[TinkerDJ]

Has anyone used a trasnformer from a microwave to build a power supply. I've looked around the net and only found sites were people are using the transformer for tesla coils and welders and such. I know that toriod coils are around but i already have a very large 1000 watt transformer from an old microwave and was wandering if it was worth it to build a power supply from it.
Thanx.

[ViperTX]

TinkerDJ,

Bubba has. One word of caution......the high voltage supplies in Microwaves can kill you, my Electrical Engineering professor was killed by one when he was trying to fix it. So, you need to make certain that you discharge all capacitors. Don't assume because the microwave hasn't been used in some time that it is safe.

Paul

[ViperTX]

TinkerDJ,

Also unless you have some exotic voltage needs, I would search ebay for Toroids...johnngo always has some excellent Toroids and they range from $35 to $48 for a 800VA unit and they are well built.
Paul

[TinkerDJ]

I have the transformer on my bench. The cap was shorted when i dissambled the microwave. The main problem with ebay toroids is that i have only seen ones in the states. Shipping to canada can be not so nice these days. Last us item i bought for 1 dollar in the us cost me 9 bucks in custom charges and 12 dollars shipping. A toroid coil is of course the best option but i already have the transformer. I will keep my options open as i am at the end of the ability of my 24 volt 5 amp power supply at this point and would like to go to bigger steppers in my next router design.

[ViperTX]

Okay, well then Bubba is the man. He will probably find you by the AM....I've forgotten his web site....something like countrybubba.com....

[Bubba]

Yep, I'm here!
Building the transformer is quite easy.
Material needed:
Transformer (you have)
Magnet wire of the proper size for the amperage you desire
Insulating "paper", I have used tablet back and gasket material
Tape (I use self adhesive glass tape from MSC)
I also use insulating varnish again can be found at MSC

Tools
Hacksaw
Hammer
Punch
Big screwdriver
Drill(press) with a pretty good sized drill (say 1/2")


The process I use to make my transformers is as follow, as usual, YMMV or you may find a better way to do it.

!ABOVE ALL, WORK SAFELY AND BE CAREFULL!

Process:
Carefully saw off one side of the High Voltage winding and be sure NOT to knick the 110V or ~3V winding. Both of the lower voltage windings have always been wound together on all the transformers that I have messed with.

Now, I have found it easier to get the rest of the HV winding out by drilling into the center of the winding mass to make room to start pulling wires out. Some will wrap on the drill. When you have a hole in the middle. start pulling the wires out of the core (one side at a time). Remove ALL the wires and discard. When you get all the wires out, you can either remove the insulating paper or leave it for use on your new transformer winding.

When you have it all cleaned out, I usually use an air hose to be sure all the little chips etc are completely blown out of the clean core area as well as any that may have dropped into the 110/3V windings (you will keep these)

When every thing is clean, I usually put the core in a vice on the bench to hold it firmly. You now want to use your insulating paper to cover ALL exposed edges of the core. You don't want to knick the insulation on your magent wire or you will not have a good transformer (ask me how I know

After wraping the insulation on the core, I use glass tape to hold it in place. There are other tapes that can be used but this is my preference.

Measure the perimiter or the core to see how long a piece of wire it will take to make one turn and then multiply by the number of turns you think you will need. My experience is .9 to 1.0V per turn. Add a few feet of wire to this "estimate". For example, for the 40V AC that I just made, I used a piece of wire about 40ft long and only cut off a little bit at the end!

Start winding and be sure to leave a "tail" sticking out to connect to your circuit when done. Thread the wire through the core and carfully lay each winding next to the last one as best you can. When you fill that layer, I then wrap that winding with a layer of tape. Continue until you have what you think is the correct number of turns for the voltage wanted.

Now we are almost ready to test the actual voltage of the transformer.

BE CARFULL AS YOU WILL BE WORKING WITH POTENTIALLY LETHAL VOLTAGE/CURRENT!

REPEATING:
BE CARFULL AS YOU WILL BE WORKING WITH POTENTIALLY LETHAL VOLTAGE/CURRENT!

