Welder as a power supply??

[Stevie]

Hi Guys

I've been busy buying up Copley servo amps; I now have 3

They need to be fed 18-55Vdc at 5amp with a 10amp max (peak draw)

I have an old welder that seems to be giving 38.9Vac the transformer is massive and can give 75amps max
If I was to get a good sized bridge rectifier and some big a**ed smoothing caps do you think it could be used??

opinions pleaseeee

[cbcnc]

At 39v ac you would get about 55v dc. That's 1.4 * Vac = Dc. There is no room there for any back EMF and might go over the max for your controllers.

Chris

[Hobbiest]

Sorry for the hijack question here Stevie. What about the transformer from an old microwave? They have big old beefy ones. Anybody know about them?

[vacpress]

I guess i would open the welder and see what is in it.. its probably something that you could modift to be more appropriate. i would guess most old welders would be fairly simple...

[Splint]

I bought a portable mig welder from a guy who repares welders. The wire feed, leads and gun had been removed. All I had to do was buy some wire and put 7 more turns on the secondry winding and hook up the 3 capacitors he threw in an it's ready to use. This worked out to be very easy and cheap (A$50 for the welder and caps), I've not yet used it but it should be ok. Being built the way it is the welder will be able to supply what ever volt/amp requirements I'll need even if I upgrade to bigger servos, it's just a case of adjusting the number of turns on the secondry and possibly adding more caps.
Splint

[Stevie]

As I need a peak of 2500 watts; the microwave trans is out; but it is a good idea; I have an old one too

Thanks for the replies guys; sounds like it might just work; i moved the slider in and the volts cut to 32; so i could keep it under 55Vdc; but I thnk the amps cut too; but I'm sure it was still over 40Amps

[Al_The_Man]

One thing to keep in mind with welding transformers is the stick type arc welder has a loosley coupled secondary to primary winding, especially when the coupling bar is all the way out. this allows the voltage to collapse on high currents, as with a stuck rod etc. In other words it is not constant-current constant-voltage as is the case with a mig welder transformer. If your load current is well below the max current the welder is rated at you may be OK.
Al

[Stevie]

Thanks Al; as the total draw will be 30 absolute max; and the welder is 30-75 I can keep it low

[Stevie]

Ok the dc will be 52.5 volts; this is 37.5Vac X 1.4
The amps are anywhere from 20-75 by the info on the welder case
The volts only max out when the rod is amost all the way out; they stay within 52.5 most of the way up
I looked today in Radio Shack and the biggest bridge was a 50V at 25Amps
Ill order one with some caps online

What would be a good Value???

[sendkeys]

[Stevie]

Thanks Keys

The amps can run on 18-55 Vdc so if it drops a touch in the bridge it will not be a problem

Thanks for the links