/diy/ - Do It Yourself

Bumping for answers. I really just want to know if this is going to melt itself or pop a breaker when I plug it in. For those who don't know, MOT stands for Microwave Oven Transformer i.e. the transformer that powers the magnetron in your microwave. This is what a finished MOT spot welder looks like.

>>129909
Can't really help, but what are you welding really? If you're building a solar car shouldn't you just make everything out of carbon fibre or other lightweight stuff?

> I would assume that since a microwave doesn't trip a breaker every time it's turned on that plugging the MOT itself in wouldn't either.
Don't know the schematics, but it probably works. As for user safety, I wouldn't count on that.
I hope you're up to date on first-aid recusitation.

The transformer should be self ballasting due to back EMF, however there will be an inrush current. Ideally circuits used for large inductive loads should be on a class C or D breaker (slower to trip). A classic exam question for C&G electrical installation required you to know this with regards to an installing an X-ray machine.

You could always use something like a 1000W halogen lamp in series as ballast, or hook it up to a variac.

>>129934
Oh, the car is already built, she's a carbon-fiber+nomex honeycomb for the shell and structural aluminum tubing for the chassis/roll cage. This is for an internal auxiliary pack for starting the car (precharging the caps, turning on the motor controller, stuff like that).

The schematic is basically 120 turns of wire in the primary => Transformer core => 3 turns of thick wire for secondary.

>>129937
I think we have a big shop lamp hiding somewhere, I'll see if we can work with that. There's also an ancient (at least 25 years old) 1.5kw genny in a storeroom, if we just used that, would the point of shorting the outlet and popping a breaker be moot?

Sorry about the delay, lost the thread when my comp rebooted. Here's the solar car:

>>129945

The generator would have it's own breaker (probably something nice and slow if it's 25 years old) and it's supply impedance would probably be too high to trip due to inrush anyway.

Id still give you a 90% chance that would could just plug it into the mains. You could use a couple of long extensions to limit the current inrush.

>>129953
Long extensions? Now those we have plenty of.

So with the current setup, to minimize the risk of something going wrong (at least on the power supply end), we should use the genny, and a long extension cord between it and the MOT? Seems simple enough. Not quite as simple as plugging it into the mains and going from there, but still simple enough. What could we do to get it to safely run straight from mains?

For power control I was thinking of wiring a lightswitch inline with the primary, flick on to weld, off to stop. Would this be a safe way to control the power?

>I would assume that since a microwave doesn't trip a breaker every time it's turned on that plugging the MOT itself in wouldn't either.

Correct.

>Also, considering this welder cost us $0 to make, is there any advantage to buying/renting a ready made spot welder?

Only the form factor of a store bought one and its buy it/use it ability.

Put something more than simple air cooling on that transformer too. It can overheat rather easily. you may want to consider submersing it into a dielectric oil (mineral spirits) and cooling the oil either passively or actively.

>>129969
>lightswitch inline with the primary

If it is between the wall and transformer it should be fine. Otherwise, you might weld it to itself inside when it turned on. You could also considering using a beefy power bar.

>>129909
What size MOT are you using?

>>129975
>Put something more than simple air cooling on that transformer too. It can overheat rather easily. you may want to consider submersing it into a dielectric oil (mineral spirits) and cooling the oil either passively or actively.

We have a couple aluminum heat sinks from old computers (from both CPUs and PSUs) lying around, would one of those (attached with thermal paste of course) and a fan be sufficient?

>If it is between the wall and transformer it should be fine. Otherwise, you might weld it to itself inside when it turned on. You could also considering using a beefy power bar.

Yeah, that's what I mean, so the schematic would go something like this:
>outlet- lightswitch->primary->transformer->secondary ->electrodes
Good call on the power bar. I'm sure there's one somewhere we can use.

Anyways, I'm off to class now, thanks for the help.

>>129979
It came from a 1kw microwave if I recall correctly.

>>129992
What amp breakers do you have for the breaker box?

>>129988
>would one of those (attached with thermal paste of course) and a fan be sufficient?

You can try. Just monitor how hot it is getting.

Dunno, they're in a locked box, presumably to keep us from being jerks and shutting off power to the rest of the rooms in the building. Which is ironic since the 600A main breaker for the entire building is completely exposed in our garage...

I'd assume 20 or 30 amps though.

>>130005
>I'd assume 20 or 30 amps though.

