# [Newbie] Using 50V cap with 30-0-30. safe?

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#### nina

Sorry for this dummy question:

I have many good electrolytic cap rated 50V. I wonder if it can be used "safely" with the power 30-0-30V ( ~ 44Vdc) ?

Thanks,

#### Jorge

I don't think they would blow on you, but their life will be shortened.

#### UrSv

Absolutely no problem at all IMHO. Go ahead.

#### nina

Jorge said:
I don't think they would blow on you, but their life will be shortened.

Thanks UrSv and Jorge.

When saying their life will be shortened. How bad it is?

#### wxn

IMHO it's more dependent on such factors as temperature and physical damage.

#### jacco vermeulen

No dummy question, very good question.

30Vac = 42.43 Vdc, with rectifier losses a 50Vdc capacitor has nearly 20% reserve for voltage surge and unloaded voltage increase.
Which is more than enough for any transformer.
Using higher voltage capacitors will only cost you extra money without returns.
Reforming the ones you have is far more important than having some with extra spare volts.

If you like the tech stuff, read some pages on (re)-forming capacitors.
Pooge posted some excellent links to some recently:

(i had a Bearded Collie named Niña, but that is Spanish )

#### Lsharptec1

Rectified and filtered DC voltage is 1.414 times the AC voltage.

So, 30 VAC X 1.414 = 42.42 VDC positive and negative rails.

Larry

#### guzzler

All the above is correct, but forgetting the fact that the transformers voltage is usually rated at maximum output. An unloaded transformer will easily put out 10-20% higher voltage than actually specced. As always when dealing with high voltage/current, play safe.

Go double the voltage rating of the transformer for some margin of safety, ie 63V

#### nina

guzzler said:
All the above is correct, but forgetting the fact that the transformers voltage is usually rated at maximum output. An unloaded transformer will easily put out 10-20% higher voltage than actually specced. As always when dealing with high voltage/current, play safe.

Go double the voltage rating of the transformer for some margin of safety, ie 63V

guzzler,

I plan to put 2.2K-10K 5W Bleeder resister in parallel with the filer cap. Will it help?

Thanks.

#### dsavitsk

nina said:

guzzler,

I plan to put 2.2K-10K 5W Bleeder resister in parallel with the filer cap. Will it help?

Thanks.

Sure, it will help some (by drawing current and thus lowering the noload voltage of the transformer), but keep keep in mind that you want to design for the worst case scenario, which here could be (60V + 20%) * 1.414 = 102V and could easily be caused by shorting one rail to ground. Exploding electrolytics are messy to clean up. I would personally use 120V caps here.

#### peranders

Paid Member
50 volts caps for 30 VAC is OK according to me but what about the load?

Do you plan to use LM3886? 2 x 28 VAC is max voltage together with LM3886.

#### phase_accurate

and could easily be caused by shorting one rail to ground.

I don't see how this could happen !!
Apart from that: Even if there were some "mechanism" doing that - a short between one output rail and ground should immediately blow the primary fuse if correctly dimensioned (for the sake of your own and anyone else's safety !! ).

Regards

Charles

#### SheldonD

quote:
and could easily be caused by shorting one rail to ground.

This would in fact only give 0 volts on the shorted side and the normal rail voltage on the non shorted side.

To give twice the voltageone would have to have the ground lifted on the shorting side and the "hot end of the shorting side going to ground..... hard to see how this could happen.

Other than a wiring error while building the amp.

The original poster has lots of capacitors so they could be wired in series to give 2x the voltage rating. at the cost of space.

Of course the capacitance would be halved.
.

#### h_andree

I was under the impression that if you have considerable
ripple a smaller (Lower voltage) cap would heat up more
and therefore the life span would be sortened.

How large is this effect?

Harry

#### Jorge

dsavitsk said:
I would personally use 120V caps here.

Not that much...

I used to design pro telecom equipment, and 63V would be Ok here.

#### phase_accurate

If I had to buy new ones I would also opt for 63 volt ones. But he already has 50 Volt types. In this case I would use them also. Since he is not building any telecom equipment that is in service 7x24 and which is also presenting a constant load ( => ripple current ) - I don't see much of a problem.
Furthermore he seems to have a lot of these so spares won't be a problem either.

Regards

Charles

#### derekyu

I wonder to know how i can set this transformer to 25v0v25v output.
thanks very much

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#### Nisbeth

dsavitsk said:

Sure, it will help some (by drawing current and thus lowering the noload voltage of the transformer), but keep keep in mind that you want to design for the worst case scenario, which here could be (60V + 20%) * 1.414 = 102V and could easily be caused by shorting one rail to ground. Exploding electrolytics are messy to clean up. I would personally use 120V caps here.
Sounds like you have been building too many headamps with virtual ground Doug

/U.

#### dsavitsk

Nisbeth said:

Sounds like you have been building too many headamps with virtual ground Doug

/U.

I think so.

In the end, I am the guy that won't leave anything I built on if I am not around as I am safety/fire paranoid. Always in the back of my head is the time I built a stereo for my wife, got it working, went to get her to show it off, only to hear her ask, "Is it supposed to be smoking?"

I'd always rather spend an extra \$0.50 to overspec something of I can.

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