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Forum Index : Electronics : MOT
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| irishron40 Senior Member  Joined: 22/09/2014 Location: IrelandPosts: 251 |
Hi. Hope ye are all well. I have two questions. What is the best way to measure the current output with my multimeter of a microwave transformer. It should have about 2000vac and 500mA output on the secondary winding. That would mean 1000watt And also I like the output current limited to about 20-25 mA. Would that mean I need a high voltage resistor of 50watt ??What would be the easiest way. Thanks a mill. Ron |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
Be EXTREMELY CAREFUL! 2kV or so at anything up to half an amp will be INSTANTLY FATAL if you touch the HV. I expect you probably know that, but MOT's are extremely dangerous when powered and you need to take WAY more care then you normally would working around 230v or so. If I were doing it, I would just use an isolated AC clamp-meter to measure the AC current. The best thing about this method, is you don't need to play with the HV wiring, you just need to clamp the meter around one of the HV leads from the MOT, then power on the oven and run you tests. Once finished, be aware that HV capacitor on the MOT can remain charged up to several hundred volts for a long time AFTER you remove the power, and if you touch that cap, it WILL bite you. Depending on the energy stored in that cap at the time, that too can be fatal. BE EXTREMELY CAREFUL!!!! Smoke makes things work. When the smoke gets out, it stops! |
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Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
When I was in the business of repairing domestic goods, microwaves were regarded as the biggest killer of us technicians. You would be far safer, for you and your test gear, to measure the transformer input current, and using that as a estimation of the output current. Is this a microwave oven you are testing? Output heating low, or not heating at all. Can you hear the transformer change noise after 1 or 2 seconds as the magnatron element heats up and it starts to draw HV current? There are 4 parts to a old school microwave HV circuit. Transformers were reliable, and can be easily measured with a ohm meter. The caps do die, and can also be tested with a multimeter out of circuit. Short it out before you go touching the thing! The HV diodes would go open circuit, and can not be tested with a typical multimeter as the forward voltage is very high compared to a typical diode, so if the cap and transformer were ok, replace the diode before the magnatron, its cheaper. The magatron filaments do go open circuit, again easily tested with a multimeter out of circuit. All these tests can be done safely. I did use a HV probe once or twice, but it was a clip on affair that was connected while the microwave was off. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hold the phone, just re-read your post. You're using this transformer for something else, arnt you? To keep the volts but reduce current, yeah a resistor will do the trick. But as its high voltage, you need a string of resistors in series to reduce the voltage across each so they dont arc across. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
Thx . Yes I'm using it for a wood burner. But 400-500mA is too much. The arc burnes wood quiet deep. That is why I only want about 20 to 25 mA.that still might give a person a good bang, but wouldn't kill him as 0.5 amp would.i would love it to be variable from 0 to 30 mA. |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
Do you mean like one of those hot-wire poker-work woodworking things? Cos if you do, there are much safer ways to get a hot-wire to do that.... Smoke makes things work. When the smoke gets out, it stops! |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
Yes a Lichtenberg burner |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
2000vac with 20mA behind it will still kill you, easily, permanently. And it will hurt a lot, and smell bad. I would suggest a car ignition coil and some sort of pulse circuit. The sparks are hot and bite, but its a short pulse, so it really really hurts, but you have a chance to flinch and escape between hurts. The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
Don't think arc from ignition coil would travel far. Think these are about 10000 v and 5 Amp |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Depends on the coil. My Commodore coil packs throw a spark about 40mm long. A few weeks ago I pulled a plug lead while the car was running to find the missing cylinder. I was using double insulated pliers, but it still zapped me, and then started throwing big sparks all over the place, with big loud cracks! Impressed I was. The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
Beautiful fractal patterns, but a dangerous hobby!  Video of home-made machine... Smoke makes things work. When the smoke gets out, it stops! |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
Gizmo .can I feed this system with 240vac. I know it works with 120vac. I would run the supply into a dimmer Then into a 250v/1-3uF capacitor From there into positive of coil Neg of coil to Neg supply Arc output from coil HV output and coil ground What is your opinion? I do want it if a 220 supply. Cheers everyone. Ron |
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| BenandAmber Guru  Joined: 16/02/2019 Location: United StatesPosts: 961 |
