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Old 11-30-2003, 02:54 PM
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Default Homemade DC welding rectifier working!

My homemade bridge rectifier to convert my AC stick welder into a DC source using SCR's is working. The SCR's were bought surplus from Princess Auto for $9.99 each. I used a 300 ohm resistor and a small diode to connect the gate of each SCR to the anode to convert them into simple diodes. I mounted them to 2.5" aluminum channel for a heat sink and have temporarily mounted it to a board for testing.

I ran a bead with some 3/32 6013 rod first to see if the diodes heated up. At 80 amps, no trace of heat. I turned it up to 100 amps and burned a full rod. Barely perceptable that it was warming up the heat sinks. I then switched to a 1/8 7014 rod and turned the welder up to 130 amps input. After burning the rod up, I checked the heat sinks. They were now warm, and the diodes were hot to the touch. That is probably the limits. I will have to use some heat sink compound on the base of the diodes to make sure they are conducting heat will to the aluminum channel. I ran the stick to the "-" side and the ground clamp to the positive output. I sure seemed effortless to start the arc and there was no tendancy of the rod to stick to the work on first striking the arc. I wondering if this is a result of the reverse polarity setup?

Next step is to experiment with using a lamp dimmer to control the input of the gate of an SCR to control the output. The lamp dimmer will chop up the ac phase to produce ac pulses at 60 hz, but of varying duration. I will run this thru a bridge rectifier to get dc pulses to input to the gate.

Ultimately the goal is to power an air cooled tig torch.

Anyone know the limits of material I can weld using 110 amps? I'm thinking that should be good to around 3/16 on mild steel or stainless.

Here's a picture of the bridge as tested this afternoon.

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Old 11-30-2003, 03:05 PM
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Default Re: Homemade DC welding rectifier working!

Looks like your getting it. [img]images/graemlins/40BEER.gif[/img] I'm interested to see how the thing as a whole ends up doing.
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Old 11-30-2003, 06:01 PM
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Default Re: Homemade DC welding rectifier working!

what is a scr?
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Old 11-30-2003, 06:23 PM
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Default Re: Homemade DC welding rectifier working!

It's a semiconductor, similar to a diode in that it only flows current in one direction. It is different in that it has a third connection, called the gate, which must have positive bias in order for the scr to function. So, essentially, it's like a solid state relay, but it only flows in one direction. You can use it as a very dependable electrical switch, that operates at high speeds. They use them on power control circuits. Sorry, I'm not an electrical engineer, just a backyard mechanic, so that's as good as my explanation gets.

High current diodes are expensive new. You can sometimes get them or scr cheap if they are surplus or pulled from service. I'm trying to have fun and scrounge together something usefull on the cheap.
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Old 12-01-2003, 07:04 AM
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Default Re: Homemade DC welding rectifier working!

With 110 amps you'll be pushing it to do 3/16"; a more realistic goal would be 1/8". Here's the chart out of a Miller handbook:


And just for good measure, the chart for Stainless:


You can push it some, but it gets slow; you have to slowly walk the puddle back and forth across the joint. The time and heat worsen the warping and distortion.
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Old 12-01-2003, 10:28 AM
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Default Re: Homemade DC welding rectifier working!

Hey Tim, what does a rectifier do? hey and O/T ,,i been way up there several times to Winnipeg back in late '90s.. stayed in that tallest building beside both rivers a close to the beer store. hey she was very nice lady her name was Mona.
ok very nice work dude!!!
DEX [img]images/graemlins/40BEER.gif[/img]
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Old 12-01-2003, 11:05 AM
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Default Re: Homemade DC welding rectifier working!

A rectifier is a device that converts alternating current (AC) into Direct Current (DC). For some welding applications, DC is necessary. DC welders tend to cost a lot more and are harder to find used. I already have an old AC unit, and am interested in trying tig welding steel, which requires DC.

Sounds like you had a good time in Winnipeg. Not quite sure if you met Mona in Winnipeg or brought her along. You must have stayed close to the Forks. Nice area to go for a walk by the river, do some shopping or have a drink / dinner.
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Old 12-01-2003, 11:18 AM
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Default Re: Homemade DC welding rectifier working!

ill show my dad your hook up. heck im still doing kinda good at stainless steal rods, self tought, at my factory when ever i can get over to shop
DM1954@aol.com stay in touch
i make glass ber bottles owens illinose [img]images/graemlins/40BEER.gif[/img]
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Old 12-01-2003, 03:12 PM
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Default Re: Homemade DC welding rectifier working!

Could you show a wiring diagram plzzzzzzzzzzzzzzzzz? [img]images/graemlins/smile.gif[/img]
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Old 12-01-2003, 05:10 PM
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Default Re: Homemade DC welding rectifier working!

I'd like to make a drawing, but I'm not sure how to do it. Are there any programs out there for drawing schematics easily? Hoping there is something shareware or at least free. I usually just draw it on paper. I did, but it look pretty ugly.
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