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Output comes then decrease the output

When we switch on the smps (230v) output comes (5v,12v,-5v,-12v) suddenly decreses the output i.e.power shutdown problem

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That's because the Electrolytic Capacitors inside the SMPS are breaking down.

Specifically Radial Aluminum Electrolytic Capacitors.

A Capacitor slowly builds up a charge, then releases it all at once.
(Although it happens in milliseconds. Millionths of a second)

Think of a large swimming pool being filled up by a garden hose, then one wall is taken down all at once.

Capacitors are used in flash units for cameras. (Slowly being phased out)

The basic construction of a Radial Aluminum Electrolytic Capacitor, is;

A) Outside case: Thin aluminum. Like a Coca-Cola can with the top, and bottom cut out.

B) Top: Thin aluminum flat disk. Has a lK or X shape etched partway into it.
This is the Vent Cover.

C) Bottom: Thin flat synthetic rubber disk. The Positive and Negative lead pokes through it.
This is the Bung.

D) There are three strips inside. (Basic construction)
1) One strip is of foil thin metal, and has the Positive lead connected to it.
This is the Conducting strip.

2) One strip is also thin foil metal, but has a non-conducting medium applied to it. It has the Negative lead connected to it.
This is the Non-Conducting strip.

3) The last strip is a paper-like medium, and is soaked with Electrolytic Paste.

The paper-like strip is laid in-between the two metal strips, and all three are rolled up tightly.
There are many layers of this construction inside the capacitor.

When the capacitor starts to fail, it's because the Electrolytic Paste inside is developing gas. Hydrogen Gas.
The gas expands inside the capacitor's case, and the build-up of pressure eventually compromises the seals.

Either breaks the lK or X shape open in the Vent Cover at the top, or pushes one side of the rubber-like Bung down.
Electrolytic Paste starts oozing out.

So much paste loss, and the capacitor operates in a weakened state. Too much paste loss, and the capacitor fails.

Capacitors are used as Filters inside the Switched- Mode Power Supply,

This is NOT an invite to open the Power Supply, and attempt repair.


Posted on Dec 07, 2012


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