capacitor question

nsisman

Active Member
Question for electrical guru's......

I have been using this capacitor as a signal bridge for about 2 years now with no problem.

Now I am having signal problems that I didn't have before.

It is a .1 micro farad capacitor ( I think that is what it's called) and it says 250v 40/085/21 on it.

My question is how do I test it ?

Should I see a short when testing it with a multi meter ? it looks open so maybe it is blown ?

Thanks in advance

Neil
 

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BraveSirRobbin

Moderator
nsisman said:
Should I see a short when testing it with a multi meter ? it looks open so maybe it is blown ?
A capacitor does not "pass" DC current, that is why you are seeing an "open" with your meter. Can you just try another capacitor? Do you happen to have an X-10 signal strength meter?
 

JohnBullard

Active Member
250 Volts? Could that be a little on the margin rating? I would think if you replace it, to bump up the voltage rating to something like 450 volts or so just to give a little more overhead for voltage spikes or surges.
 

BraveSirRobbin

Moderator
JohnBullard said:
250 Volts? Could that be a little on the margin rating? I would think if you replace it, to bump up the voltage rating to something like 450 volts or so just to give a little more overhead for voltage spikes or surges.
Oh man, good point John, I totally missed that one!
 

Guy Lavoie

Active Member
Checking out a capacitor is usually quite easy if you have an analog Volt/Ohm meter. It is harder to do with a digital one. What you do is set it to the highest ohms scale that you have and then connect the probes to the (previously discharged) capacitor. Be careful not to touch both probes when doing this or else you'll possibly alter the test results. As you cennect the probes, watch the meter needle. It should quickly swing over close to 0 ohms and then start to rise towards infinite resistance as the capacitor charges. Then (again, without touching the capacitor terminals with your fingers) inverse the polarity of the probes. Now the needle should swing towards 0 ohms even faster (and might even go past 0 ohms since the cap's voltage is now being added to that of the meter, showing effectively negative resistance) and then should rise in resistance again as the cap charges. If that test works and the resistance rises all the way to infinity, then the cap is not blown or shorted. With a digital meter, you might see an increasing resistance as a jumble of values on the screen, or it might not appear to show anything that makes sense.

For the voltage rating, I used a 600 volt rated cap. The voltage across the two phases of a home's electrical system is normally 240 V (RMS). The peak voltage is 1.4 times that, or about 336 volts. This is the voltage that the cap must withstand. A 450 volts cap should be just fine too, but don't go below that.
 
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