Introduction: Easiest Microwave (MOT) Salvage/Re-use Tutorial

These videos show you how to take any broken microwave and get the transformer out of it, tear it down, and rebuild it according to whatever project you have that needs one. These form the heart of many other projects, which I cover (or will cover if not out yet) in other tutorials.

Hopefully every question you might wonder is explain in detail in the videos. I give plenty of alternative options for tools and methods.

Part 2:

Part 3:

I'm a big proponent of criticism and feedback. My first video had some audio issues. I made parts 2 and 3 several months later after feedback and I think they're much better. Please leave comments if you have any suggestions for improvement, criticism, things you thought were poorly explained, etc. I read it all and it helps me make better tutorials in the future.


batchit (author)2015-12-30

Hi Matt, great tutorial, just wondering if
you could clarify something for me, going through the videos and
comments I couldn't find the answer. The winding direction for the new
secondary should it be in the same direction as the primary, ie: say if
the primary winds clockwise if viewed from above the secondary goes the
same way?

And also does it matter if the primary is on top of the secondary or vise versa?



Winding direction does not matter unless you're combining multiple transformers or windings together (in which case, you want the end result to be matched so they add, rather than opposite so they subtract to net zero). Think of a sine wave. The direction you wind determines whether you start at the top, or the bottom. It alternates 120 times per second, so, unless some quirky circuit requires that precise of timing and polarity, it makes no difference. To keep my head straight, I usually try to wrap in the same direction, good habit, less confusing if you want to combine transformers later to make it more powerful.

Order of primary or secondary doesn't matter at all. The transformer doesn't have a "top" or "bottom" and it's operation is unaffected by gravity, which is the only way "top" and "bottom" could be defined. Put it on however is convenient. I usually put the primary on first (trapped at the back of the E) because I'm more likely to change the secondary some day, or have to adjust it to fit. Operation-wise it doesn't matter.

Thanks for that, isn't amazing when something is explained in simple logic and common sense comes into play. The only difference is that in Australia we run on 240V @ 50 Hz.

bendigonian (author)2015-11-07

Just as a PS to the last note. That's a fourth way of determining turns required. Measure the voltage of the secondary as I described. Unwind secondary and count turns, you could probably just use a pedometer mounted offset on the unwinding spindle if you haven't got some better turns counting mechanism.

I'd probably use a cordless drill with a 13mm bit of dowell as a bobbin to unwind onto. Those secondarys look like they've got a lot of turns on them. (By your estimates of average primary turns, there should be 1100 odd turns on the secondary). Then divide secondary original voltage by number of turns on original secondary to give turns ratio.

Then multiply by required voltage to give new required turns. Then work out amperage anticipated and look up a copper wire amp carrying chart to get best wire thickness. You could also do a voltage drop test with length, amperage and gauge to give heat rise if you're really feeling gung ho.

Also means you've got a nice little spool of small gauge wire for when you build that Tesla coil you always wanted to...

bendigonian (author)2015-11-07

Great tutorial, just what I need to make a decent cnc servo linear power supply and get away from switched supplies (and save 2-300 bucks as well). Got a dozen MOTs from local tip shop for AUD$2 each, already out of case, including a monster that looks like it's 1500 watt at least (weighs over 8kgs).

Wanted to test the secondary output which lead to some confusion 'til I discovered that MOTs ground one secondary lead to the core (and then the case to I suppose).

As the MM only went up to 600vac and the secondary is probably around 2000vac I made up a resistor string of 90Mohm to boost the MM impedance from 10Mohm to 100Mohm. Had to series eight resistors in the end out of salvage as I didn't have a 90mOhm resistor to hand.

Worked a treat. Secondary was putting out 1890vac, and the two other little secondaries (tertiaries, quaternaries?) were putting out 3vac and 9vac, probably to rectify to 5v for a logic board and 13v for the fan motor....

I thought that maybe the meter could have handled it without the resistor seriesed with the probe, but sadly not! Fizzle, pop, smoke. One dead multi meter. Oh well...

