Tuesday, November 16, 2010

13.8V 20A power supply

...Well it's more like 13.7V and probably 20A power supply since that's the transformer secondary current rating.

I have this project for finish since 2002, it was for some time built but I didn't like the original schematic and now I changed. The original schematic was based on a LM7812 but that creates problems isolating the output transistors (2n3055, what else!). Now it's a more elegant solution based on a 7912 for voltage "control".

here's the schematic:

It is advisable to put an over voltage protection (due to circuit nature and in case any of the "gnd" diodes fail) circuit but for now it's ok for me.
This is a ground regulation circuit so makes it possible to have the collector of the 2n3055 transistors to case avoiding the mica insulators. The output capacitor is critical, without it the 7912 has a tendency to oscillate. The 2n3055 base connected capacitor should be 2.2uF (if tantalum) per amp (on the 7912) or 10 times more if electrolytic, so a 22uF should be enough but the more the better...

Now the working unit under load test (FM mode, high power setting on the TS-50) to a dummy load (didn't measured the input power):

Some more crappy photos:

The ugly "control" board:

The finished "product":

For now there's no need to put any fan (that's not an optimal dissipator position) because on a normal qso the output transistors don't get enough warm).
Have fun!

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