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Registered Member #1733
Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
I could could run a search on this, but there are thousands of threads with "transformer" in it and I'd have to spend a week sorting and reading.
I am looking for a transformer option that can deliver about 30 to 40V @ 6 to 8A. At my disposal are a MOT (stock windings,) an APC UPS transformer, smaller transformers salvaged from various boards (multi-tap pri and sec,) and I can pickup torid transformers (0V to 240V 50/60Hz Pri, 42VA Sec) for $10.
I've never used a MOT, I was looking at it last night, and psyched myself out of using it. Probably the most intelligent course of action for me today. I could re-wind it, but have never done so, and would prefer to have a better understanding of the requirements of design before I attempt it.
The transformer from the UPS is about 2/3 the size of the MOT, but it's not wound the same. Looks like an E core setup. It will run from mains, just not directly. The current draw is more that 15A, so it will blow any breakers or fuses protecting that circuit.
From a physical size point of view, I like the torid transformers. They are within my spec range, but I'd want to use 2 of them, in close proximity to each other. I would imagine that they would need to be shielded from one another, but believe the diamagnetic shield would alter the performance. I think I'll pick them up anyway - I like stock-piling parts.
Once again - I fall to the wisdom of those on the board.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
You can put toroidal mains transformers as close together as you like since there is negligible external magnetic flux, the only considerations are electrical insulation and heat. Stacking one on top of another isn't a problem, just make sure that the method used for mounting doesn't make a complete electrically conductive turn (through the hole in the center)
A transformer from a UPS output should be ideal for your purpose - just use it 'backwards' i.e. the secondary winding that gives the output is now the primary.
You may (probably will) have large 'inrush' current on initial power-up. There are many ways to handle this, my favorite is a series resistor that is short-circuited by a relay contact, the relay coil should be powered by the dc output, with a zener in series if necessary.
Registered Member #1526
Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216
It would be really quick and easy to wind a secondary on a MOT for 35V. As long as you leave the primary on there`s nothing involved, just wind until you reach your voltage. You won`t even have to fret over insulation. I`d do that.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
If you use an MOT, you would probably want to wind a couple more extra turns on the pri so that it draws less current when there is no load. I think it would reduce heating of the core too.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Myke wrote ...
If you use an MOT, you would probably want to wind a couple more extra turns on the pri so that it draws less current when there is no load. I think it would reduce heating of the core too.
If he does not wind extra turns, the MOT might as well overheat just sitting there powered! I should try that sometimes, but I believe the core gets EXTREMELY hot (>150C) if the MOT is not cooled...
Registered Member #1733
Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
Sulaiman wrote ...
You can put toroidal mains transformers as close together as you like since there is negligible external magnetic flux, the only considerations are electrical insulation and heat. Stacking one on top of another isn't a problem, just make sure that the method used for mounting doesn't make a complete electrically conductive turn (through the hole in the center)
A transformer from a UPS output should be ideal for your purpose - just use it 'backwards' i.e. the secondary winding that gives the output is now the primary.
You may (probably will) have large 'inrush' current on initial power-up. There are many ways to handle this, my favorite is a series resistor that is short-circuited by a relay contact, the relay coil should be powered by the dc output, with a zener in series if necessary.
The torids are not going to do the trick. I powered one up last night, the voltage is OK, but no current. I sent an email to the manufacturer for the specs (these are custom jobs) and they are not up to the task. Max voltage/current is 18V, 2A @ 230V input.
The UPS transformer probably has a short in it. Would explain why the UPS it was in would not turn on. I ran it from mains in it's original configuration (forwards if you will) and that lasted about 5ms before blowing my protection outlet (a custom steel box with an internal 15A breaker, filter, etc...) That box literally went up in smoke.
The MOT - well, still looking at it. This thing is just huge for the intended purpose (driving 2 flybacks to ~35 or 40kV.) My goal was to make this portable. My project-box specs have already been thrown out once because of the heat sinks required on the IRFP250's. Putting a MOT in the plan just made this bigger and heavier. Ughhh.
Fraggle wrote ...
It would be really quick and easy to wind a secondary on a MOT for 35V. As long as you leave the primary on there`s nothing involved, just wind until you reach your voltage. You won`t even have to fret over insulation. I`d do that.
Myke wrote ...
If you use an MOT, you would probably want to wind a couple more extra turns on the pri so that it draws less current when there is no load. I think it would reduce heating of the core too.
