Monday, January 27, 2020

Small 12V Buck PSU

How does one build reliable electronic circuits?  By starting with a reliable and clean power supply.

The three terminal linear voltage regulators have been almost obsoleted by three terminal switching regulators.  It is now quite trivial to make a very efficient small power supply by using one of these modules - available from Mouser and Digikey.

I tend to buy interesting parts when I see them and the little bag may lie around in my workshop for years until I find a use for them in some or other widget.  Here is a little 12 Volt switcher circuit which took me about 2 hours to find the parts in the depths of my Junque Bochs, and build on strip board:

The circuit starts with a self resetting fuse against over current, has a MOV for protection against high voltage spikes, an anti-dumkopf diode, a switching regulator and a couple of electrolitic smoothing capacitors. The big capacitor has a tiny one accross it, to quench high frequency EMI. The two little terminal blocks make for easy wire-up.

The capacitors are overrated in voltage by a factor of three and the regulator is overrated in current capacity by a factor of five, so the circuit should last a long time - no stress.

The regulator doesn’t need a heat sink, since it is very efficient and won’t get warm.

If you want to use a switcher with a VHF radio receiver, check the noise with an oscilloscope/spectrum analyzer and if necessary, add another little EMI capacitor, or mount it in a separate aluminium enclosure for shielding.



Through hole parts with a double sided circuit board, where the bottom is a ground plane, will ensure low noise operation. It is easy to build and easy to repair if it ever breaks. For vibration and shock, the big parts should get blobs of RTV to hold them.  A quick spray with V66 or similar conformal coating, will protect the parts against corrosion.

That is about it.

It Just Works!


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