Is this capacitor polarized?

I have to replace a small capacitor that had a lead ripped out when the tv was dropped and the pcb broken.

It's a cylinder about a half inch high, a quarter inch in diameter, with two leads at the bottom, and a narrowing near the bottom, like a waist line, but it gets back to full diameter at the bottom.

It's black and says on it:

extra info TL [in an elongated circle] 50v4.7uF --

CD71

40/085/10 N --

There is lead coming out next to the N and next to the 4.7uF.

Does the N mean that that is the negative lead?

If not, does that mean the capacitor is non-polarized?

BTW, what happens if I use a polarized cap where a non-polarized was intended, or vice versa? Will a reverse voltage puncture the dielectric? Even if it is less than 50 volts, like the rating here?

Thanks.

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Reply to
mm
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On Mon, 19 Feb 2007 19:55:37 -0500, mm put finger to keyboard and composed:

All my Googling suggests that "CD71" is a bipolar or non-polarised aluminium electrolytic. Yours seems to be a standard temperature type, ie -40C to +85C. The "10" may refer to the tolerance.

- Franc Zabkar

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Franc Zabkar

Thanks. OK, there are no polarity markings on the pcboard, so I guess it is not.

The cap is on the other side of a resistor that is near the flyback, and the other side of the cap goes to the pcb ground. I wish I could say more than "near the flyback" but it's very hard to see. Later today, I'll try to unscrew and move the pcb from the plastic frame it's mounted to.

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mm

Thanks. I didnt' realize CD71 would be the part number.

Since I need 4.7uF, I have two polarised 50v 2.2uF caps of the same appearance as the one I need to replace, that I was going to connect in parallel, with the negatives connected together.

If instead I connected them in parallel with each negative connected to the other's positive, would that give me the equivalent of a non-polarised cap?

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Reply to
mm

In series, either positive to positive or negative to negative (back to back.) You need to compute the value that results (formulas and calculators abound on the web...) but with just two, you'd end up with half the capacitance so you need a couple of 10 uF caps (each rated at

50 volts).

However, series back to back capacitors are not (IMHO) a good solution--better to go out and get the right part instead.

Reply to
PeterD

I remember that now from 10 or 20 years ago, although I never knew the details you give below.

Thanks to you and Peter.

One more question for now. Like I said, the tv must have been droped, even though there is no damage on that part of the case. I'm soldering jumpers across all the broken traces.

But my supply of spare caps is small, I guess because I only inherited a few and there are so many possible kinds to have.

So it will take me a while to get this one little cap. Can I apply power to the tv without damaging anything if I have nothing where the capacitor under discussion should go?

Or a bigger or smaller non-polarised one temporarily?

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Reply to
mm

PeterD wrote in news:lg1mt2tc5n34le0lauqcqc31qgl82n50u4 @4ax.com:

Should there not also be a couple of diodes, one across each capacitor, to prevent applying reverse voltage to either?

I seem to remember that electolytic do NOT like reverse voltage applied. It removed the 'insulating layer' that has been electroplated on one of the plates (that also serves as the dielectric). This tends to lead to high current flow and breakdown. Since there is another capacitor in series, the current will be limited, and the dielectric may reform when polarity is right again, but I would not expect it to be very reliable.

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bz

Personally, without knowing the purpose of the cap, I wouldn't go ahead and apply power. If it was in some kind of snubber circuit for instance, the effect of it not being there might prove catastrophic to some following semiconductor

Arfa

Reply to
Arfa Daily

OK. Thanks.

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Reply to
mm

In theory I guess, that works, but in practice, I don't think that it would prove to be an issue

You are right that again, in theory, electrolytics do not like reverse polarity, but I think that the trick to this is the duration / duty factor of the reverse polarity. Electrolytics are subjected to AC across them in many circuit locations, and sometimes, in output stages for instance, the level of reverse voltage that they are subjected to can be quite substantial, but it does not seem to cause any long term problems, except when a cap with a poor ESR or insufficient basic voltage rating is used. One particular hifi that I used to repair a lot of, springs to mind in this regard. Both of it's output coupling caps used to fail open circuit, quite regularly. Once they had been replaced with low ESR 105 degree types with twice the voltage rating, there was never any further trouble. However, even though caps in this sort of position do suffer reverse voltage across them, the average duty factor is 50%, and the durations of the reverse excursions are relatively short, all of which, I think, contributes to the scheme working by and large, reliably.

As I said elsewhere, another area where electrolytics are subjected to a severe reverse pounding, is in the voltage multipliers for VFD supplies. In these circuits, they have transformer derived line-power frequency AC of high levels across the multiplier input cap. Sometimes, ultimately, these do fail - it's very common in Aiwa's for instance - leading to a dark display. There also used to be a very popular Sony tape deck that was part of a stacking system, where two normal polarised electrolytics were in the VFD filament supply lines. These used to fail and wreck the display tube. Sony supplied a revised type that was brown with gold writing, but it was still a normal polarised type. However, the revised ones did not re-fail, so there doesn't seem to be any hard and fast rule that designers stick rigidly to in this regard.

Arfa

Reply to
Arfa Daily

Well, lucky me. This is one of 46 capacitors that Radio Shack sells, and it's even one that they normally keep in the store, AND according to the webpage, it is in 3 different stores near me.

That's a good thing, because I was having a lot of trouble keeping myself from turning the tv on.

There is a store I know has every such part, but it's 6 times as far away.

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Reply to
mm

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