Dr. Kilovolt wrote ...
If he does not wind extra turns, the MOT might as well overheat just sitting there powered! I should try that sometimes, but I believe the core gets EXTREMELY hot (>150C) if the MOT is not cooled...
So my MOT primary looks like 18AWG magnet wire. Each layer has 8 or 10 turns, and I don't know how many layers for a total turn count. This sucker is about 15 years old. There are 2 secondary coils. One looks like more 18AWG, 12 or 15 turns. The other is the HV secondary, all nicely covered. This MOT came with the rest of the microwave, which worked fine before I took it apart. I kept everything except the case, so I have the big AC fan too. If it was good enough to cool the MOT before, it should still do the job.
Side question - There were 3 or 4 disks, with 2 leads each, on various parts of the microwave. One on the top of the cooking chamber, on the side, on the side of the magnetron... What would these be?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Zenador wrote ...
The UPS transformer probably has a short in it. Would explain why the UPS it was in would not turn on. I ran it from mains in it's original configuration (forwards if you will) and that lasted about 5ms before blowing my protection outlet (a custom steel box with an internal 15A breaker, filter, etc...) That box literally went up in smoke.
Are you absolutely sure you have used the higher voltage winding, the original secondary, as your primary?
Measure the resistances to be sure. The highest resistance winding should be your primary.
Zenador wrote ...
...so I have the big AC fan too. If it was good enough to cool the MOT before, it should still do the job.
Yeah, if you don't mind a lot of noise coming off your PSU.
Zenador wrote ...
Side question - There were 3 or 4 disks, with 2 leads each, on various parts of the microwave. One on the top of the cooking chamber, on the side, on the side of the magnetron... What would these be?
Registered Member #1733
Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
Dr. Kilovolt wrote ...
Are you absolutely sure you have used the higher voltage winding, the original secondary, as your primary?
Measure the resistances to be sure. The highest resistance winding should be your primary.
Nope, not sure... I'm getting another transformer today. My boss told me to toss out another rack-mount UPS today. And I will, right after I cannibalize it.
Dr. Kilovolt wrote ...
Yeah, if you don't mind a lot of noise coming off your PSU.
Meh, It's a work in progress.
Dr. Kilovolt wrote ...
Thermal fuses.
Thanks... I'll have to figure out the threshold, and if it makes sense, stick one on the MOT. If it blows, make it shut the sucker down.
Edit:
I grabbed the transformers from the UPS. There were 2, with the secondary coils in series.
Secondary wires are 600V 10AWG Primary wires are 600V 16AWG.
The black primary lead (HOT) has a fuse inline - 250V 20A. The UPS was an APC SmartUPS 3000... 2700 Watts, 3kVA.
Primary Leads Secondary Leads Info from top
I don't have a DMM at work, so I can't measure the resistance of the coils. These things weigh more than my MOT.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Well if you decide on using the mot, i have done this several times. Just get a sharp chisel, and set the mot on its side, so that when you hit the chisel against the secondary windings, it is aimed toward the other side of the secondary, not the primary. If you aim it toward the primary, when you finally cut through the secondary, you might find yourself cutting into the primary, which is no good(of course, i mean come on). You will have to do this 2 times, and once you get half of the secondary cut away, just get something flat that will fit inside the core, and hammer and push the rest of the windings out.
After you get the secondary out, wind 10-20 turns extra for the primary. Use the same thickness wire as the primary if possible, or just use anything that doesn't take up too much room and can handle the current. After that just wind about the same amount of windings as you want the output voltage to be. After this, power it up, check the voltage and make an estimate of how many more turns you might need to get the voltage you are going for. When you are winding, just put the wire through the core, and pull it tight, then do the other side. Make sure you do this neatly and half all the windings flat, so that you can fit plenty of windings on there.
Remember, when you rectify and smooth, you will get slightly higher output voltage than what the mot originally put out.
Make sure that when you are winding, that you do not scrape the wire for the windings against the core, as it is sharp and will cut right through the insulation. Use as thin insulation as possible to allow more room, but thick enough to keep from scraping the insulation away and shorting against the core, that would be bad.
Use this to find what wire gauge to use for the current you want. The less current you want, the smaller wire, and the easier it will be to fit the windings you need onto the core.
Hope it helps, and if I was unclear on any part(probably just about every part i am sure) just ask, i will show ya what i mean.
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