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Modifying A Switching Power Supply Into A Variable Power Supply (A May Communicator Preview)

An Inexpensive Project To Power Your Amateur Radio (Or Other) Gear

By Daniel Romila VE7LCG

I was faced with a problem that many radio amateurs have: Getting a power supply capable of fulfilling my needs today and for the reasonable future. I decided that switching technology is the way to go, and that 12 Volts is my main voltage requirement. Regarding power, I was limited by my budget, because the more current a power supply can deliver, the more money I would have had to spend. I established 42 Amps would be enough, and that means 500 Watt capacity.

The above moment of decision was in 2016. So, now – in 2018 – I can tell you about the results of my selection of power supply and the modifications I made.

Whenever I was looking at eBay, Amazon and the Chinese websites (,,, etc), I was searching for 12V at 42A. 

The size of such a brick, at 12V and 42A is 115 x 215 x 46 (mm). The current price  around $38 CAD - about $29 US (shipping and taxes included).

I modified the circuit somewhat and replaced the trimmer resistor with a linear potentiometer. That extends the voltage range from the original 9 – 14 Volts to 4.5 – 15 Volts. Here is the result:

I successfully used this power supply for my Kenwood 7950 HF transceiver. I had very short power wires, so I put the power supply on top of the transceiver, on its speaker, because I use headphones... and there was no noise. I also used it for various audio experiments with operational amplifiers installed on a breadboard at around 30 cm from the switching power supply.

It ran cool, and despite my tries, I was not able to make this power supply run hot under various heavy loads. Under maximum power of a little less than 150 Watts. The fan starts to rotate automatically and it is silent.

A comparable digital power supply made for Amateur Radio would cost well over $150 CDN so this is a budget solution.

The full article is featured in the
May SARC Communicator

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