Diode current rating - HF Dual MIG 151

[serious_lee]

I have a feeling one of my diodes in the bridge has gone south in my HF Dual MIG 151. Does anyone know what the rating is/should be? It's a 120 Amp machine, so I was guessing that 60 Amp diodes would be sufficient operating in full bridge configuration. I still need to verify the part number if I can actually read it, but it would be nice to hear from experienced guys on this.

Thanks,
Everett

[Bluewelders]

60amps are about as big as the common press in diodes go.
If you can get a set of 60 amp ones of the right polarity and plenty of volts, it should work.
You might check with harbor freight, on the cost for a new bridge unit.
Their replacement parts are usually inexpensive.

[timrb]

Presuming the 120A comes from the bridge output, then each diode in the bridge must be rated for the full operating current, not 1/2 of it. In a bridge one pair of diodes conducts the full current on 1/2 wave, and then the other pair conducts on the other half. If your 120A is RMS then the peak current conducted by any of the diodes will be about 170A.

Tim

[papaof2]

If there's space in the box, you might consider a 200 amp bridge from Ebay. This one is about $60 delivered:
http://cgi.ebay.com/ws/eBayISAPI.dll...STRK:MEWAX:VRI

John

[serious_lee]

Thanks for the feedback! I was under the impression that these ratings are continuous current ratings therefore at 50% duty cycle they would form a bridge rectifier with twice the continuous rating of the single diode. Bigger is probably better anyway if I could fit it, but that rectifier on eBay is about half the price of the welder! HF sells the replacement unit for 9 bucks but 6-8 weeks is the estimated delivery time!!

[Bluewelders]

Your rectifier is just one of the eight diode ones isn't it ?
One plate with four reds and a plate with 4 blacks ?
These would probably work, but A full set of diodes is probably a lot more than $9.
http://www.diotec-usa.com/B75PF.PDF

[serious_lee]

I ended up ordering the 200A one John mentioned above since the seller accepted a best offer from me and I had a $10 off coupon for eBay. I will post pics in the Harbor Freight "How-To-Mod" thread. Hopefully I can get it to fit.

[Bluewelders]

When I did the original mods to my welder, I used thermistors to test positions for the rectifier I made.
It had to fit with the new capacitor and still be cool.
I made sure the new fan supplied ample cooling to the transformer and the rectifier bridge.
That is why it is positioned at an angle and is the size that it is.
http://weldingweb.com/attachment.php...1&d=1241530315
So as long as you can make it fit in,and stay cool, it should work.