AC to DC welder conversion.




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  1. #1
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    Default AC to DC welder conversion.

    I have an old AC welder, and while I have a nifty new DC inverter welder, I don't want to just chuck the old 230v AC oil bath welder away.

    We also have crappy voltage on the farm: 360v 3Phase and around 200-210v AC 1Ph. Tshwane doesn't want to replace the pole transformer unless it's totally broken.

    So DC makes more sense so that I can get a decent arc from a rectified consistent voltage.

    I did some Youtube "research": apparently a nice single phase bridge-rectifier will convert the AC welder to a DC welder?

    I doubt if my old oil bath can push more than 50A: will a 72A single phase bridge rectifier do the job?

    http://za.rs-online.com/web/p/bridge...fiers/0192865/
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  2. #2
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    Default Re: AC to DC welder conversion.

    I'd love to hear what the experts say since I'm in the same boat. Have an old oil bath which has done very little work.
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    Default Re: AC to DC welder conversion.

    I had more of a look and it seems a 150A / 1600V bridge rectifier would be safer.

    The problem is that it seems nobody in ZA sells these things.

    What I'm planning on doing is running the + and - from the oil bath to the B-R, which I will mount either on the oil bath casing or on a wheeled trolley frame, using a wooden insulator and an aluminium plate heat-sink , and then to a plastic hobby box which will have female connections for the new style Dinse quick-connect system.

    That oil bath welder is bloody heavy: probably around 50kgs. So I'd have to put it on a wheeled trolley anyway.

    I have an old wheeled trolley that used to be a braai, I'd just need to put some angle-iron floor in it and then work out where to put the connector box.
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    Default Re: AC to DC welder conversion.

    You can add multiple bridge rectifiers in parallel to split the current.
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    Default Re: AC to DC welder conversion.

    Quote Originally Posted by Tracer87ZA View Post
    You can add multiple bridge rectifiers in parallel to split the current.
    beyond my electrical capability

    A 140A oil bath welder, and assuming then a 150A bridge rectifier is sufficient:

    2 x 75A bridge rectifiers? What voltage?
    3 x 50A? Again, what voltage?

    It will make the whole thing very untidy.
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  6. #6
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    Default Re: AC to DC welder conversion.

    OK Jelo you do this and tell me if it works!
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  7. #7
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    Default Re: AC to DC welder conversion.

    The rectifier is going to convert the AC cycles into rectified cycles at the output - The signal is not pure DC, at the positive terminal the output will look like a sine wave where the negative cycles are inverted (vice versa for negative terminal) . If the input is a varying AC, the rectified output is also going to vary. The rectifier is the first step in an AC to DC conversion. What is needed next is very large capacitors to "filter out" the AC components ...

    I am not familiar with what you guys want to achieve, but to my mind the rectifier on its own will not do the job ...

    Quote Originally Posted by jelo View Post
    I have an old AC welder, and while I have a nifty new DC inverter welder, I don't want to just chuck the old 230v AC oil bath welder away.

    We also have crappy voltage on the farm: 360v 3Phase and around 200-210v AC 1Ph. Tshwane doesn't want to replace the pole transformer unless it's totally broken.

    So DC makes more sense so that I can get a decent arc from a rectified consistent voltage.

    I did some Youtube "research": apparently a nice single phase bridge-rectifier will convert the AC welder to a DC welder?

    I doubt if my old oil bath can push more than 50A: will a 72A single phase bridge rectifier do the job?

    http://za.rs-online.com/web/p/bridge...fiers/0192865/

  8. #8
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    Default Re: AC to DC welder conversion.

    Quote Originally Posted by jelo View Post
    beyond my electrical capability

    A 140A oil bath welder, and assuming then a 150A bridge rectifier is sufficient:

    2 x 75A bridge rectifiers? What voltage?
    3 x 50A? Again, what voltage?

    It will make the whole thing very untidy.
    An ac welder is a stepdown transformer, thus you should have a voltage well below 100V. The rectifier might be able to handle 150Amp, but don't think the connections can. Cable should be at least 25mm square to cope with the current.
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  9. #9
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    Default Re: AC to DC welder conversion.

    Here is the waveform that I tried to explain:

    http://www.electronics-tutorials.ws/diode/diode_6.html


    Quote Originally Posted by Jaco Versfeld View Post
    The rectifier is going to convert the AC cycles into rectified cycles at the output - The signal is not pure DC, at the positive terminal the output will look like a sine wave where the negative cycles are inverted (vice versa for negative terminal) . If the input is a varying AC, the rectified output is also going to vary. The rectifier is the first step in an AC to DC conversion. What is needed next is very large capacitors to "filter out" the AC components ...

    I am not familiar with what you guys want to achieve, but to my mind the rectifier on its own will not do the job ...

  10. #10
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    Default Re: AC to DC welder conversion.

    Quote Originally Posted by Jaco Versfeld View Post
    The rectifier is going to convert the AC cycles into rectified cycles at the output - The signal is not pure DC, at the positive terminal the output will look like a sine wave where the negative cycles are inverted (vice versa for negative terminal) . If the input is a varying AC, the rectified output is also going to vary. The rectifier is the first step in an AC to DC conversion. What is needed next is very large capacitors to "filter out" the AC components ...

    I am not familiar with what you guys want to achieve, but to my mind the rectifier on its own will not do the job ...
    well the AC welder provides full current then zero then full current then zero then full current then zero...........

    Yes, I've seen you need capacitors if you want to smooth the flow, but I'd be happy with a sine wave that fluctuates between 70% and 100% : that's already better than the AC supply.
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  11. #11
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    Default Re: AC to DC welder conversion.

    I have done this with my AC welder. Yes it does spark a bit easier and I can weld thinner plate without burning holes as quickly but the overall effect was not what I expected. The biggest problem with the diodes that you use is not the forward current over them, Allthough you do need to keep this in mind. The major problem is the peak reverse voltage for the times that you tap the rod to the item. That spark can cause havoc up the line. You think that you are just sitting with 70V, as in my case, when you can easily have spikes going back into the rectifier of 500V. I will take a picture of my setup and list what diodes I used later this afternoon, The diodes came out of a UPS that we had at work that was thrown out.

  12. #12
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    Default Re: AC to DC welder conversion.


    Sorry forgot to post. I cant make out what the top diodes are as I have been spray painting in my garage and they got a bit of overspray on them

  13. #13
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    Default Re: AC to DC welder conversion.

    I would guess a person that has to buy all parts out of pocket would find it cheaper and.more economical to buy an inverter welder.
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  14. #14
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    Default Re: AC to DC welder conversion.

    Don't know if this will work but try and find an older forklift charger and use the diodes and heatsinks from it. Or keep the oil bath welder for gouging.
    Last edited by HobieDave; 2018/01/25 at 06:33 PM.

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