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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I have an 8kw ferrite welder transformer, rated at 32v output at 250 amps wound with all Litz, and it has another Litz winding which i assume is to kick up the voltage to help start an arc. It is made out of 5 E cores and is completely intact.
I also have a pair of 75 amp 600v halfbridge brick IGBTs, and a pair of 150 amp 1200v halfbridge bricks. I 8 fast stud diodes along with their heatsink, two large 1uf (iirc 600v or 800v) high current caps from a Miller welder. I have other misc bits, such as 4 big 1250vdc 500vac 10uf film caps and heatsinks and parts for a 4 output supply for gate drive.
I have been wanting to experiment with homemade welders, and i have so far made a 60hz raw AC stick welder, which actually works quite nice, outputting 4kw using two large MOT cores. What i would like to do with the transformer is maybe arc or TIG welding, though that would require me to buy a torch. 4.8kw is the highest power plug i am able to use, 240v and 20a. Lower power is fine, though.
My main obstacle is switching topology. Would a simple fullbridge inverter work alright, with adjustable duty cycle? I am not sure how most inverter welders work. Also, does frequency matter too much? Just something my IGBTs can handle, like 30KHz?
I have done research, and have found very little. One page that i did find has been pretty useful: And a document from that page: really makes me think it could be as simple as an open loop oscillator and fullbridge. I know there are many considerations such as inductance everywhere in the circuit, like all HF circuits, but the theory is simple enough.
On that page, there is this: schematic. Is that really all this guy used for a simple TIG welder? This is like what i had in mind, something with variable duty etc except with an IC i know better, like a TL494.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
I have a small hobby Tig welder that I have been trying to get round to repairing, no circuit’s for it though. But it uses a double switch forward converter, each switch is actually 4x 10n60’s. Driven by a L6386 from a Uc3845D. Looks like a micro controller, controls the pulse width so it isn’t easy to see how the control feedback circuit works. But would have thought it would have used some kind of constant current control. Don’t know the switching frequency, but the ferrite transformer isn’t that big, say 55mm square.
There’s a large tape wound ferrite inductor after the rectifiers (it’s a dc only welder) but no smoothing cap of course, also there’s some sort of hv pulsing in it for starting the Arc.
A lot of it including the output board is made with surface mount components, which I thought wouldn’t have been the best way to build a welder, but I guess it must have been cheap.
I would have thought the key thing when building a welder is to make sure the current is limited so it can work when the output is short circuited, without blowing up.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
How to make a TIG welder? Dismantle two and try to put one back together again. :P
The ones I've seen the guts of have been two-switch forward converters. (What the coilgunners call "diagonal half-bridge".)
If you use a full bridge, it makes more efficient use of the transformer's winding and core. But you have to use twice as many IGBTs, and not only that, twice as many diodes on the output side (full-wave rectification) to get this benefit. And those 250 amp Schottky diodes are really expensive.
Some accountant at the welder company will have added up the costs of copper, ferrite, silicon and so on, and decided that the two-switch forward converter was the overall best solution.
As a hobbyist working with salvaged parts, economics doesn't matter and you can build any topology you want.
And yes, there should be constant current control by PWM. Easy enough with a TL494 or whatever. Sometimes with a hobbyist design made out of junk, it'll work open loop, because the transformer will have enough leakage inductance to limit the welding current to a safe (or even useless :) ) value.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Electra wrote ...
I have a small hobby Tig welder that I have been trying to get round to repairing, no circuit’s for it though. But it uses a double switch forward converter, each switch is actually 4x 10n60’s. Driven by a L6386 from a Uc3845D. Looks like a micro controller, controls the pulse width so it isn’t easy to see how the control feedback circuit works. But would have thought it would have used some kind of constant current control. Don’t know the switching frequency, but the ferrite transformer isn’t that big, say 55mm square.
There’s a large tape wound ferrite inductor after the rectifiers (it’s a dc only welder) but no smoothing cap of course, also there’s some sort of hv pulsing in it for starting the Arc.
