AC to DC, spot welding board

[SentinelAeon]

Hello, i salvaged a transformer from my old microwave, among other things. Since i got a spare spot welding board i thought i could power it with the transformer. The problem is that the board obviously requires DC, i was shooting for enough windings to get around 12V.

What would be the easiest way to convert AC to DC, preferably using the spare parts i got laying around from old power supplies.

 

[SentinelAeon]

The easiest way it seems would be a rectifier. But will it be able to handle high enough current's in the 100-200A, even if it's just for few ms ?

 

[Nigel Goodwin]

You already know the answer - however, if you have to ask such a question you shouldn't be playing with anything as dangerous as an MOT - it's a highly lethal device.

As for a rectifier, you need a LARGE one.

 

[SentinelAeon]

I agree, it's very dangerous Right now i have it hooked up as a spot welder for metal

 

[rjenkinsgb]

The rectifier would have to be built out of something like these:

1pcs 400A 100V Schottky Barrier Module Diode MBRP400100CT new #A6-14# | eBay

Find many great new & used options and get the best deals for 1pcs 400A 100V Schottky Barrier Module Diode MBRP400100CT new #A6-14# at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

Paralleling the two diodes in a module makes it capable of 400A (on a good heatsink).

Four such modules in a bridge could take 800A continuous, or rather more in short pulses.

It could be cheaper overall to buy some high current supercaps & use a normal DC supply?

(That's what my Chinese spot welder uses, two monster 3000F in series; input The power source is 12V from a 5A power supply via am internal regulator to charge it to 5.5V, though it does take quite some time to reach full charge for the first weld!)

 

[SentinelAeon]

Yeah, those aren't cheap.

I already have a little battery powered spot welder, its good for 0.12 and passable for 0.15. I really wanted something heavy duty for 0.2 that is instant use. I have 2 2.7V 500F caps, but this board has a flaw and its easy to burn mosfets when voltage drops. Thats why i thought it would be perfect to run it from 12V source.

Now, i could just make arduino controller and use that 12V AC, its easy to program. And for the spare board i guess i could just buy a car battery or some small lipo.

Smaller 20A diodes are dirt cheap though, 50pcs for few bucks. Could i just parallel a bunch of them and add some heatsink with a fan ? I don't expect i will need more than 200A to spot weld 0.2, and its miliseconds of current going through for each weld

 

[SentinelAeon]

What about something like this ? Pretty sure i can find similar priced one thats 200A ?

link

 

[danadak]

Lots of youtube videos rewinding these transformers for low voltage.

https://www.youtube.com/results?search_query=microwave+transformer+spot+welder

I would start out with a xfmr from a higher wattage microwave, not the small
shelf type, in order to get the currents needed. Maybe on youtube someone has
measured the current for their case.


Regards, Dana.

 

[ChrisP58]

Yeah, those aren't cheap.

I already have a little battery powered spot welder, its good for 0.12 and passable for 0.15. I really wanted something heavy duty for 0.2 that is instant use. I have 2 2.7V 500F caps, but this board has a flaw and its easy to burn mosfets when voltage drops. Thats why i thought it would be perfect to run it from 12V source.

The most likely cause of the behavior you describe is the mosfet gate voltage falling to a level to low to keep the mosfets fully on.

I would look at the board and see if you can separate the control voltage from the power voltage. running the mosfet gates at 10V or greater will likely solve that problem.

 

[SentinelAeon]

The problem at hand is converting AC to DC, everything else is taken care of

 

[SentinelAeon]

I have a cheap battery powered spot welder and its fine for up to 0.12-0.15. One thing i was wondering, its run by a small lipo. And to the best of my understanding, apart from pcb traces and thick wires, there is nothing limiting the current. It is almost like ur making a short everytime the spot welder does its thing.

So stricly talking about safety .. how safe is this ? To do the spot weld is max 50ms of short. So lipo will handle it safely ?

I am asking because i have a bunch of 20A 18650 liion batteries that i could use to power spot welding but i would never think to connect them to spot welder without some sort of resistor. I know wires and traces add some resistance but still its very little. And shorting my cells like this everytime i spot weld wouldn't feel safe.

What do u think ?