Evander12345 (author)2015-11-03

Hello, I've been having some trouble with my transformer. Instead of two welds, one on one side and another on the opposite side, it looks like its made of two interlocking chunks. It was salvaged from a microwave, and when I look at all these videos online on how to cut the weld apart, it doesn't match any. on the label it says OBJY2 and below that, EBJ60664602. Any and all help is appreciated.

Okay, I looked that up. You are right... The welds on that model are in peculiar places in the pics I found.

Two possibilities:

1 - the welds are in weird places but the transformer us a normal not. Cut where the E and I join and follow the tutorial as-is.

2 - the transformer is "interleaved", which means the E and I pieces plates aren't each in a big 100-piece stack that are the welded after, they're alternating every single layer. This is the better way to make a transformer, but harder, and it's almost impossible to disassemble (and then reassemble) without damaging it. In this case you'll have to cut the secondary off and thread your new secondary as per the tutorial "hard way".

Take some pictures in good light (bathroom), post them on Imgur and link the album here, I'll try to help more.

Here are the pictures you asked for

Thanks again for the help with my strange transformer

oooooh (author)2015-10-10

If it was a book I would say it not only contained information I needed it was also an easy enjoyable read. Thank you!

etkoehn (author)2014-03-14

So how many wraps would i need on both P and S for 120v in and 24v @50+amps and size of wire

Which part of the tutorial are you stuck on?

I don't mean to be rude, but, I took the time and effort to create a tutorial so that people like you could learn, and then answer your own questions, not just ask me every time you need an answer. I think it explains exactly how to answer this question, so, let me know where you got stuck and I will do my best to assist further.

I watched vids again and think I got the amps figured out please help if I'm wrong. If i want 24v @50amps on Secondary (=1200Watt) and using 120v on primary amps will be around 10amps(1200watt). So I would need to find approximately 12 pound MOT

I know you said voltage ratio= turn ratio and MOT's are at the bottom limit of the P wraps. Does wire size effect that? Would less wraps with larger wire increase Power? Or would adding more wraps on primary and secondary increas power?

That leads me to wire size on both primary and secondary, mainly secondary if I find a MOT with correct power rating(1200Watt). If I am pulling 50amps @ 24 what wire size do I need on secondary so I don't burn them up.

As for "not just ask me every time you need an answer." I'm lazy why not have someone else do the work for me :P I would appericate your help though .


mitchells365 (author)etkoehn2015-09-22

If you increase both wire size and number of turns on the primary it will be able to transfer more power to the secondary without overheating. Power = Voltage * Current, and in order for a coil to work at a higher voltage it needs more turns, while in order for a coil to work at a higher current, you need thicker wire. Therefore more powerful transformers will take up more space and cost more. That is why substation transformers are multiple feet across, they are made to handle a hundred thousand watts or more. If you are serious about designing a transformer you should have a good understanding of Ohm's Law and circuit design theory. You have to be good at math to work with electronics. You also should have some common sense so you don't electrocute yourself, always turn off the power before working on a circuit. Only specially trained people can work on live circuits safely, and even they are taking risks by working with live circuits

1200 watts is correct.

No amount of turns or any ratio effects power. Adding more turns increases voltage but lowers amps, and power is Volts x Amps, so it always stays the same. The limitation is magnetic, set by the size of the core. The only thing that affects the max power of a core is the frequency of power, which is very complicated and expensive to change (building a fancy inverter power supply or using a "Variable Frequency Driver [VFD]"), and even then you have to use a core made out of special materials, so forget I mentioned it.

For wire size, pick the largest you can, to fit ~24 turns in. An MOT is already pre-engineered, so you know whatever was figured out originally was something that would work. Wire size and max amps scale with size, so, by simply picking the thickest you can, you know it'll be about right.

Thinner wire will work, but will tend to overheat and melt. So that effects duty cycle. Use the thickest you can.

DragonfireFX (author)etkoehn2015-04-22

5 turn

JamesA17 (author)2015-06-04

I am rewatching your videos and I believe I am answering some of my own questions (please correct me or compliment me)

1. Fundamentally it doesn't matter if your two MOTs are from different wattage microwaves. You can use the "winding magic" to make a MOT from a 900W microwave, and a MOT from a 1100W microwave to produce ~equivalent voltage.

