The last one I bought, a 150 Amp unit, was from Sealey's spare parts service.
Had Chinese writing all over it.
Was about £25 posted to my local distributor.
- Thread starter Mike_H
- Start date
Had Chinese writing all over it.
Was about £25 posted to my local distributor.
The numeric value shown in Diode Test mode is the voltage dropped across the diode when it is conducting ( the forward voltage ) in millivolts. Typically 0.4v to 0.7v - 400 to 700 millivolts.
When these press-fit diodes go short-circuit, the lead to them - the pigtail - often breaks off or melts, so in fact that bad diode showing open-circuit may actually be shorted.
So the new rectifier arrived today. I’m not an electrician, and I’m probably about to make that blindingly obvious, but here goes anyway
the new part is marked up on the terminal, which seems helpful, but only if it’s correct.
Remembering school physics class, over 35 years ago, I think that DC current flows from positive to negative, but if I test the diodes in the new rectifier, it seems to be flowing the other way. If I put the positive probe on the negative side of the diode, only then do I get a flow. Same on the old rectifier, so my logic is wrong somewhere. I’m very happy to be educated about why I‘m wrong.
if I am wrong, then the old and new part have the connections the other way round - negative is the top plate on the new one, but the bottom plate on the old one, based on direction of bolt fitting, but that isn’t a big issue… I think . The bolt seems insulated from the plates, so it’s not providing a path to earth.
other than that, I’m thinking it’s black earth lead to negative, red to positive, and yellow to AC. Does that make sense?
the new part is marked up on the terminal, which seems helpful, but only if it’s correct.
Remembering school physics class, over 35 years ago, I think that DC current flows from positive to negative, but if I test the diodes in the new rectifier, it seems to be flowing the other way. If I put the positive probe on the negative side of the diode, only then do I get a flow. Same on the old rectifier, so my logic is wrong somewhere. I’m very happy to be educated about why I‘m wrong.
if I am wrong, then the old and new part have the connections the other way round - negative is the top plate on the new one, but the bottom plate on the old one, based on direction of bolt fitting, but that isn’t a big issue… I think . The bolt seems insulated from the plates, so it’s not providing a path to earth.
other than that, I’m thinking it’s black earth lead to negative, red to positive, and yellow to AC. Does that make sense?
I think that I have seen those hand-written labels before, on replacement rectifiers delivered by Tec-Arc. If this is one of theirs, then I am sure the labelling is correct - they do know what they are doing!
You can confirm the labelling by setting your meter to the Diode Test range, then putting the red meter lead on one of the "ears" marked AC. Then put the black meter lead on "+". The display should indicate conduction - i.e. a forward voltage reading of 400 to 700.Then move the red meter lead to the other "AC" tag, and still have the black meter lead on the same "+" tag. The meter should again show forward conduction, 400 to 700. These two tests confirm that whichever of the two AC tags has a positive-going waveform, the output current will be on the "+" tag.
Yes, you are correct: the two yellow AC output wires from the secondary of the transformer go to the "AC" tags, and the black heavy cable goes to the "Neg" tag, then through the front panel as the work return cable & clamp.
According to the attached circuit diagram ( Cebora Article 838 - rebadged as Snap-on ), this MIG is one of those that have the inductor ( choke ) in the positive circuit - it can go in either the pos or the neg lead. So the red-sleeved solid wire from the inductor goes to the "Pos" tag. The other side of the inductor winding goes to the MIG torch. ( Torch positive for solid wire with gas.)
The two mounting studs are isolated from all the rectifier connection tags and the cooling fins by the shouldered washers. This enables them to be safely bolted to the sheet metal case of the welder. Both sides of the DC welding output, positive and negative, are fully-floating from the case, and from AC ground. The so-called "Earth" cable is the negative return from the workpiece; it is not an AC earth. Other than the two bolts, no part of the rectifier should make contact with the case.
You can confirm the labelling by setting your meter to the Diode Test range, then putting the red meter lead on one of the "ears" marked AC. Then put the black meter lead on "+". The display should indicate conduction - i.e. a forward voltage reading of 400 to 700.Then move the red meter lead to the other "AC" tag, and still have the black meter lead on the same "+" tag. The meter should again show forward conduction, 400 to 700. These two tests confirm that whichever of the two AC tags has a positive-going waveform, the output current will be on the "+" tag.
Yes, you are correct: the two yellow AC output wires from the secondary of the transformer go to the "AC" tags, and the black heavy cable goes to the "Neg" tag, then through the front panel as the work return cable & clamp.
According to the attached circuit diagram ( Cebora Article 838 - rebadged as Snap-on ), this MIG is one of those that have the inductor ( choke ) in the positive circuit - it can go in either the pos or the neg lead. So the red-sleeved solid wire from the inductor goes to the "Pos" tag. The other side of the inductor winding goes to the MIG torch. ( Torch positive for solid wire with gas.)
The two mounting studs are isolated from all the rectifier connection tags and the cooling fins by the shouldered washers. This enables them to be safely bolted to the sheet metal case of the welder. Both sides of the DC welding output, positive and negative, are fully-floating from the case, and from AC ground. The so-called "Earth" cable is the negative return from the workpiece; it is not an AC earth. Other than the two bolts, no part of the rectifier should make contact with the case.
Thanks for the full and patient explanation.
Yes it’s a Tec-Arc rectifier. Now fitted and seems to be working, although I haven’t loaded any wire into it as yet. I convinced myself it was right by testing the old part and seeing the same diode test results.
Yes it’s a Tec-Arc rectifier. Now fitted and seems to be working, although I haven’t loaded any wire into it as yet. I convinced myself it was right by testing the old part and seeing the same diode test results.