A lot of it including the output board is made with surface mount components, which I thought wouldn’t have been the best way to build a welder, but I guess it must have been cheap.
I would have thought the key thing when building a welder is to make sure the current is limited so it can work when the output is short circuited, without blowing up.
Thanks! Explanations of 'real' welders will definitely give me an idea of how i should go about this...
If you use a full bridge, it makes more efficient use of the transformer's winding and core. But you have to use twice as many IGBTs, and not only that, twice as many diodes on the output side (full-wave rectification) to get this benefit. And those 250 amp Schottky diodes are really expensive.
Well, do you think performance would be too awful with just normal fast Si diodes? 2v or so drop is not that bad, i won't be running 250 amps. Maybe 100-150. But them i don't know if i do in fact have any diodes made for that current, and i don't know what the actual drop would be until i looked at a datasheet for whatever diode... 2v at 100 amps is still sorta kinda 200w of dissipation though, so i can definitely see why shottky diodes would be best. I have a couple dual squirrel cage fans and plenty of heatsinkage, and efficiency doesn't matter to me.
And as for current limiting, provided the core does not limit enough, how would i go about that? Inductors after the diode? How would that work though if it's DC... I have 2 'things' that are each 2 inductors with a pretty small E core wound with about 3 turns of thick litz, and held together with a metal clamp. I assume it came from a welder, but what would it be for? Limiting currrent? Or smoothing?
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Steve McConner wrote ...
The ones I've seen the guts of have been two-switch forward converters. (What the coilgunners call "diagonal half-bridge".)
I'm considering doing a forward converter instead of a boost buck converter. Would a standard 1:2 60hz ferrite transformer work for the step-up work, or does the core have to be a ferrite that can handle the 35-50khz switching frequency?
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
You have to view the output inductor as part of the convertor, like the inductor in a buck convertor so it will store energy when the switches are closed and supply the load when the switches are open, it will have a d.c. component as well as a ripple current.
Current must be sensed somewhere like by using an ammeter shunt, to generate a feedback voltage for the PWM chip, like that used for the normal voltage mode control. Maybe it is also possible to do it by using a peak current mode ic, I’ve never built one so don’t really know. For the two switch forward convertor the duty cycle has to be limited to 50% I believe.
Keep in mind the different ground potentials when sensing the current on the secondary side.
I guess using regular hi speed diodes, would be cheaper for testing at least.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I was typing fast at work. What I meant to ask was would a standard silicon steel transformer meant for 60hz mains operation work for the step-up portion of a follower converter, or would one need to use a ferrite core for the high frequency switching action instead?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
IamSmooth wrote ...
I was typing fast at work. What I meant to ask was would a standard silicon steel transformer meant for 60hz mains operation work for the step-up portion of a follower converter
No. The core will be molten in seconds.
A boost converter is more efficient than a forward converter, but the output voltage won't go below the input and there's no isolation.
If you need output voltages both above and below the input, but don't need isolation, then you should check out the buck/boost or SEPIC.
wrote ... Keep in mind the different ground potentials when sensing the current on the secondary side.
This is a huge hassle. So much so that I would pay the 50 bucks for a Hall effect current transformer, to get an isolated measurement. I'd expect a good quality commercial welder to sense current this way, too.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Steve McConner wrote ...
This is a huge hassle. So much so that I would pay the 50 bucks for a Hall effect current transformer, to get an isolated measurement. I'd expect a good quality commercial welder to sense current this way, too.
That makes me wonder... Would it be best to use a hall sensor CT, or a linear shunt plus analog meter? I have both, but i did not even think about a current meter! Is there any real advantage of using a hall effect CT over a typical passive CT for over current limiting?
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Hi just found a simple circuit of a tig welder on another forum, this uses a half bridge, with current sensing on the primary side. so a regular current transformer.
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Practicality of making an inverter welder.
