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I just began being interested in electronics and I got my hands on a Chinese bench power supply APS 3005 that didn't work - only CC red led was on.

I didn't find a schematic for this PSU, but it's the same as on this video.

I found a shorted diode, which I changed with a new one and I also found that there is a LM7812 voltage regulator where I measured that it ouputs only 7V, but between input and ground, there is 18V.

I disconnected the output terminal from the circuit and output was still the same value. So I bought a new LM7812 and to m surprise when plugged in again only 7V was at output.

Why is that it never outputs 12V? Could it be that both the old and the new LM7812 are bad?

Here is a picture of the regulator with output disconnected from rest of the circuit and between input and ground is 18V.

LM7812

Edit: So I apologize, the problem wasn't the 7812, but my multimeter which showed lower values. When I tried another multimeter it showed 11V, which is fine, since the multimeter is for higher voltages. Now I have to find why the power supply is always only in CC mode.

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    \$\begingroup\$ Do you have a small capacitor (e.g. 0.47 μF) that you could put across the input of the 7812 to make sure it is stable? Ref: LM7812 and decoupling capacitor value. Also, are you sure that your multimeter is reading correctly? \$\endgroup\$
    – Andrew Morton
    Commented Sep 19, 2020 at 11:00
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    \$\begingroup\$ The 7812 needs capacitors on both input and output to be stable, and at least some light load (5 to 10 mA) to work stable. If both the old and new 7812 operated identically, they are not broken, but the problem is on the board that the 7812 gives power to. \$\endgroup\$
    – Justme
    Commented Sep 19, 2020 at 11:08
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    \$\begingroup\$ Ill get batteries for my another multimeter and try that. Also there are capacitors on the board which are not shorted. But they are kinda big, 470uf 35v on the input and 220uf 25v on the output. Here is photo of the circuit where is the reulator and the capacitors: imgur.com/gallery/8GlQt00 \$\endgroup\$
    – TomCrow
    Commented Sep 19, 2020 at 11:43
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    \$\begingroup\$ @TomCrow It could be a problem having such long wires leading in to the regulator without a bypass capacitor, as mentioned in the answer I previously linked to. \$\endgroup\$
    – Andrew Morton
    Commented Sep 19, 2020 at 12:03
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    \$\begingroup\$ Soo, the whole problem was my multimeter. Thank you all for your answers, Iam learning something new everyday. Now I have to find the real problem why its always only in CC mode. \$\endgroup\$
    – TomCrow
    Commented Sep 19, 2020 at 17:21

1 Answer 1

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So I found that the LM7812 is perfectly fine and that my multimeter is broken and showing lower values. It really outputs 12V.

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