Introduction: Custom DC Power From Microwave Oven Transformer
This instructable puts together a few disparate concepts already in circulation.
Microwave oven transformers are awesome. But 2000 volts of kill-you are not too useful.
Lots of people make welders, but I have not seen much on the way of simple, useful power supplies.
This will be a quick overview on how to rewind and make a PSU from a MOT
BE CAREFUL WHEN WORKING WITH HIGH VOLTAGE DEVICES AND MAINS VOLTAGE.
Make sure you know what you are doing before diving in!!
I am assuming you have general working knowledge of electricity, if not, read up.
Step 1: Get a MOT
Any will do. This is a big old 900-watt one.
Step 2: Grind the Welds
All of these cheap Chinese-made MOTs have a little bead of weld down their sides.
Get your angle grinder and grind them away.
A few taps from a hammer should then break it apart.
Step 3: Remove the Coils.
Different MOTs will have different coil arrangements.
There are 3 coils. 1 is useful.
The low voltage windings are the thick copper ones with the 2 terminals sticking out.
The red cloth insulated one is a high amperage element that works with the magnetron.
The thin windings make the 2000 volts that run the magnetron.
In this case, I had to remove all of them.
Clamp the transformer, open side down, to a surface. Add some spacers (in my case, i used two pieces of metal).
A hammer and a wooden wedge make short work of this. You need something soft to hit the windings with, like a rubber mallet. If you smack the insulated copper with the hammer you will ruin it.
Be gentle with the low voltage windings!!! if you scrape the insulation or damage them they won't be any good.
The others don't matter.
Step 4: If You Had to Remove All the Windings
Now put the low voltage coil back on.
Not much to it, just fit it on and drive it back in.
Step 5: Voltage Test
Now you can do a little investigation.
You need to figure out how many turns you are going to need for your new transformer.
Usually its one to one ratio. One turn is one volt. May not always be the case.
What you want to do is put 10 turns of insulated wire in the core.
Clamp the core together, and get a power strip with a fuse (in case there is a short).
Plug the original low voltage coil to the power strip.
If all is well, you should hear the core humm a bit.
Now use a multimeter to check the voltage you get on the 10 turns you put in the core.
And do the math to figure out how many turns you will need for your desired voltage.
Consider consulting Wikipedia for the basics, that's all you need.
Step 6: Now the Tedious Job of Wining Your Own Coil
Get the enameled wire you stripped from a damaged Tesla coil secondary, or any that you have on hand.
Any insulated wire will do, so long as you can fit the right number of turns, and it can take the number of amps you will be pulling from it.
I suggest a jig and a hand drill.
Make the wooden core to be a bit bigger than the transformer core, since they have to match, and you want to easily fit the wires in and not have them rub against the iron core and short out.
The pics i am using are from 2 different builds one for a 500v output, and one for a 36v output.
For 36 windings, you can just do it by hand.
For 500, you want the jig, and magnet wire, regular plastic insulated won't fit.
you might get some tangles... be patient
Step 7: Fit Your Secondary on the Core!
Allways throw on several extra turns to dial in the desired voltage by measuring and cutting to get it just right.
Once on the core get some wood, or cardboard and fit it in between the windings and the core to tighten things up.
Clamp it closed, measure voltage and adjust as necessary.
Once you get your voltage throw some spot welds on, or big bolts or hose clamps and keep that sucker together and tight.
Then varnish the whole thing. Oil-based anything will do the job.
Step 8: Rectify
I am linking a video from This Old Tony (awesome channel on youtube, go watch and subscribe).
Even though he is talking about a CNC router PSU, the video taught me how to calculate the capacitor i would need to smoothe the DC coming out of a full bridge rectifier.
You only need the smoothing capacitor if you are doing finer electronics. To run any old motor the DC straight from the rectifier (the square thing with 4 terminals)will be fine. Rectifier on amazon, this is probably overkill for any job you may want.
This Old Tony explains better than i could how to do the rectifying and the smoothing, so i'll leave it to him.
Step 9: Finishing Up
You might need a heat sink for the rectifier if the current daw of your project is high.
Other than that, you wire things up from the transformer to the rectifier, from the rectifier to the capacitor(if needed), and then you have DC output for your project.
When you wire things up, ground the core and the enclosure if it is metal.
Be careful and happy making!