|
Low Voltage MOT PSU
Low Voltage Bench supply from 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, and buying a bench supply which can handle some real power
is even more expensive. More often than not a crude, simple,
industrucable power supply is what's truly required, and for this
application nothing beats a good old iron beast. MOTs (Microwave oven
transformers) are readily
available, rated for around 1 kW, and easy to rewind. Quick and dirty version The
first supply presented here is actually the most recently built one,
being just a little weekend project. Still, I think it outperforms any
supply I've built so far, simply by virtue of being so simple and
versatile. It's also much better constructed than the second supply.
The idea here is to rewind a transformer with several voltage taps, and
then provide a rectification and filtration circuit. Optionally a
voltmeter can be added as well. This way a number of voltages can be
selected, simply by choosing which tap to rectify. With a voltmeter
built into the supply, you'll always know what the voltage is, even as
it sags with load. This is a feature I missed sorely on the supply
below. The electrical circuit is so simple I haven't drawn it, but if
you need to see it just look here for the equivalent.
The
first step to rewinding a MOT or similar transformer is to remove any
exsisting windings. You need to keep the primary winding, and remove
the secondary. For the amount of work involved unwinding the secondary,
I just use a hacksaw and cut off the part of the winding that sticks
out. After it has been removed, I use a piece of wood scrap and hammer
to punch the secondary out of the transformer. The finished product can
be seen below. When rewinding a MOT, you don't have much space to put
turns on unless you disassemble the transformer, which I no longer
recommend. There is an easy way to overcome this however: Cut lengths
of wire roughly 4 meters long, all the same length, and twist them
together. You'll want about four wires if using 18AWG enameled copper
wire, this should fill most, if not all of the winding window. Once
done, you'll be left with one short length of braided wire, which you
can then wind on the transformer. Doing this saves you four times the
work! Once on the transformer, you may choose to keep the wires in
parallel for more current, in series for more voltage, or a combination
of both.
Short Circuit Protected Version Of
course, simply winding and rectifying a monster transformer is not
always such a
good idea, as all it takes is one failure and everything down the
line is ruined. This can be remedied by using some simple protection
circuitry. The short circuit protection was designed by Tim
Williams.
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 in the event of a load failure. A neat
bi-color LED will switch between
green when everything is OK and red when a short is present. An
overcurrent condition is detected by measuring the voltage across a
resistor as mentioned earlier. Once this voltage exceeds the base
threshold of a 2N3904 transistor the flip-flop is triggered, and will
remain on until reset manually. In the set condition, the main power
mosfet is opened, which prevents current flow to ground. This in turn
"shuts off the power" to any circuit down stream, preventing further
damage. Keep in mind that current may still flow from the 50V line to 12V or 5V
By 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 up to a kilowatt!
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 overcurrent level is decided by the 0.024 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
With only one 0.047 ohm resistor the
peak current allowed is;
0.6V / 0.047 = 12.7 A
Rewinding the MOT
The MOT is the heart of the power
supply. They are often welded shut, and must be cut open with an
angle grinder to be rewound. Once you have ground the welds down, it
will still be glued together with varnish. Give the “I” section a
knock with a hammer and will break it off. Be sure to hit the
laminations sideways, my
Dad and I learned the hard way. 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, and if possible use enameled wire, not plastic coated. I
used 18 AWG plastic coated wire, which is all I had at the time.
If you want to achieve a specific output voltage, wind a few turns,
measure the voltage, then divide by the number of turns. Keep in mind
that the output voltage per turn decreases as you add more turns to the
core. Outer turns may only generate 80% of the voltage you'd get from
an inner turn. For this particular supply you're 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. Welding the transformer shut again would not be a bad idea. 
No, I didn't
use one piece of wire.
Comments
|