Never assume. Call and ask about it. 30amp would be perfect. They probably lock it to keep people from stealing breakers.

http://www.teralab.co.uk/Electronics/Spot_Welder/Spot_Welder_Page1.htm

looks like others have done this with no problems

http://www.youtube.com/watch?v=7571Qe6LJbQ

I want to do this too. Though it seems several people are using 2 transformers when they can't find a single large transformer. I have a transformer out of a big stereo. It is about 4.5" x 4" x 4.25" and is a Tamradio "34L104HNR" which I can only find a catalog entry on the database.ul.com site.

Any ideas or comments about this one?

good luck on your...
" I'm on the UK Solar Car Team..."
"...the UK Solar Car Team..."
"...UK..."
I hope America beats your face in... and also that you get your welding issue cleared up.

1. That's going to be some serious current just to assemble a battery pack. If your EE hasn't expressed issues with dumping 100 amps into the top of a battery then you might want to start looking for a replacement. (I am assuming you aren't using lead acid here..., if you are it's probably not that big of a deal then.)

2. Why haven't you or the EE just hooked up a multimeter and read the resistance on the now primary side of the MOT? That would at least give you a base line reading of current. Most everyone is right you are going to get an inrush of current. If the resistance is really low add in something (like someone said a light) in series to reduce the overall current.

3. If you are making nimh or li-ion battery packs with specific types of cells like 18650 or general cylindrical cells there are specific battery spot welders designed just for this purpose. Pros are faster, safer, easier. Cons are availability and cost. From what I've read a lot of RC hobby people just solder them together...however you do have some odds of ruining a cell doing that.

The main issue when spot/tack welding something like an 18650 is only having the current flow through the connector plate and not through the battery. So you need a ground ring on the battery, the buss strip to weld on, and the welder on top.

>Your EE isn't the brightest bulb in the box.

>holy shit, this is still here

>>130745
UK as in University of Kentucky, look at the picture of the car I posted.

>>130760
The primary Li-Ion pack was soldered together (I think, I wasn't around for it's creation), this is for spot welding connectors to NiMH cells together to make a 12v aux pack for starting the car. The plan is to wire the output of the secondary to two electrodes placed close to each other, press them to the battery, turn on the switch, make the weld, turn off switch, remove battery with newly attached tab. Wash, rinse and repeat. We're going with the MOT instead of a purpose built machine is because this cost us $0 to make, and we're only going to use it once or twice. Dunno why we didn't think to measure the resistance of the primary, brain fart moment there.

>>130824

As long as you have one of the secondaries on the battery terminal (should be touching with copper or brass since those don't really weld up) and then the other secondary on the tab it might/should work.

Since you have everything you might as well try it and see what happens. But from a pure /diy/ standpoint I have always liked the idea of doing it from a capacitor discharge or inductor discharge point of view. You get a nice clean pulse and can have it assembled and read to weld before any current goes through it.

>>130745

>" I'm on the UK Solar Car Team..."
>"...the UK Solar Car Team..."
>"...UK..."
>I hope America beats your face in...

I've always heard it's dreary and rainy most of the time in the UK... That sounds like an exercise in futility.

>>130891
>That sounds like an exercise in futility.

Only for an improperly built solar car.

>>130894
I want a car powered by cloud cover.
If you must it can be powered by dampness.

The issue is that when the transformer core saturates its effective primary inductance will fall allowing a large current, however the resistance of the primary may be such that this current is lower than the breaker. I recomend testing the resistance of the primary and using this as the maximum current that will flow. You will also need not to run this for long periods of time as I imagaine that transformer will get very hot. Don't go over the 13A limit of uk mains plugs as your doing this in an institutional context and making a proper spot welder with a big capacitor bank is only a few days work. I don't see an inrush current issue.

Why the fuck would you risk the damage when spotwelders are cheap as fuck?

>>130955
>Don't go over the 13A limit of uk mains plugs

Read comprehension...
>UK as in University of Kentucky, look at the picture of the car I posted.

From: >>130824

Damn people this thread isn't even that long...

>>130824

Worse case this doesn't work, just try and solder the things together. Tin the battery and tin the tab, put together, heat and done.

Like I said before you might, MIGHT kill one of them, but nimh is pretty durable compared to li-ion.

>>130963
go away buyfag, no one wants you here.

>>129909
OP how'd your spot welder turn out? We need updates.

>Holy crap, this thread is still here...
>>133034
Our next meeting is tomorrow at noonish. I'll hopefully have the electrode jig done up by 1-1:30ish. We should be welding by 2, if all goes well. I've got a bunch of dead AA's for practicing on, so we can figure out weld times and whatnot before going to the real pack. I'll take some photos of the finished welder and post em up when I get back home.

