Need circuit to measure capacitance?

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Without a resistor, I don\'t see how.

z wrote:

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.

no no, sorry, I _do_ have a resistor, in fact, lots of them... please continue...

555 oscillators generally produce a frequency proportional to the reciprocal of the value of the timing capacitor. In other words, the period of the oscillation is proportional to the capacitance. If your frequency meter also reads out period, the correct value of timing resistors can make the display read out capacitance, directly. If not, you take a frequency measurement and take the reciprocal with a calculator.

--- View in Courier:

VCC>--+-------------------------+ | | [Ra] +---------+ | | |_ | | +-----------O|D Vcc|--+ | | _| | +--[Rb]-+----|TH R|O-+ | |__ | +---------+ +---O|TR OUT|-----| COUNTER | | | GND | +---------+ [C] +----+----+ | | GND>----------+---------+

1.46 C = ------------- (Ra + Rb) f

-- JF

"Homer J Simpson"

** No capacitance meters in sight there.

Talk about the blind leading the blind ......

....... Phil

But it'll keep him amused for hours.

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???

http://www.siliconchip.com.au/cms/A_102408/article.html

I have seen the links, I like them all' thank you. Anyone knows what is typically used to measure caoacitance in dmm's, is it the tau=rc approach or the 555 with counter approach?

Also, is there a particular method which suits smaller capacitors 1pF to 1uF or larger capacitors 1uF to 20000 uF

Is this accurate for all ranges from 10pF to 20000uF ?

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Nope.

^^

^^

That is *NOT* what you asked for.

Ed

Hehe. In the earlier case, it's a dynamic range of only 3 orders of magnitude or so. In that latter case, it's only increased to a mere 9 orders. ;)

Jon