However, it would be to my benefit to use the largest MOTs available because the more physical space you have the larger the chances are to use thicker/more reliable wire or improve cooling.

2. Still no clue.. any help on this would be awesome


4. Where would you recommend to get scrap aluminum? I want to melt/pour/cast aluminum into molds, but having a hard time thinking of where to find it for free (besides beer/soda cans.. I am already starting a collection)

1 - Correct.

This is in the script of the followup video I haven't filmed yet.

In short, you have 2 choices: A - Match voltages each to your target voltage, join in parallel, and have the currents add up. Or, B - Match wire thicknesses (current), join in series, and have the voltages add up.

In Case A you would wrap, I dunno, 30V each, and use different wire sizes for whatever ~30 turns would yield. The voltages have to match really, really closely, as they'll fight each other to balance if not. Sometimes you have to tweak the primaries by adding turns just to get the two secondaries to match.

In Case B it's not as big of a deal, you just coil and go. Maybe the big one fits 16 turns and the small one 14. Together they make 30. Also, if you're a little high or a little low you it doesn't matter, just pack turns in.

2 - Phase is tricky. Trial and error is easiest. Basically, you've got a 50/50 chance of getting it right on the first try. If you get it wrong, you pick one of the primaries and swap the wires. With 2 separate transformers you're okay to do trial and error.

Connect your big transformers and measure the output of the secondaries. If you measure almost zero volts on the output, your voltages subtracted rather than added. Flip the wiring on ONE (only one) of the primaries. If you measure the right amount of volts you're set.

Think of it like two guys pushing a car. If one of you is pushing on the front bumper, and the other on the rear... the car is going nowhere unless one of you is stronger, in which case it's still only barely moving. You need to both be pushing from the same side, so one of you needs to switch. If both of you switch positions, it's still wrong.

2a - Correct. Power generation is 3 phase and is each phase 120' out of synch. Also correct that home circuits are 180' out of phase. Here's how:

Residential wiring in the US is "split phase". What that means is, when the 3 phases from the power plant make it to a community, they take each of those those phases and give them to a different block. So each block gets only 1 phase. So residential is 1 phase wiring. Into your house comes 1 phase at 240 volts. This is on a center tapped transformer, so, one side of your panel is -120v to 0, and the other side is 0 to +120v. These are 180 degrees out of phase. Most plugs in your house are all 120v, and it doesn't matter which it is because it's relative to itself. Some appliances like your stove and clothes dryer use 240V, which means they ignore the middle tap and use wires from -120v and +120v, which is the full 240v difference.

Which matters if you're running your welder from 240v, or, if you intend to run them from 2 separate breakers so you can get a full 15amps out of one, and a full 15amps out of another.

But for the questions you're asking, also phase matters even if power wasn't that complicated and you only had a single 120v ac signal. That's because if you're hooking up two things to it, you can still hook them up backwards to each other. Both would work on their own, (like pushing a car from the back or the front), but the polarity of a coil determines whether you start at the peak or valley of the AC cycle.

3 - I do my best.

4 - If your area has deposits for empty beverages, don't melt them down. They're worth like, 50x as much as empties. If you area does not have them, go ahead, but pop cans are awful, there's about 50% waste when you melt.

A good source of scrap aluminum is old hard drives. Just bash out the platters and electronics, the rest is cast aluminum (which means it's a good alloy to cast again). Save the big neodymium magnets! Broken equipment works well too. Most air compressors are aluminum, they're junk when they get cracked. CPU heatsinks are aluminum (or copper) too, but they're expensive and if you need heatsinks for anything (like your welder, or to epoxy to your transformer to help it keep cool) then don't, metaphorically, whittle an ornate coffee table into a walking stick. Just grab a stick.

Broken aluminum bike parts are a good source too. Cargo racks, frames, seat posts, handlebars, rims. If they don't hold a magnet (steel), they're probably aluminum. Call up a bike shop and ask them to set aside their aluminum scrap for you. Every major city has some non-profit community bike place that turned donated junk bikes into working bikes, find yours and ask them, they'll for sure be tossing away junk frames. Bike parts break all the time.

