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Sigma25 regulator blown

sigma25, sigma26, sigma24

Sigma25 regulator blown

Postby lugili » March 26th, 2019, 3:16 am

I try to build a sigma25 board for 12v DC out. When connecting to my 12v transformer (10VA) there is some smoke coming out of the 7812 regulator and that was it.
Transformer measures at 16v with open secondary circuit, is the inrush current too high and blows the regulator?
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Re: Sigma25 regulator blown

Postby amb » March 26th, 2019, 8:22 am

First, are you sure that it’s a σ25 board (rather than a σ26)? Also, is it a 7812 as opposed to a 7912 (or some other device)? What brand and full part number?

What do you have connected to the output?
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Re: Sigma25 regulator blown

Postby lugili » March 26th, 2019, 10:33 am

I am sure it is a sigma25 and a 7812 regulator. I have nothing connected, also no led. Might this be the problem?
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Re: Sigma25 regulator blown

Postby amb » March 26th, 2019, 11:03 am

The LED (or lack thereof) shouldn’t be the problem. Is your transformer still putting out the correct secondary AC voltage?

Also check to make sure there are no solder bridges, and your board isn’t sitting on something that might cause a short circuit.
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Re: Sigma25 regulator blown

Postby lugili » April 14th, 2019, 1:29 am

I am still puzzled what happens here. I understand now that the regulators are not blown, they just get very hot in about 15 seconds with no load attached.
The ones I use are 863-MC7812ACTG from Mouser, the recommened replacement for the ones that are mentioned on your site, look fine to me. Also C4 gets very hot, although there is no load attached. It all seems to me as if the Voltage over the regulator is not right.

I tested the transformer and outputs 16VAC with no load attached and about 13.5VAC when a 22Ohm resistor is attached (drawing about 8.3VA) from the 10VA rated transformer. Looks all not too bad for me. But voltage is a bit too high. But as there is no load attached there should not be an overheating of the regulator as no current is drawn.
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Re: Sigma25 regulator blown

Postby amb » April 14th, 2019, 1:39 am

C4 is most likely installed in reverse polarity and has been damaged (it shorted), causing an excessive amount of current draw from the 7812. Replace C4 and install it in the correct orientation, and your problem should be fixed.
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Re: Sigma25 regulator blown

Postby lugili » April 14th, 2019, 5:59 am

That is exactly what happened! I have 5 sigma 25 built and I did not care about polarity of C4. The ones that have the right polarity (I stopped testing after the first one did not work worried I destroy them all) are working fine. Thanks, Ti Kan, great help as always.
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