Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
Well, this is what I came up with. Not exacly simple. It should work up to 100uF. Sorry, 1000uF was too hard to handle without some active filtering usign this principle.
This is how it works. The oscillator built with IC1A produces a frequency selectable from the ganged switch of 40Hz, 500Hz, 5KHz or 20kHz, depending on the range.
There are 4 ranges: 100pF, 10nF, 1uF and 100uF.
This square-wave signal triggers the one-shot built with IC1B. The unknown cap is connected at TP!, TP2, with the + at TP1 if it happens to be polarized. The unknown cap then modifies the period of the one-shot. The resistor in the one-shot is also selected depending on the range.
The values chosen should produce a one-shot period that is about 90% the oscillator period for the maximum capacitance in each range. For example, if the range is 1uF, then connecting a 1uF cap should produce about a 1.8ms output pulse (at 500kHz, T=2ms so 2ms*90%=1.8ms).
The output pulses of the one-shot are then filtered by R15, C8 and applied to the amplifier. That results in about a 0~6V output from the filter. Next, the opamp amplifies this filtered signal with a gain of about 1.6 to produce a 0~10V output.
To adjust it: get good known caps.
For eachj range before each measurement, with no cap connected, adjust R14 for a zero output. R14 should be available on the panel, for adjustment. Once the voltage is zero, connect the cap. Then adjust the corresponding gain pot (R2, R16, R17, or R18) to obtain 10V at the output. Continue for the other ranges.
Now you can measure caps: select a range, adjust the zero, connect the cap and read the voltage. The voltage is directly proportional to the cap's value. For example, if you are using the 1uF range and you measure 4.7V then the cap's value is 4.7/10*1uF=0.47uF.
If the meter indicates more than 10V, switch to the upper range, because the cap is too large.
The best way to test and asjust the circuit is to use a scope and see exactly how things operate, make the corrections, etc.
Just two more things: I think you will be better off with the CMOS version of the 556 and the 12V supply should be stable, as the amplitude of the output pulses and ultimately the output voltage of the circuit depends on it.