Car parts are often aluminum too, but I'm not a car person.

Alternatively, aluminum is only $1/lb as scrap, go to a scrapyard and ask to buy some from their lot.

awesome sauce..

Okay yeah the first part you explained about the phase within a 2 MOT welder was pretty clear and straightforward (that idea I understood even before, but thank you for califying)

The whole 3 phase to 3 different blocks was completely knew knowledge and enlightened me. Thanks!

Unfortunately I don't live in a large city, but I do live on an military base which throws stuff away all the time. So I'll start looking around, maybe i'll take a trip to my local dump and see if I can't stand the stink long enough to salvage some stuff.

Headed to the big city this weekend, hopefully to buy a second 1100 watt microwave for $25.. i know i know I could find it free, but meh I don't have time to look ha

Again thanks. I am sure I'll be back again with questions. And thanks for the quick response.

Ugh. $25 each for a microwave you're turning into scrap? You can *buy* a crappy stick welder for like, $50. You're whittling furniture into walking sticks.

JamesA17 (author)JamesA172015-06-05

forgive the typos and mispelling.. wow.. I guess I am so used to swype on my phone, when I hop on a computer I just type whatever the hell I want and expect my computer to figure out what I am saying! LOL

Here are the list of corrections:

thank you for clairifying*

completely new* knowledge

JamesA17 (author)2015-06-04

Hello Matt,

Big picture: I want to build a welder and/or electric arc furnace to weld/melt.

I know I literally have just signed up for an account, but I have been scouring the internet for about a month to find an answer to the following questions, and can't quite seem to find a direct answer. (in my experience that usually means there is an obvious and simple answer that I am just unaware of) I was hoping you'd be able to help.

1. Does the wattage of the microwave matter?

I found a deal on an 1100W microwave and bought it (have yet to tear it down). My question now is, should I find another 1100W microwave in order to have a "balanced" design? Or if I buy a 900W microwave, will they average out to a 2000W welder/arc furnace?

In other words: if I bought a cheap 900W microwave, and salvaged that for the MOT, and combined the MOT from the 1100W microwave, would it make that much of a difference vs using TWO 1100W MOTs?

Can I make up for the 200W less by winding things differently or some magic like that?

2. How can you determine the phase of a socket?

I learned that power generation plants use 3 phase generators usually (right?) so I am extremely confused by two things:

2a. if power generation is by 3 phases, shouldn't the phase by separated by 120 degrees and not 180? (I have read other instructables about connecting the 2 MOTs in a phase that is off by 180 degrees and confused how that works)

2b. how can I detect the phase of a socket?

Thank you kindly!


peterc4 (author)2015-05-14

The tutorial and comments are great. I'm interested in building a spotwelder for very small work - smaller than battery-tab welders. Yours is by far the best instructable about transformers that I have seen. Pity I didn't land on your tutorial first.

Yours is just the right balance - perfect amount of math to get me going - without scaring me off. Very helpful. Thank you for taking the effort to make this!

Any quick thoughts on fine tuning output? A timer came up in the comments. Commercial micro spot welders have a bunch of fiddly controls which are probably beyond my ability to DIY, but I think I could manage a timer circuit.

Thanks for the kind words.

Most timing circuits are based on TRIACs. So, think about an AC wave. For most power grids, AC power cycles 60 times a second (that's 120 times it crosses zero). TRIACs are turned on by a trigger and then are locked on until voltage drops to zero (which it does every 1/120th of a second). So, you can design a timer that holds the trigger down for an amount of time... but the smallest iteration is going to be 1/120th of a second. For most things that's plenty brief enough and this is simple and cheap, so most duration timers use this method.

If you need even less power, it's probably still easiest to still use the same timer but do something to drop the power. One thing you could do is just use thinner wire on your secondary. It will have more resistance, waste more heat, and eat up some of your power.

You can use a variac to selectively limit the voltage going into the primary too, variacs are fairly cheap.

I have other solutions to this but they're projects that take too long to type out and are badly explained in text.