Scrape the ends of the wires that you have just wound and using an ohm meter, check to make sure you have continuity of the winding.
Also, check to make sure that you DO NOT HAVE continuity between the winding and the core. (In the former case, the transformer will not work, and in the second, you will see magic smoke in the following test

Set you volt meter to the appropriate scale and connect to the ends of the winding that you just made.
Using a FUSED power cord, hook it to the transformer and get ready to plug it in. Before plugging in, be sure there is nothing that will short out the windings of the transformer or anyone will get shocked.

BE CAREFULL AS YOU WILL BE WORKING WITH POTENTIALLY LETHAL VOLTAGE/CURRENT!

Now plug the power cord in the wall there should be a slight "chunk", but don't worry at this point. If things seem wrong, UNPLUG the cord immediately and find out why.

If all seems well, read the voltage on the meter. Is it the desired voltage that you want???
Hopefully yes, but if not; UNPLUG THE CORD and either add a winding or two or (?) to increase the voltage or remove a winding or (?) and test again.
Repeat as necessary until your voltage is correct.

If you have surplus wire, cut off the excess again leaving enough to allow you to connect to your circuit. I then spray the winding with my insulating varnish and let dry at least overnight before using the transformer.

Ok, now that you have the main winding for your servo, you may want to add another winding for say your 5V/12V controls and breakout board.

Repeat the above process as needed to wind additional winding(s) as necessary using the appropriate sized wire.

WIRE SIZING:
Based on what I have found, wire sizing is based on approximately 750 to 1000 CM per amp of current and having said that, I find that #14 does quite well on my setup of 3 servos each being fed by a gecko with a 5 amp fuse on it and haven't blown anything yet:})

If you work cleanly and carefully, you can make your own transformer this way and have the voltage(s) and current ratings that you want:cheers:

Let me know if I can help further.

Bubba

[trilect]

Microwave transformers reworked like this are also fantastic for audio amplifier projects since you can almost never find the exact voltage transformer needed for the raw DC supply.


Also not sure what voltage you might need but check out used ACME step down transformers. Most of them have 480v 240v inputs and multiple 120v outputs. Wire the 480 volt primary input to 120 and you get 30V out at one of the windings, plus its in a nice metal box. I have (4) 800 VA acme transformers in my scrap heap.

[TinkerDJ]

Awesome post bubba. Thats exactly the info i needed. I have a 240 step down transformer that i was using on another project. That one has been used in reverse and i'm using to step voltage up. But i only have the one. I had a very nice step down transformer from 240 to 24, 12, 6 taps on it but. I gave that one away becuase at the time i wasn't into cnc stuff. After i get the next power supply figured out i can start work on my new stepper drivers. Thanx for the input guys.

[Bubba]

TinkerDJ
Your more than welcome. Just trying to give back for all the help I have gotten here and elsewhere
This is one area that is fun and sure can save a ton of money if you are careful and take your time. Also as stated earlier, you can design to the voltage the YOU want and not have to accept something "close".

Bubba

[lerman]

TinkerDJ
Your more than welcome. Just trying to give back for all the help I have gotten here and elsewhere
This is one area that is fun and sure can save a ton of money if you are careful and take your time. Also as stated earlier, you can design to the voltage the YOU want and not have to accept something "close".

Bubba

Bubba -- you did a pretty good job emphasizing safety. Two more tests I would add, although probably not necessary are if you were working carefully:

1 -- Measure the resistance between the 120 volt winding and the core. It should be approximately infinity.

2 -- Measure the resistance between the coil(s) you wound and the 120 volt winding. It, too, should be infinite.

Ken

[Bubba]

Ken,
Good points, but in all of the ones that I have delt with, the 120 winding and the one that you wind are in different "windows" and you should not have a problem.

