z said:
Does anyone know a circuit to measure electrolitic capacitors in
the range 10uF to 20000 uF ?
I have a 555 ic , a 5v supply, and a frequency counter, could I
make a good circuit?
You can find a data sheet for the 555 easy enough. Go to digikey or
mouser or google the part number, ne555. Look for the multivibrator
circuit diagrams. Astable multivibrator is an oscillator. Monostable
multivibrator when you trigger it turns on for a set time, then turns
off.
In either case the values of the cap and resistors in the circuit
determine the timing: the frequency of the astable or the time period
of the monostable. So if you have a circuit where you know the values
of the resistors and you know the timing (this is where your frequency
counter comes in), then you can calculate the value of the capacitor.
The datasheets have the formulas for that. It's not totally accurate
for various reasons. For one thing, the 0.693 (or its reciprocal 1.44)
cited in the formula is the natural logarithm of 2, which assumes the
circuit follows the physics of ideal circuit elements. But voltage
drop across transistors in the chip etc. render it less than accurate.
Even with high precision resistors this is probably not a real accurate
way to measure capacitance. Really when you use a timer circuit like
this, for whatever purpose, you have to calibrate it. So it would be
to your advantage not to assume the formulas in the datasheets will
give you an accurate result. Start with a known cap and find its
frequency (or monostable time), and then other caps will be calculated
in proportion to that based on the timing results you measure for them.
Another thing, when you are using biggish electrolyics, you may get
pretty low frequencies. Trying to make the resistors real small in an
attempt to get around this may not work very well. So for really big
electrolytics, you may want big resistors in a monostable you can
measure with a stopwatch. Leakage in the electrolytics will cause some
error in that case, but you get the same thing with an astable anyway.
For little caps like ceramics you need to use an astable and measure
frequency.
There are other circuits you can build to measure capacitance that are
a lot more versatile and accurate than using a 555, but it sounds like
a fun and educational project.