Thanks for the explanation of how a timer works - all new to me - I'm not sure how I managed to get through school w/o even a basic electronics class.

120th of a second is seems to be fine enough a timer will be in the cards once I get the transformer sorted out.

I rigged a foot-pedal on/off, I can get a pretty quick tap for testing - I'm not popping any breakers so far. Buzzes like crazy - I thought I was in phase, but maybe not. It measures 250A initially then it falls off a bit. 2 turns of 4G auto battery cable.

I might try the Variac route before the timer. I know someone who offered to loan me a Variac.

I was initially thinking of making an adjustable secondary coil - a Variac on the primary coil would do the same thing (??). but a heck of a lot less complicated.

Its a work in progress - thank you again.

If you were using a single MOT, phase doesn't matter, there's nothing to be out of phase to. And yes, always switch or control the primary side. Switching hundreds of amps on the secondary is almost impossible.

You can probably buy a timer cheaper than make one. It can be relay or triac based. A relay might not be fast enough since it mechanically moves. In any case, you control the primary side (and usually control that switch with a small DC voltage in the timer's output).

Generally you don't want to use a variac to lower the power, you get better welds with fewer negative sideffects by doing a fast weld. You only want to drop the power if you're overburning the weld and it's already the minimum time.

pega5 (author)2015-05-05

pozdrav.ja sam pokusao napraviti punkt aparat od mikrotalasne,skinuo sam trafo.izvadio sekundar i namotao zicu precnika 5mm i dobio izlazni napon na sekundaru 4,9v a elektrode sam uzeo od lemilice precnika 4,8 i pokusao da zavarim 2 lima debljine 1mm ali nije islo,trazim savjet sta bih trebao da uradim,hvala

MattsAwesomeStuff (author)pega52015-05-05

Google says you are writing in the Bosnian language. Its translation is poor. You probably understood my video because I used universal subtitles.

I cannot understand your question. I understand some parts but not enough to know what problem you have and what you are trying to do. Sorry. Maybe ask a friend who speaks English to communicate your question.

matthewp4 (author)2014-11-17

What should I do if I have a 1200 watt and a 850 watt transformer

simplyconnected (author)2012-06-25

Matt, I was looking for your spot welder thread but I don't see one yet.

I'm retired from Ford Motor. Our Body Shop transformers are 50-kva to 100-kva and fed with 460-volts. The primaries are fused at 400-amps with 4/0-AWG wires. These transformers are water-cooled at 2-gal./min.

A typical 1/4" spot weld in two thicknesses of .020" sheet metal requires 20,000-amps at seven cycles (and ~4.5-volts).

The secondaries produce so much magnetism that carbon steel bolts on the jumpers are disasterous. They heat, loosen, and burn off. We must torque 1/2" stainless steel bolts, washers, and nuts very tight. They last because stainless isn't magnetic.

I realize you aren't making 1/4" buttons and that your duty cycle is very low, but still, spot welding requires a lot of power, tip pressure, and a good timer for 'squeeze', 'weld', and 'hold' times. The only way to tell if a weld is good is by destroying it. (just my 2 cents)

You don't see a thread because I haven't finished the tutorial yet. It's only half-filmed. I should just make a TL;DR version to get people started.

Spot welders do not need good timers. I have one, it works without. Most home and comercial (non-car factory) use do not timers of any sort. They have a thumbswitch for power and a lever for pressure. Braindead easy stuff.

Like many things, you can add features and complications if you are doing mass production or want to take a human element out of it. But to anyone building their own spot welder from junk, this does not apply.

Also, you can make your own timer mostly from microwave parts too.

Thanks for the feedback though, it's nice to have a frame of reference for what the big boys do.

+1 on the TL;DR spot welder tutorial. The vast majority of spot welder tutorials are made by people that have just enough knowledge to be dangerous. A thrown-together tutorial from you would be more valuable than all of those other tutorials combined.

