Power Supply 12.6V 25A from old Microwave Oven Transformer

Dig out that old microwave oven transformer you’ve been saving and take a look in your junk box. It’s time to build that battery eliminator you need for the mobile rigs.

Although the transformer is probably good for 50A with a little air over the secondary winding, I’ve limited this design to 25A due to the semiconductors available. The output voltage is set to 12.6V and the overcurrent is set to 28A. There are no external controls beyond an on-off switch.

This is a low cost design. The parts chosen are not optimal, they reflect the contents of my larder. None of the parts are terribly critical. The mosfets can be any low RDSon N-channel parts, and you can parallel as many as you like. The IRF-150 devices that happen to have on hand are rated for a VDS of 100V and their RDSon is 0.055 Ohms.

However devices with a 30V VDS rating would work as well. The mosfets are not isolated from the chassis, that is at earth ground, which makes the construction and thermal management a little easier. The regulation is fair, about 0.25% from zero load to 25A. The ripple is less than 75mV at 25A.

Features? Not many! The output is slot adjustable from 10.5V to 14.5V. The over current circuit uses a hall effect sensor that is adjustable from 15A to 35A. At turn-on there is a delay before the output voltage comes up as the overcurrent circuit initalizes. On an over current fault the power supply shuts down for a few seconds then it restarts. This keeps the mosfets cool. Over voltage protection may be added to the next revision.

I’ll post details of the transformer rework, hall effect current sensor, and more photos eventually. I’ve also designed an over voltage protection circuit to be posted later.

Correction to schematic, the bulk capacitor value is 25,000 uF. About 1,000uF per Ampere of load current is the goal. Not 2,500uF.

Update:

I’ve used the power supply for some time now. The transfomer magnetizing current is quite high since I knocked the shunts out of the core. So the transformer gets quite warm and needs the fan blowing on it even if the load is light. On the other hand even with a heavy load of 25A it doesn’t get sensibly warmer. I’m going to build another transformer for this project but leave the shunts in this time.

The regulator works super, very clean and well behaved. The over current circuit is too fast! When I turn the rig on the inrush latches the power supply off. If I start the power supply with the rig on, then the over current doesn’t trip, even when running 100W in CW.