A question for anyone who tests capactiors. Do you ever use a Megger to charge it up and watch the needle swing to a higher resisitance value as the plates reach their farad rating?
Is this method 100%. I know guys who swear by it.
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A question for anyone who tests capactiors. Do you ever use a Megger to charge it up and watch the needle swing to a higher resisitance value as the plates reach their farad rating?
Is this method 100%. I know guys who swear by it.
It's 2002 AD, man, buy a Fluke model 12 for eighty bucks, and it'll give you an accurate, direct read on microfarads.
I used to have a capacitance meter. It measured shorts, shunts, capacitance and one more thing I can't remember.
I have heard that if you can't get a steady reading on a feildpiece or fluke capacity reading the capacitor is bad. Enough if the value is bouncing around in the accepted +\- 10% area.
Thanks dense dave for your helpful comments. Ive got a fluke and i know they give you a farad rating , but then that wasnt my question was it arsehole.
To answer your question. That method will tell you there is capacitance in that little silver box. The real question you what to be able to answer is: Is the capacitance within 10% of the required value? The method you discribe will not tell you the answer to that question.
Quote:
Originally posted by reggie
Thanks dense dave for your helpful comments. Ive got a fluke and i know they give you a farad rating , but then that wasnt my question was it arsehole.
OOO, a little sensitive, there, reggie? Quite the little smartass, aren't we?
HMMM. A megger. When a simple meter will do the trick? One that you already possess?
I suppose that renders both of us assholes, now doesn't it???
Lighten up. Not just your loafers, now.
Thanks superheat. Not that this was a big issue but this was a friendly debate at work. A couple of guys claimed that the Megger was the only 100% way to test it.
I don't understand how you can use a megger to test capacitors.
The megger I have sends out either 500 volts or 1000 volts. Most capacitors Iv'e seen are rated at 440 volts?
I feel i must confess to charging capacitors with a meggar and then when discharging them using the spark to light a leak detector when i had no matches.
Adapt,improvise and make do
Boys, boys, boys!
You are all right! Your nice little Fluke of Fieldpiece is fine for measuring a capacitor's rating, albeit at meter voltages instead of system voltages. (A problem) :cool: Some capacitors will be shorted at operating voltages and behave properly for a meter at it's voltage.
Therefore, a megohmeter is a nice backup to the Fluke to test at higher voltatges, 500 is just fine and won't hurt a 370v rated capacitor. The available current from the megger won't burn anything.
A third test is to use a decent ohmeter and charge the capacitor in both directions. It must, absolutely must work the same both ways to be a good one. Testing one way is not sufficient. It will charge up and show a higher and higher resistance before exceeding meter range and showing infinity. I like using my finger to discharge the capacitor before repeating the test. One finger across both terminals so the spark does not go through your heart (muy imporante):(. Some capacitors are fused and shorting with a screwdriver will blow the fuse.
You also must check from both terminals to the metal case of the capacitor. Occasionally a capacitor will be fine at the terminals and be shorted to the case.
Are you saying that my Fluke might tell me the Cap is good, but the cap is really shorted?
I like to check ampdraw. It is easy and with a calculator it is accurate.
could you explain this method superheat? Thanks.
John K.
Just to throw another spanner in the works i read an article recently that said your current draw *14 will equal the microfarad rating of the capacitor.
Wouldn't it be easier and quicker just to replace a questionable capacitor instead of trying to determine an exact measurement??
They are cheap enough in most cases.
A dual trace ocilliscope can give you the exact information you may be looking for.
amps = measured volts times micro farads divided by 2650. Thats is the volts you measure at the cap not the max rated volts printed on the side of the cap. I've got a pocket table around here someplace that has all that calculated so you can look it up. If somebody tells me how to post it I will. I was thinking maybe I could copy it in table form in a spreadsheet or something. But I do not know how to post it here.