Islam Safir (author)2012-05-29

thanks for the fast response, well after a quick search i found out that it's already been done using this transformer here in this video,
it's not that hard to separate the core I from the rest of the core and yes the T column goes all the way to the bottom.
I think it's doable but the only downside is that as the primary coil has about 4 turns per volt then to achieve the 2 volts needed for a spot welder i would need about 8 turns of heavy gauge wire that can handle 500 amps which about 3/4 of an inch in diameter seems unrealistic

Neat, thanks for doing the research. Okay, so, if it's a continuous (ungapped) core, it should work just fine. Just some extra grinding to get it apart.

Moving on, the rule for a spot welder isn't "1 volt", it's really more "1 turn". Even the big welders that weld car bodies are still 1 turn IIRC. So, you're not looking to add 8 turns. You should still add only 1 turn, 8x as thick as you would have.

500 amps is not a lot of current for spot welding. It's about the bare minimum for the thinnest of materials and smallest contact areas.

Figure you should fill half your transformer with the primary winding (about the same thickness as the wiring of the socket its plugged into, no point in going thicker, so, generally re-using the primary is fine). The other half, roughly should be the secondary. In a spot welder, the secondary is a gigantic single turn. It can be solid copper, stranded copper, scrap bits of pipe and sheet soldered together at the ends or hoseclamped into a bundle, doesn't matter.

A spot welder doesn't need any voltage really, since the output is a short circuit with no load. The only resistance in the circuit is the wires and metal material being welded itself, all in contact.. This yields only the tiniest fraction of an ohm, which means the tiniest voltage can cause massive amps to flow, which is what does the welding. Current = Voltage / Resistance.

Lastly, if you're looking up current carrying capacities of wire, ignore that. It's off by an order of magnitude usually. Spot welds take fractions of a second, up to about 5 seconds max. Then there's lots of time to reposition before the next weld, so your effective duty cycle is tiny. For welders with high duty cycle, the electrodes have hose/tubing nipples on them so they can be water cooled.

Look at a picture of a spot welder some time and look at how thick the arms are. 1" thick bar is quite common even on a smaller welder.

WOW great information , i kept thinking about the math and theories and ignored the practical facts altogether and was about to be dismayed . but reading through your comment makes perfect sense now . but i already took it apart though the way you showed on here with an angle grinder took only 2 passes on each weld line , actually the primary coil used 1mm wire and the secondary coil used 1.5mm wire and it was the bigger one here is pictures of it dismantled. i think i will use the bigger coil as primary and maybe take the second coil and unravel the wire and combine it into the secondary but i still don't know how long that might be and what thickness i would use to be safe as secondary coil, thanks for your help , i can't wait to see your next instructable of making a spot welder.

The shorter coil looks too small. I'd use the bigger one. On a spot welder, I wouldn't worry too much about the details. Pick whichever one would fill about half the the core height and use that one as is. I wouldn't bother trying to add more wire or turns.

The spot welder video is scripted and half filmed, it just takes time. I think I will have to reduce the quality and thoroughness of my videos, 'cause, at this rate, 2 or 3 a year is so slow it's frustrating for me. I thought I'd get the next video out in 2-3 weeks, it's been 4 months.

well from the picture looks small but the smaller coil is about 2 pounds the bigger one is about twice that , and yes i'm going to use the bigger coil for primary as is , but i meant take the other coil "the smaller coil" take it apart and use the wire bundled as a one turn for secondary after insulating of-course .
I know exactly what you mean about editing videos it takes so much time but the quality of editing isn't really important as the quality of information .
so don't feel that it needs to be perfect because nothing is .

Islam Safir (author)Islam Safir2012-06-04

just wanna give an update on my build, i unraveled the primary coil which had the thinner wire "1mm" and wrapped it around a 22" piece of wood and then cut it in one end so i ended up with about 105 strands of wire 44" long , twisted them and insulated with shrink tubing and it filled the transformer exactly 1 turn with no room to spare , i get 0.4 volt which means 2500 Amps its so enormous amount that it's just ridiculous , i still have to strip the tips of the wire "if you have an easy way to do that plz share" and then connect copper electrodes and make proper housing for the transformer.

jtechian (author)Islam Safir2014-03-18

On cleaning the varnish from the ends, try dipping the ends in paint remover for a bit. Then wipe clean and use flux to tin it with solder.

