Testing diodes

[mike 109444]

Mr seadog think you will find that they are in groups of 4 acting as one large diode

 

[mike 109444]

Swedge reason I asked about the capacitor(s) was that I had a similar problem where after a bit of use one of my welders would drop in power/penetration. This I eventually found to be the terminal/riveted on top of the capacitor was getting so hot it was becoming loose.

 

[Seadog]

From what I can see it's a bridge. There are two two separate leads from the transformer suggesting two ends of a winding. Apologies if I'm wrong swedge. I'll have another squint at the pictures when I get in from the pub.

 

[Seadog]

Right, it's a strange setup. I could only see three sets of diodes in the pictures so went hunting for a diagram. Here's what I found -

It seems a strange way of doing things but it obviously is done for a reason. The negative has two arms to the bridge , each running through the choke, hence the two wires on the lower right of the main picture. The diode test is still valid but you need to remember that there is a separate pair of diodes in circuit. That, and the diodes seem to be connected in pairs too, since there are 12 diodes that I can see. If there is a fault you'll only be able to tie it down to a pair of diodes without dismantling.

 

[swedge]

If I can and find any faults at all that would be good enough for me to replace the rectifier

I don't want to replace it if it's not faulty

Thanks again for your help

 

[DTS]

I would check the ac at each switch setting and the dc out by triggering the m/c 1st, probably use a scope too. Be careful, it may well kill you if you don't know what you're testing.

This guy's done a video on fitting new, better diodes to the cheap welders.

http://www.instructables.com/id/Mig-Welder-upgrade-diode-replacement-in-bridge-rec/

 

[swedge]

Not done much this week full of the cold and a swollen hand due to an infected cut

Anyway electrician is gonna have a look at it on Monday for me but I've found out I can't get a replacement rectifier

I have seen universal ones but these are usually 2 or 4 plates

Mines has 2.5 lol, since there's a smaller one at the top

Also my welder is 180amps and when looking at universal ones they usually go 170-200

Which one would I fit to mine if I needed one and how would I sort out the plate count difference

Thanks

 

[swedge]

A company called technical arc ltd said they can make me one the same as that for £80

 

[NotANormalCoder]

For what it's worth, common problem on these machines is the connection just behind the earth lead outlet, inside the machine. There are three fairly heavy wires bolted together and usually covered in heat shrink tube. The bolt works loose over time and the lead terminals arc and burn. Cut off the heatshrink and check it's all tight. If not, strip and clean, replacing the bolt if it looks as if its got hot.
Because of the strange wiring used, if this part is bad, low power after a short while of welding is common.
Dave

 

[swedge]

Thanks I'll check that tomorrow

It's definitely getting worse, it would nearly hold together 2mm steel today

A fart would have broke the weld

If the connections are ok in the above post do I bite the bullet and get a replacement rectifier for £70 or do I send it off and get it checked which will cost a lot more than £70 as the closest is 50 mile away

What do you guys reckon

 

[swedge]

For what it's worth, common problem on these machines is the connection just behind the earth lead outlet, inside the machine. There are three fairly heavy wires bolted together and usually covered in heat shrink tube. The bolt works loose over time and the lead terminals arc and burn. Cut off the heatshrink and check it's all tight. If not, strip and clean, replacing the bolt if it looks as if its got hot.
Because of the strange wiring used, if this part is bad, low power after a short while of welding is common.
Dave

Thank you very much for this info

I originally checked the voltage using this nut as a earth point and everything seemed fine with it

I had a closer look today and the connection was very gritty and I could turn it slightly

I undone the bolt which actually snapped, the endsbof the wires weren't smooth and didn't look like they had been making a good connection, the smaller of the 2 wires had actually basicly snapped at the connection so I remade that

Filed the ends of the connections and the welder is now fully working

Thanks for all your help

Lesson learned

A connection that looks fine dosent mean it is fine lol