I think most people on this board goes overboard for safety and accuracy on advice but this one just seems scary to me just asking for it them Sparks going to jump out there and get you If you're not really really experienced at this kind of stuff I would not do it and I'm an electrician Human beings are flawed and can make mistakes no matter how careful they are Just my two cents I apologize in advance if I offended anyone be warned i am good parrot but Dumber than a box of rocks |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
Thx Glenn. I'm quiet aware of the caps and mot,s. I even know how to test all.lol. Must be 2 years now since I started with electronics, and still love it.think sparks are amazing lolol |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
I'm glad you are enjoying your electronics path.  I think the rest of us are just concerned that you get to continue to enjoy that path. I've been working with electronics for THIRTY years now, and I still consider a powered MOT not something I would really be prepared to play with. There is no more sure-way to kill yourself then playing with one of those things unless you have YEARS of experience with HV work. SPECIFICALLY HV work, not just domestic 115v or 230v AC. Even then, that does not make HV any less dangerous.Have to agree with BenandAmber on the point they make above. Make no mistake - MOT's are lethal. EDIT: [Quote]can I feed this system with 240vac I would run the supply into a dimmer Then into a 250v/1-3uF capacitor From there into positive of coil Neg of coil to Neg supply[/Quote] It would probably explode in your face. Remember that an ignition coil is designed to work from 12v DC, NOT 220v AC. You would either blow up the coil, the dimmer, or both. You would most likely also be putting yourself at risk just from the 220v side of things with an arrangement such as that. Please don't.... A dimmer is not designed to pass the current that the coil would suck from the setup. Most ignition coils only have a series resistance of a few ohms, and they can suck six or seven amps from the battery when running. One of the reasons they are oil-filled, is to help dissipate the heat generated by the coil primary. And besides that, the back EMF from an ignition coil would probably destroy the dimmer when you turn it off. Sure, you could fit a snubber diode, but still.... I get the strong feeling that you have not quite had enough experience with high-voltage work, to really be playing with it. If you DID, you would not be suggesting connecting 220v AC to an automotive ignition coil, to say nothing of the MOT's for a moment. As I say: I have had 30 years experience, and even I would not play about with the secondary on an MOT. It could very well be the last experiment you do - ever. Ultimately, it is indeed up to you what you do, but nothing is served by your electrocuting yourself for the sake of an experiment.  I apologise if you are offended by this post - really not my intention - honest. I'm simply concerned for your continued existence. Smoke makes things work. When the smoke gets out, it stops! |
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RFburns Regular Member  Joined: 21/07/2017 Location: AustraliaPosts: 43 |
I don't often post here but always interested when someone starts talking about using MOT's (microwave oven transformers) as most people still don't seem to realize that these thing are in fact a saturatable reactor (not a true transformer unless you remove the shunts and then one end of the H.V secondary is grounded normally). They usually have a winding ratio of between 15:1 and 20:1 and only normally do 50% duty cycle in there normal use . I have used at H.V, many of these and also repurposed many for other L.V use. In normal use the HV circuit is a voltage doubler with the magnetron being one of the diodes, you may consider it a dead short. Ordinarily the current would soar; but for the split core design (shunts) of the transformer which makes it a saturatable reactor, self limiting in the current department. However I would suggest Grogster is correct when he says "I get the strong feeling that you have not quite had enough experience with high-voltage work, to really be playing with it. If you DID, you would not be suggesting connecting 220v AC to an automotive ignition coil, to say nothing of the MOT's for a moment." with raw AC anywhere from 1100v to 2500v these things take no prisoners! Strong like horse smart like tractor! |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
You don't offended anyone. You are right in what you are saying. Wires won't be held,and switched on by a 2 switch deadmans switch |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
I know people do use a 120vac input at a coil and no problems with the setup I described earlier. That is why I asked would 220vac be ok also. What would be best way if I do wanna use ignition coil with current control. I could also connect a poor man's current limiter for savety |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
OK, then. Your life, your decisions. Best of luck. I HOPE we can continue to see more posts from you in the future...... Smoke makes things work. When the smoke gets out, it stops! |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
Lol . I didn't say I was gonna do it. Just asking for wiring advise |
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