That is 2 turns, not one. You could fit 2x as much wire in there as you did, if you only used 1 turn.

To join the wires: Separate them about 2" back. Lay them out flat and separate like a comb. Then scrape them a bit with a butter knife, wire brush, or something else. Not much, then take a thick piece of copper wire and wrap it around them tight, 3 or 4 times at the back of the 2" , very very tight, crushing them together. You could also use a hose clamp.

Then, heat it up using a blow torch, or stick it on the element on your stove. Shove solder into the copper wrap. If the wire was scraped, the solder will work its way into the wire and melt the insulation back. Alternatively, just blowtorch the wire ends completely, from every side, incinerating the enamel. Wait for it to cool, brush the comb part on all sides with a wire brush, maybe use acetone or vinegar to remove the oxidation, and then secure them all together with a hose clamp.

Another thing I like to do is then wrap the end of a copper pipe around the last 2", with an extra 2" sticking out. Bash the copper flat/rectangular and then torch that and fill it with solder. That gives you a nice heavy mounting lug that you can drill a hole in a put a bolt through.

AlpineLED (author)2013-06-21

Hi Matt, I love your teaching and want to build pro custom spot welder. I used to work on large spot welder and TIG welder. I'm hearing impaired trying to understand CC but words are not perfect. I have MOT left over in stock. Would if two match core as E to E with two stock copper primary away from center connect wire together input will be 120 VAC. Will install 4/0 flex welding cable wrap around three or four turn in the center of core. Will this work better than one E-I core? Also like to add rheostat control not sure how proper wire hook up. Please explained.

The CC is exactly what I am saying. It's a script. I previewed it after uploading. Translations may be wrong, try switching your CC language to English.

I'm not sure I understand your question precisely enough to give a useful answer. Your English is not good enough and I am scared of giving an imprecise answer to you that answers a different question than you asked, or answers it in a wrong way.

For a spot welder: 1-turn. Always 1-turn on the secondary. More turns is bad. If you have 4/0 welding cable, if there is space, add extra turns but join them together as one big turn.

Rheostat control will not work. Too much power for a rheostat. It would melt.

I will try to be more helpful if you re-explain your questions so I can understand more, that's all I know how to answer right now. Sorry.

YouTube have ready CC english I used most of the time when it available. When people talking on video recording sometimes garble will effect to YouTube software.
The spot weld I want to build more power than single transformer. I saw your video that show two core of "E" together in one. I notice the two separate primary on each side. The middle many turns to get more voltage is that correct?
The idea of your video, What if I use 4/0 cable wire two to four turns to get more voltage = more amp? The 4/0 insulation rubber might be too thick to fit in the transformer. I may have to removed it and use teflon tape or better to stay flex ro wrap around the "E" core in the center. Now back to primary using 120 vac at 20 amp. Is it okay to have two primary either be series or parallel connect to input of 120 vac with 20 amp?
I've seen DIY spot welder in YouTube using different ways. But for safely, I want to make sure using common sense before I plug to 120 volt. Most MOT are using 15 amp to 30 amp depend on single transformer size. if double transformer may be more than 40 amp.
You're right single Rheostat control may burn up from 120vac. What can I use to control varable because Stainless steel use less amperage than other steel depend on thickness of metal.

Umm, re-read what I wrote last message. I manually typed up the script myself and submitted it to Youtube so that it was *exactly* what I was saying when I was saying it. And then I spent 45 minutes previewing it to make sure it's good. So... it's good. If the words in the CC are not clear to you, that is not a software error, that is what I actually said, so, that's my fault for explaining poorly. It's the same thing the people with average hearing heard in the audio.

"More voltage more amps" ... normally yes. With a spot welder, no.

This is a common question so I have explained it in my spot welder video which I have not finished. In short: Your current limitation is not due to voltage and resistance, it is due to magnetic limitations of the transformer in terms of wattage. The amount of voltage in 1 turn through the circuit is actually sufficient to put something stupid like 50,000 amps through it. So, ~50,000W. But the transformer is only ~1000 watts, so, you'd never see it.