I tried to emphasize the safety aspect as I realize there may be somebody that is not aware of potential problems (historically speaking, I have been hit with 550V AC and lived to tell it and don't want somebody else to go throught that!)

we might add, check the filament winding to core also (all I have delt with have the filament wound in the same window as the 110 winding)

The last one I wound had also had the HV winding grounded to the core. I had to be sure it was isolated from any new/old windings before proceeding.

Bubba

[gmfoster]

Buba

I rewound a microwave transformer and it was as you say quite easy. However I found the transformer less than satisfactpry when I got done as it ran very hot even with no load. I blamed this on the fact that the oven manfacturer had used wire feed welder and made a pass across the laminations. I assume ths was to keep hum down. However the transformer would run too hot to keep your hand on with no load. I still have and for intermittent useage it would be great.

One other thing on this transformer there wern't windows the secondary was wound over the primary.

This was a large older US made Microwave. What was your transformmer from?

Also I used teflon insulated wire as I didn't need the room saved by varnished wire.


Garry

[Bubba]

gmfoster,
all of the transformer that I have wound, have had the HV winding in a seperate "window" in the core.
Yes, all have also had the core "welded" and they do run a little warm, but not so warm that you can't hold you hand on them.

Personally, I still feel it is a viable way to make a transformer for our purposes.

[gmfoster]

gmfoster,

Personally, I still feel it is a viable way to make a transformer for our purposes.

I didn't mean to imply it wasn't(sorry if it sounded that way) its just the core I had didn't work out. That is why I was curious as to what types of Microwaves you have disassembled? I assmed that the newer lighter imports would have a lighter transformer, which may well not be correct. I would be interested in trying one of the "window" style frames as they would seem a lot easier to strip.

Garrry

[Bubba]

All of the ones that I have had so far have been BIG and OLD with heavy transformers mostly in the 1500 1750 watt range.

Haven't been particularlly looking for any of the newer ones to see what they have in them yet. You have peaked my interest now and will keep my eye out to see if I can find one out on a trash heap somewhere and tear into it

[Dylwad]

Alright,

Found what appears to be an 800VA transformer from a microwave, wound it with dual 16 gauge magnet wire, 33.8 volts AC output.

Heres the setup,

33.8 VAC secondary
25 amp 600 volt bridge
47000MF 80V capacitor

sound about right?

Do I need a bleeder resistor? if so anyone know the value?

I needed a 46 volt 12 amp power supply, and this looks like Im well in range.

Any thoughts?


Dylan

[ViperTX]

Well as long as you don't touch the capacitor or unplug the leads to the motors, etc. you won't need one.....but I would certainly use one....start with a 10 K 1/4 watt resistor and measure the capacitor voltage 10 sec after you turn off the AC supply....if the value if around 1 volt DC then you should be fine...measure after a minute....

[Bubba]

Also, if you are going to work on the power supply, don't just trust the bleeder!

Measure the voltage with a meter FIRST, and then double check it. Resistors have been known to fail or you may not have watied long enough. Then as a final check, take a piece of INSULATED wire with stripped ends and short it across the cap. Don't be suprised if you still get a SNAP as it discharges the remaing amount.

ALWAYS THINK AND PLAY IT SAFE

Remember, it only takes 5ma across the heart to KILL.

Never get in a hurry. Check, and double check. Safety pays (and your loved ones will appreciate it also)

If you think I am a nut about safety, then so be it, I appreciate the complement :cheers:

[Dylwad]

Thanks guys, ill try the 10K and see what happens.

Calculated filter cap value for 10% ripple is 21000mf, I bought 2 47000mf caps on ebay this morning, should i use one or both?

Dylan

[Dylwad]

Decided to run a few tests on the transformer. I dont know enough about electronics to know if the test was good or bad, so im asking you guys.

33.8 VAC no load
31.4 VAC @7.8 amps
30 VAC @ 11 amps


Ive had it running at 11 amps for about an hour now, The coil stays at 50*C with the cooling fan on.

This sound like a reasonable voltage drop for laminated transformers? There is some space left between the secondary coil and laminations, probably 15-20% of the "hole" is empty, Would filling this up give less voltage drop?

Dylan