Low Voltage MOT PSU

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Low Voltage Bench supply form a MOT (Microwave oven transformer)

Getting a proper bench supply is necessary for any hobbyist, and can often be a limiting factor when experimenting. Buying a high tech bench supply is very expensive however, and buying a bench supply which can handle some real power is even more expensive. Therefore making your own bench supply is a natural solution. MOTs (Microwave oven transformers) are readily available, and are rated for around 1 kW, making a good basis for a bench supply. Because of their size they are also easy to wind. Of course, simply winding and rectifying a monster transformer is no good idea, as all it takes is one failure and everything down the line is ruined. So some short circuit protection is needed as well.

With a little work, one can get a reliable and safe supply for very little money. Using two separate windings and a little extra circuitry, the supply is able to give a regulated +5 and +12 volts and an unregulated 50 volt line. Depending on the components used, the 50 volt line can supply a kilowatt or more! Not bad for something homemade. Remember that this supply is not protected from a short between +50V and the regulated +5V and +12V lines. The tripping current is decided by the 0.075 ohm resistor, which will develop a 0.6 voltage difference from real ground when 25 amps pass through it. The size of the current sense resistor is determined by the transistor turn on voltage divided by the max current, so just Ohm’s law. In this case,

0.6V / 25A = 0.024 ohms

Rewinding the MOT

The MOT is the heart of the power supply. They are often welded shut, and must be grinded open to be rewound. Once you have ground the welds down, it will be glued in place with varnish, giving the “I” section a knock with a hammer will break it off. Don’t hit it with the laminates, my Dad and I learnt the hard way, but rather on the side. Once the top section is off, remove the secondary winding however you like, but be very careful with the primary winding – you will need it later. Once the transformer is disassembled, it is time to wind. Pick a wire size which will allow for 50 turns, I used 18 AWG. The winding process is trail and error, but you can estimate by winding a few turns first, and measuring the voltage on them. Divide the voltage by the number of turns and you have the volts per turn number. Figure out the rest. You are aiming for 13V AC on the low voltage line and 35V AC on the 50V line. Once you are ready to test the transformer, it is necessary to clamp it tightly; otherwise the mains buzz will shake your bench apart. Here is how I did it. The rest of the clamp can be used to house the electronics. Note the depth of the grinds, much further is not necessary.

(No, I didn’t use one piece of wire.)

The short circuit protection can switch very quickly, because Tim Williams designed it. I added a few changes, but nothing worth mentioning. It’s a discrete flip flop which switches the power MOSFET off when a short occurs, and closes it when the reset button is pushed. Pretty self-explanatory and simple, but it will save you a lot of work. A neat bi-color LED will switch between green when everything is OK and red when a short is present.

(Another ghetto control panel, made from a computer PSU case.)

Some pictures of the finished PSU.

 You PSU can look this good too!