipment often rub off. By being able to measure these unmarked
components, this project will prove useful to the constructor,
vintage radio enthusiast or antenna experimenter.
The common 555 timer IC forms the heart of the circuit (Figure
Three). Its function is to charge the unknown capacitor (Cx) to a
fixed voltage. The capacitor is then discharged into the meter
circuit. The meter measures the current being drawn through the 47
ohm resistor. The 555 repeats the process several times a second,
so that the meter needle remains steady.
The deflection on the meter is directly proportional to the value
of the unknown capacitor. This means that the scale is linear,
like the voltage and current ranges on an analogue multimeter.
The meter has five ranges, from 100pF to 1uF, selected by a five
position two pole switch. In addition, there is a x10 switch for
measuring higher values and a divide-by-two facility to allow a
better indication on the meter where the capacitor being measured
is just above 100, 1000pF, 0.01, 0.1 or 1 uF.
Component values are critical. For best accuracy, it is desirable
that the nine resistors wired to the Range switch have a 2%
tolerance. If 0A47 diodes are not available, try OA91 or OA95
germanium diodes instead. Construct the meter in a plastic box;
one that is about the size of your multimeter but deeper is ideal.
The meter movement should as large as your budget allows; you will
be using it to indicate exact values. A round 70mm-diameter
movement salvaged from a piece of electronic equipment was used in
the prototype. The meter you buy will have a scale of 0 to 50
microamps. This scale needs to be converted to read 0 to 100 (ie
20, 40, 60, 80, 100 instead of 10, 20, 30, 40, 50). Use of white
correction fluid or small pieces of paper will help here.
The components can be mounted on a piece of matrix board or
printed circuit board. Use a socket for the IC should replacement
ever be needed. Keep wires short to minimise stray capacitance;
stray capacitance reduces accuracy.
Calibrating the completed meter can be done in conjunction with a
ready-built capacitance meter. Failing this, a selection of
capacitors of known value, as measured on a laboratory meter,
could be used. If neither of these options are available, simply
buy several capacitors of the same value and use the one which is
nearest the average as your standard reference. Use several
standards to verify accuracy on all ranges.
To calibrate, disable both the x10 and divide-by-two functions (ie
both switches open). Then connect one of your reference capacitors
and switch to an appropriate range. Vary the setting of the 47k
trimpot until the meter is reading the exact value of the
capacitor. Then switch in the divide-by-two function. This should
change the reading on the meter. Adjust the 10k trimpot so that
the needle shows exactly twice the original reading. For example,
if you used a 0.01 uF reference, and the meter read 10 on the 0.1
uF range, it should now read 20. Now switch out the divide-by-two
If you are not doing so already, change to a reference with a
value equal to one of the ranges (eg 1000pF, 0.01uF, 0.1uF etc).
Switch to the range equal to that value (ie the meter reads
full-scale (100) when that capacitor is being measured. Switching
in the x10 function should cause the meter indication to drop
significantly. Adjust the 470 ohm trimpot so that the meter reads
10. Move down one range (eg from 0.01uF to 1000pF). The meter
should read 100 again. If it does not, vary the 470 ohm trimpot
until it does. That completes the calibration of the capacitance
meter. Now try measuring other components to confirm that the
measurements are reasonable.
With care, an accuracy of five percent or better should be
possible on most ranges.