Testing diodes

[swedge]

Yesterday I was welding up some large axel stands for use in a 4 post lift

Welded up the bases on full power (7-180amps), welded fine then all of a sudden it's as if the power settings had been turned down but it obviously wasn't

It's done this when I first bought it but I thought it might have had something to do with using c02

I got myself a bottle from boc and it fixed itself (think it was coincidence)

I don't use it a lot but now it's doing it again

I ran a test bead today and while doing it it's as if the power lowered slightly mid weld, the start of the weld still wasn't what I'd expect and previously had at 180 amps from this welder

It seems be intermittent but right now it's faulty all the time

Is there anywhere I can send the rectifier for testing? As I don't fancy having to cut and solder the diodes in and out

Or do I bight the bullet and get a replacement rectifier, id just like to make sure it is that first

 

[Barking Mat]

Have you blown it through with a compressor?
Contact spray on all knobs?

 

[swedge]

Yeah I've gave it a blow out, unplugged and plugged in all connectors

 

[WorkshopChris]

Post 18 the blue disk (mov)is that just a dust spot right next to the aluminium rectifier plate?

 

[swedge]

You mean this one? I can't see post numbers on my phone lol

 

[WorkshopChris]

Yes thats the one , sometimes the post numbers show if you turn the phone through 90 but it all depends on software.

 

[swedge]

Does anyone know of anywhere I could send the rectifier for testing?

 

[mike 109444]

As the diodes are much the same as the ones used in some car alternators you MAY get an auto electrical place to test them BUT as they are all connected together they may not be able/willing to. You could try technical arc. Link http://technicalarc.co.uk/ One of the members of the forum works for them "Rosepud" but have not seen him post for a while. Might want to get a quote for the testing before splashing the cash.
Before you do send it off try dropping down one power level and see if the power on that setting also fads off. Reason being it could be a burned contact in the rotary switch that is failing to pass enough current under load. BUT DON'T RUSH TO TAKE THE SWITCH APART COS it will end in tears..... There are lots of people that have come onto forum saying "I have taken my switch apart and now can't get it back together working"

 

[mike 109444]

Oh forgot to ask does your machine have a very large capacitor or a bank of capacitors ??

 

[swedge]

Thank you it's has 1 large I think

 

[Seadog]

If you have a diode test option on our DMM then there should be no problem testing it yourself, now that it is disconnected.

Connect the positive lead to the - terminal. Then, in turn connect the negative to the A/B and B/A terminals. You should get a reading of 0.7 of maybe a little larger. Now connect the negative lead to the - terminal and perform the same test. There should be no reading or OL displayed, depending on your DMM. Now connect the negative lead to the + terminal on the rectifier and test the A/B and B/A. Once again should get a low reading of around 0.7 and OL when you reverse the meter leads. You can pretty much assume that the rectifier is OK if all checks out. If any read short circuit in both directions, or open circuit in both directions, then that diode is bad.

 

[swedge]

Sorry to sound stupid but could you point out where each terminal is

 

[WorkshopChris]

Sorry to sound stupid but could you point out where each terminal is

Your AC is coming in from the brown colored transformer on 2 wires and looks like it comes out as DC on the 2 wires coming down towards the yellow coil.
From what I can make out in the picture.

 

[swedge]

If you have a diode test option on our DMM then there should be no problem testing it yourself, now that it is disconnected.

View attachment 114631 Connect the positive lead to the - terminal. Then, in turn connect the negative to the A/B and B/A terminals. You should get a reading of 0.7 of maybe a little larger. Now connect the negative lead to the - terminal and perform the same test. There should be no reading or OL displayed, depending on your DMM. Now connect the negative lead to the + terminal on the rectifier and test the A/B and B/A. Once again should get a low reading of around 0.7 and OL when you reverse the meter leads. You can pretty much assume that the rectifier is OK if all checks out. If any read short circuit in both directions, or open circuit in both directions, then that diode is bad.

When you say a/b and b/a terminals you mean the diodes?

 

[Seadog]

The front two terminals are the A/B and B/A in my diagram. The other two terminals are positive and negative. It doesn't matter which is which, you just need to interpret the readings from your meter using what I have written. You do need to disconnect all of the wires from the rectifier before you test it otherwise you will get false readings.

 

[swedge]

So after I remove the cables do I just use the point where the cables were attached?

 

[Seadog]

That's correct.

 

[swedge]

Perfect so it kind of tests all diodes on that circuit at once? Am I right? Lol

 

[Seadog]

You're testing each one individually if you follow the sequence I outlined.

 

[swedge]

Right I think I get it now, I shall report back tomorrow evening

Thank you