>>133067
See,
>>132419
for one just made.

Also, that sounds good. Post pics!

>>133067
What are you using for the electrodes?

>>133077
Either big nails, or screwdrivers is the plan right now. We'll see how things go tomorrow.

Spot welding is resistance welding. As in, electrical resistance. I can understand tacking inert bits of metal together (and I hope that's what you're planning to do), but using metal-melting levels of resistance on batteries? That's a very bad idea. You'll blow the cells doing that. If these are little packs of NiMH or L-ion batteries, solder will be fine (and easier to remove blown cells later, if need be).

If you just really need to weld something electrical, try using an oxyfuel setup. Gas welding is one of the oldest methods out there, and it uses no electricity at all. If any of your classmates have farmers in their families, there's a decent chance you could borrow their oxyfuel set for long enough to make your welds.

Conversely, TIG is a great way to join thin metals, but you'd be hard pressed to find somebody with the skills and equipment to do that (especially for free).

Check into oxyfuel. It's worth at least a few minutes of research.

>>133164

>but using metal-melting levels of resistance on batteries?
>If you just really need to weld something electrical, try using an oxyfuel setup.

If that ain't a troll I don't know what is.
>Don't use electricity, use a 1000C+ flame.


And btw, if properly done tack or spot welding on a battery is completely safe. You are not passing the current through the battery, just one end terminal at a time. Very little residual heat, quick, and fairly effective.

>>133129
>big nails or screwdrivers
The electrodes should be copper. If you expect to weld two sheets of steel together using a high current, then how can you not expect steel electrodes to be welded to the workpiece?

Copper electrodes are used because of its lower electrical resistance and its high thermal conductivity. The two properties prevent the electrodes from becoming welded to the workpiece.

>>133164
>You'll blow the cells with resistance welding
>oxyfuel weld batteries

You can't possibly be this stupid.

Spot welding would not be the industry standard method for tabbing and joining cells into battery packs if it were not the most appropriate way to do it.

Electricity does not flow where there is no path for it. It doesn't go into and fuck up the battery's internals when the entire circuit encompasses a tiny spot on the tab and terminal. On the other hand, heat is a battery's worst enemy. A spot weld discharge is very quick and causes negligible heating to the battery beyond the point being welded.

This is precisely why it is used over all forms of thermal welding or soldering, the heat of which will spread to the battery internals and damage them.

It is unnecessarily hazardous to solder to a battery, and downright foolish to use a gas welder on one.

>>133164

not everyone has an oxy fuel rig dude, that shit is expensive, spot welders are really cheap, require almost no experience, no adjustments, are very safe

>>130970
Right!? I mean even if you missed that, troll senses should be tingling with
>UK team
>THE team representing all of UK
>looking for advice on 4 Chan

>>133075
Good news and bad news. Good news, I got the welder mostly done, just need to add electrodes. Bad news, the general consensus is now against using the MOT welder and finding someone with an actual spot welder to use. It's a bit of a letdown that 5 hours of work are for naught...

>>134027
Build it anyway. I built mine in like 4 hours of real work and that was only to test to see if it would work. And, it works. Here's my thread, which has some details on what I used,

>>132419

>>134077
Thanks for the link. I'll probably finish it anyways, just for the hell of it. I probably spent 3 hours doing real work, and 2 hours of dicking around and working on other things on the car (like reinstalling the brake calipers).

>>134177
Do you have any pics of what your MOT welder looks like and how you are doing the electrodes?

>>134180

I didn't take any, unfortunately. If this thread is still here Monday evening, I'll be sure to get a few. The secondary was rewound with 3 turns of 4AWG stranded wire (shit's stiff as hell, had to brace the MOT against my feet and pull to get enough force to wind it). The MOT itself is mounted with 4 screws to one end of ~18" of 2x6 or 2x8 (scrap wood). Immediately in front of the MOT is a ~10" piece of 2x4, mounted vertically. Halfway up that is a piece of 1x4 to act as an arm. There's a countersunk hole drilled in the middle of the 1x4, and a screw run through that into the 2x4 to act as a pivot.

One wire will run over the arm and down to be a fixed contact. The other will be mounted to/through some PVC to allow for flexibility in the distance between spots.

>>134188
Pics will be good.

The 12AWG I used (12-3 has 4 wires in it) doubled over made eight 12AWG wires I was bending around. It was a real bitch too.

I'm planning on using PVC too.