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Basic Electronics » Mysteries of Capaciators in Reverse

December 08, 2011
by dgikuljot
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So I have a question that deals with the mysteries with connecting capaciators in reverse. My math teacher is always restoring/ rebuilding old amplifiers. On one project he encounted a 4700uf 35v polarized electrolytic cap. According to him there is a label on the cap that says the red electrode is the positive one. But we traced the circuit and noticed the red electrode goes to chasi ground. And the unit has worked perfectly for 40 years.

So the question is, could a capaciator used in the power supply connected in reverse work perfectly for 40 years or could this just be a case of the manufacture mislabeling the capaciator. Also the cap is run at 21v while it is rated for 35v. In other words is it necessary a polarized capaciator will always fail if connected in reverse, or is it possible in some application it will work normally for the rest of its life.

Thanks!!!

December 09, 2011
by BobaMosfet
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We'd have to see a picture of the capacitor. They can be labeled either way, although I usually see the negative lead labeled, not the positive.

The basic difference (and there may be others) between polarized capacitors and non-polarized (aka 'bipolar') capacitors is that polorized caps are usually electrolytic based. If you connect them in reverse it causes it to react chemically improperly and it can develop enormous pressure inside. Some polarized caps have a 'release' to let this pressure off, some don't. I either (presurre release or not) case, the capacitor is usually ruined if connected wrongly.

I would believe the capacitor was either mislabeled, your teacher misidentified the leads, or the capacitor may have failed in some way so as to still allow functioning, but this would be a one-in-a-million shot I would think.

BM

December 23, 2011
by huzbum
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In most cases I'm familiar with, the negative terminal is marked, but anything is possible.

Also, while it's usually the norm with battery operated devices, chassis are not always negative grounded. Sometimes, like in the instance of radio's and amplifiers (especially tube amps) there are multiple voltage sources present (one AC source can be broken into separate AC voltages with a transformer then rectified into DC by diodes, which would be stored in a polarized capacitor to flatten out the waveform), so you could have one source negative grounded and another source positive grounded.

You would have to follow the circuit very closely, paying close attention to any diodes, and I bet you'll find that the capacitor is not reverse charged, but just configured in a way you don't expect.

December 23, 2011
by killercow
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And after following the connections around, try am a digital voltmeter. Black to a known ground point, red to both sides of the cap.

Just my 2cents,

Killercow...Moo!

December 23, 2011
by BobaMosfet
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killercow-

Let's see some pictures please. I think it's important to understand what the capacitor is doing, and then try to understand the context of how it is connected. That capacitor is there for only a few reasons- time control, filter, phase control, or bypass. The type of voltage on it, AC .v. DC will help us understand it's purpose.

In order to evaluate it properly we must see the rest of the circuit that it is part of. You say you trace the connections--- but what components are between the capacitor and the positive or ground rails? Tracing connections with a voltmeter won't be entirely accurate either, as a capacitor is most used as an AC component.

This capacitor could be used for filtering, timing, voltage shift, phase control, etc. Without enough information we cannot accurately provide an answer.

BM

December 28, 2011
by dgikuljot
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Thanks for the replies guys. I will try to get some pictures for you guys from him.

December 30, 2011
by mongo
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In the old radios, electrolytic capacitors were used as an AC shunt to ground. This was necessary to gain the voltage level shift across the speakers to be in the audible range. Many amps that had bad capacitors would just give a tiny sound. Without it, a DC level might build on the speaker line. Ground by the way, isn't always necessarily negative.

January 15, 2012
by carlhako
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I have hooked up a couple of electrolytic capacitors the wrong way before by accident. Every time in my case they have been smaller ones and they go off with a bang pretty quick. Similar to a cap gun, usually your circuit around the capacitor gets covered in fluff stuff from the insides of the capacitor. This may not happen with larger ones but it definitely gives you a scare when your not expecting it and your head is close lol.

January 15, 2012
by Rick_S
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Big ones make a louder bang with a lot more "fluff".

I went to DeVry for a couple of semesters (before I ran out of money) back in the mid 80's. One of our labs was to build a power supply/function generator box. A classmate of mine got one of his large filter caps in backward and it scared that daylights out of everyone in the lab when he plugged that thing in. LOL He had a big mess to clean up from all the fluff.

The incident definately made everyone else double check their caps before plugging in thier box.

Rick

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