Is there a simple capacitor matching method?

[Sanaka]

Is there a simple circuit/setup one can build that will match pairs of capacitors?

I'm thinking something where you clip in two caps, and if they are a reasonable match, say a couple percent or so, a light goes on or goes off or some indicator is activated. Relative matching rather than absolute value being the important criterion, and 1uf to 1000uf being the important range.

The reason is so that signal path caps that impact frequency response/bass rolloff are pretty close for L/R stereo channels.

Thanks!


Peace,
Sanaka

[johnnyx]

Put the caps in the arms of an ac driven bridge circuit, like a Wheatstone bridge, so that when the caps are the same the output to a meter is a minimum.I'll try to attach a diagram to explain it.

[Sch3mat1c]

It'll be friendlier if you swap R1 and C2, although you'll need a real low frequency for large values (like PSU caps) anyway.

In either case, when R1=R2 and C1=C2, the meter will show no current (i.e., balance).

Tim

[Stocker]

If all you want to do is MATCH capacitors...

There is a circuit available online though I have yet to build it, called IIRC the C-beeper or e-beeper or some such. the most exotic part is a variable capacitor that shouldn't be too hard to get. The point of the device is that there is a sound produced when a capacitance is connected to the case as you hold the other end of the capacitor. The sound is supposed to vary drastically with changes in capacitance, so matching becomes a VERY fast and easy excercise. The actual value of the caps is completely irrelevant to the device, other than changing the capacitance slightly changes pitch of audio output.

I intend to build one of these after I build a multimeter that is mostly done, and maybe before a good variable oscillator...but I will build one. If you google and can't find it (and want to) I will try to dig it up for you.

[Sanaka]

Thanks johnnyx and schematic,

That's pretty much perfect I had run across "Wheatstone bridge" but all the info I saw was for resistors and strain gauges. In your circuit johnnyx, would a plain cheapo 5v or 9v wall wart suffice for the AC driver?

Stocker: I meant to mention that I had seen the Curious C-beeper. It's a way cool concept, but optimized for small, like <1uf, caps. Plus more complicated than I need right now.

Peace,
Sanaka

[johnnyx]

A wall wart would be ok, 50Hz (or 60Hz in USA) would be fine for the larger caps found in crossovers. Btw, the resistors can be reduced from the value shown - 10k is just the default value given by the drawing program. You could elaborate it a bit and work out some way of determining when the caps are within say 1% of balance, otherwise you could be there for ever trying to find two identical ones.

[Stocker]

Now all you need is a set of matched resistors!

I have some high-precision (unmatched as yet) wirewound 10Ks though... Hmmmm...

[AndrewT]

Hi,
make one of the resistors variable with a pointer and scale.
Adjust the pot to achieve balance and note each reading on the caps. Quite soon you will have some that are quite close, then you can be more precise.
regards Andrew T.

[gmphadte]

Many cheap DMMs have capacitance measurement facility

Gajanan Phadte

[Stocker]

Many of us do not want to purchase additional DMMs!

The cost to quite a few of us will be $0 for this type of project.

I just did 3 measurements. I now have 2 N.I.S.T. traceably matched resistors (0.00x% variance ) and one that's within 0.02%. Now for a little transformer...

5-digit ohmmeter at work does a good job of measuring 10k's to the ohm!