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Re: Verify capacitance



Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>


> I was wondering if someone could clue me into how to verify capacitance
> of any given capacitor. This is the Mfd rating correct?
> I have read many a page referring to some pretty specific numbers and
> was wondering how people came up with these. Also, I would like to check
> my dual capacitor set up and compare itís real value
> to it calculated one.

For the saltwater bucket capacitor? And your DMM doesn't measure
capacitance?

A very crude, non-accurate, but simple way comes to mind: get a 12V
battery, and a couple of small polypropylene/styrene/-whatever caps
(16V?), say 4.7nF, 6.8nF, 10nF. That's nano-Farads. Connect your
multimeter accross your bucket cap, to measure voltages below 20V. Don't
connect the 12v battery to anything.

Now connect for example the 6.8nF ("reference capacitor") to your 12V
battery, then remove very darn quickly and stick it to your bucket cap
as fast as possible - don't touch the leads, otherwise this "measurement"
will give totally wrong values. If the voltage reading of the first 1..2
seconds on the DMM is about half of 12V, i.e. around 6V, your bucket cap
has approximately 6.8nF capacitance at low voltages.
 (for high voltage, and not firmly-pushed-together buckets, 
 the capacitance will change compared to this low voltage value)

For any arbitary reference capacitor, if the final voltage is half the
inital voltage, the bucket cap has a capacitance equal to that of the
reference cap.

If you know the reference capacitor's capacitance (10nF? whatever), and
the final voltage which you measured accross your bucket cap, you can
calculate, approximately, the capacitance of your bucket cap:

  charge on the reference cap

  => Q_inital = C_reference * 12V
  
  ref cap moved and connected to bucket cap, charge evens out,
  the final voltage drops while total charge remains the same as before
 
  => C_reference * U_final + C_bucket * U_final = Q_inital
  => C_reference + C_bucket = Q_initial/U_final 
  => C_reference + C_bucket = C_reference * 12V/U_final 

  => C_bucket = C_reference * ( (12V/U_final) - 1 )

Use nanofarad range reference caps because your bucket cap can be expected
to be in the nanofarad range as well (== 0.001 microfarads range).


Hope this makes sense to you...

Maybe I should stress once more that the capacitance you get with this
method will be "slightly" ;o) off from the actual capacitance. But as
you don't have a DMM with capacitance measurement this is probably the
easiest way to go...


cheers,

 - Jan

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