Verify a(maybe) dying diode caused by a bad capacitor???

satu...@yahoo.com

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Mar 12, 2007, 4:31:09 PM3/12/07

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I have just fixed an power supply by replacing a bulging capacitor.

The capacitor is connected to diode. And the diode becomes hot and
makes a black spot on the board since the faulty cap.

I check the diode with a multimeter and it tests great.

The repaired power supply( cap replaced) has been tested for several
hours and no problem yet.

Question: Is there any way to check whether the diode is working
100% ?

Thanks!

Charles Schuler

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Mar 12, 2007, 4:39:44 PM3/12/07

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<satu...@yahoo.com> wrote in message
news:1173731469.5...@v33g2000cwv.googlegroups.com...

Diodes are cheap. It was stressed. Replace it.

Message has been deleted

da...@drumheller.org

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Mar 12, 2007, 5:45:33 PM3/12/07

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It seems like the repair worked, but the hot diode concerns me. Your
description is a little hard to understand. Did the diode get hot
before or after the capacitor was replaced?

In any case, the black mark by the diode isn't a good sign. Whether
the mark was cause by the diode overheating or leakage from the
bulging capacitor, I would replace it. As another poster tersely
pointed out, diodes are inexpensive.

For a definitive measurement, don't test the diode in place. Rather,
unsolder one of its leads from the circuit board and then test it with
your voltmeter's continuity checker.

-Dave

Jeroni Paul

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Mar 12, 2007, 6:43:38 PM3/12/07

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By your descriptions it looks like your supply is working fine. It is
normal for some components to warm up and if it were faulty you would
see some problem with your supply. If it works, don't mess with it.

Sam Goldwasser

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Mar 12, 2007, 7:12:49 PM3/12/07

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da...@drumheller.org writes:

If it tests good in-place, it won't test bad when removed. :)

However, the inverse (??) may be true.

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Arfa Daily

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Mar 12, 2007, 8:53:17 PM3/12/07

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"Jeroni Paul" <JERON...@terra.es> wrote in message
news:1173739418....@t69g2000cwt.googlegroups.com...

I see lots of bulging caps in the power supplies of cheapo DVD players - I
probably repair 2 or 3 a week on average. I have found it very rare indeed
for the feeding diode to actually be faulty. Usually, it's just a case of
replace the cap, and that's it. It is not at all uncommon for the diodes in
these supplies to run very hot, and discolour the board. Sometimes, if there
is room, I bend a small piece of metal around the body of the diode, with a
bit of heatsink paste in there as well, to help cool it down a bit. Bear in
mind if you do replace it that it will be a Schottky type ( I'm assuming
that this is a switch mode PSU ??? )

Arfa

James Sweet

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Mar 12, 2007, 10:38:08 PM3/12/07

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Is the power supply being overloaded? If it's not and the output voltage
is correct, I wouldn't worry about a warm diode.

Eeyore

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Mar 13, 2007, 3:17:20 AM3/13/07

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Jeroni Paul wrote:

> By your descriptions it looks like your supply is working fine. It is
> normal for some components to warm up and if it were faulty you would
> see some problem with your supply. If it works, don't mess with it.

Bad advice.

Some components may have been stressed by the fault The charring near the diode
indicates this.

There's NO WAY to prove the diode hasn't had its reliability compromised through
overheating so it should be replaced as a precaution.

Graham

Eeyore

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Mar 13, 2007, 3:19:02 AM3/13/07

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Arfa Daily wrote:

> Bear in mind if you do replace it that it will be a Schottky type

Or a fast-recovery type.

Using a standard diode will guarantee long term failure although it might work
for a bit.

Graham

Jeroni Paul

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Mar 13, 2007, 3:18:45 PM3/13/07

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Eeyore wrote:
> Bad advice.
>
> Some components may have been stressed by the fault The charring near the diode
> indicates this.
>
> There's NO WAY to prove the diode hasn't had its reliability compromised through
> overheating so it should be replaced as a precaution.

The charring proves that the diode has always run hot and it is likely
the way it is meant to be. A board is not charred in just a few hours
of operation.

Messing and replacing these diodes is a good way to risk the power
supply, and given that it runs hot I would be very careful with the
type I replace it with, get it wrong and you may blow the supply again.

JW

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Mar 14, 2007, 7:42:34 AM3/14/07

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On 12 Mar 2007 19:12:49 -0400 Sam Goldwasser <s...@red.seas.upenn.edu>
wrote in Message id: <6w8xe2a...@red.seas.upenn.edu>:

>da...@drumheller.org writes:

>> For a definitive measurement, don't test the diode in place. Rather,
>> unsolder one of its leads from the circuit board and then test it with
>> your voltmeter's continuity checker.
>
>If it tests good in-place, it won't test bad when removed. :)

Unless the in-circuit test is turning on a junction of another
semiconductor that's in the same circuit, and the diode is open.