What you do by increasing the voltage however, is reduce by half the cross section of the wire, and thereby cut in half any current you did get.

Power = Volts * Amps
If power is limited to 1000W, and resistance in a short circuit of heavy copper wire is effectively zero, *lower* volts actually allows more of that wattage to appear as amps.

Backwards to normal, if you think about resistance being the limit (usually is).

Also, the thinner wire will overheat quicker, drop more voltage on the way to the workpiece, etc. Just use 1 turn, always 1 turn for a spot welder.

Can you connect two primaries in series or parallel. Short answer: Yes. Long answer: If they're not matched, you'll melt them down very quickly. Also if you connect them or the secondaries wrong, you'll melt it down and probably cause a fire hazard, perhaps instantly. There is a very simple procedure for this that I've written, that I'm too lazy to post here since it's in my spot welder video... which you can't see yet because I haven't finished it. Not very helpful, I know.

As for control... not a big deal. Control your weld by controlling the *duration* of your weld. Thin metal? Quicker weld. At some point this is tricky because it's faster than you can physically flick a switch.

There's 3 or 4 things you could use, but, again, I'm not comfortable explaining them in brief because I'm afraid I'd give you just enough information to hurt yourself and not enough to be productive. There's lots of little things that can go wrong that I would need to teach you to think about first.

Sorry for being a bit useless. And sorry for the delays, making tutorials is a lot of work for little benefit and I have to be in the mood for it.

achand7 (author)2013-06-01

Hey guys I cant find any MOT around my area so I am creating my own transformer by combining similar sized transformers.So I was going to make Primary coil.I needed to know what is the average thickness of wire used in Primary coil of MOT to draw required current from 220v wall socket. Please state the correct wire gauges that you find in Primary coils of the 220v MOT or 110v MOT. I am constructing for 220v primary

16g for 110v.
Probably half that cross section, so ~19g for 220v (every 3 gauges is doubling of cross section, every 6 gauges is a doubling of diameter [4x cross section)].

But, it depends on the duty cycle and other stuff.

jasshopper (author)2013-05-27

what would happen if the primary winding is plugged to the ac 120v without the iron core?(just curiosity)

The iron core helps focus and direct the magnetic field. When a transformer is trying to transform too much power, it "saturates" the core, turning it, increasingly (and in the case of these cores, rather sharply) back into air. Well not turning iron into air, but iron that's magnetically equivalent to air. As if there was no core there at all.

With any voltage applied, current (amps) is limited only by the resistance. Some of that resistance comes from the wire (but this is very little, that's why we use wires to transport electricity, it has almost zero resistance). The rest of the resistance comes from "reactance", which is inductive (coil-based) resistance.

This reactance is what allows us to hook up what is essentially a dead short across the 120v socket and not instantly melt the wire from the zero resistance. Think about if you shoved a paperclip into a socket, no load, no resistance, it vaporizes the wire.

Now, there is about 120 feet of wire in that 120 turn primary, which is a fair bit, so there's a little bit of resistance, but not much. Also, even though it's an air-core coil, it's still a coil, so the reactance also isn't quite zero.

So, now that you have the pieces, here's your answer. It would probably melt the wire, but would probably not flash vaporize it. It would get hot very quickly (one or two seconds) and probably trip your breaker or blow your fuse.

I do not suggest trying this, because there's also a chance that the resistance is in some sweet spot between the wire being a safe temperature and enough current flowing to trip the breaker. Suppose you stay under 15 amps... meanwhile the breaker doesn't trip but that copper is getting hot enough to be a fire hazard..

could there be a problem like this if the winding is not perfect, like overlapped?

Not that I can think of. "Overlapped" doesn't matter, but if the wire insulation wears through and shorts to itself or the frame, well, that would be a fire hazard.

jasshopper (author)2013-05-25

i have a transformer which is almost like this (tall but slim) and i want to make a power supply for my zvs driver. so if the input voltage is 220v AC how much turns should be there on the primary and secondry if i want 36v (min 10A) out of it and what should be the awg of both the wires.(i want to wind both primary and secondry).

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