No Arc but voltage

[zxz1]

I was finding it harder to weld with my 100E Mk2. There was spluttering and the bead would build up in a nasty pile. Then it would not work at all. The feed works but no arc.

After reading a lot of posts I have checked the AC and DC voltages, 34 and 24 respectively. The DC is measured across the wire tip and the earth clamp. Voltage but no arc should mean a loose connector. I have checked but obviously missing something. Any ideas anyone?

 

[rtbcomp]

Where exactly did you measure the AC voltage?

 

[zxz1]

Where the two thick leads come out of the transformer and connect to the rectifier plates.

 

[rtbcomp]

It sounds like a rectifier diode has blown, normally the measured DC on open circuit should be greater than the RMS AC.

Remove the rectifier and test each diode in turn (disconnected at one end). Use the ohms range, they should be high resistance one way round and low resistance the other.

 

[mike 109444]

I was finding it harder to weld with my 100E Mk2. There was spluttering and the bead would build up in a nasty pile. Then it would not work at all. The feed works but no arc.

After reading a lot of posts I have checked the AC and DC voltages, 34 and 24 respectively. The DC is measured across the wire tip and the earth clamp. Voltage but no arc should mean a loose connector. I have checked but obviously missing something. Any ideas anyone?

If you are getting 24 vDC between the tip and the earth clamp then you SHOULD have some spark! even with limited current.
Try loading the wire back up and try to run a bead ON the earth clamp. Could be you just have a poor earth connection/clamp jaws (the serrations of the clamp can become very mucky and cause intermittent connection.

 

[zxz1]

That's interesting, the DC should be higher than the AC? How does that happen? On second thoughts, I probably won't understand the explanation!

Tested the diodes. Open circuit one way, reading about 730 the other way. Test done using the diode function on my test meter. One thing, the diodes are paired, that is they are in parallel. If one of each pair failed, would that make a difference to the meter reading? I really don't want to separate them from their connector ring to test them individually.

Yes I have already tried touching the wire to the clamp, nothing. It does not make sense.

About to re-assemble the rectifier and try to see if I get a spark again, just for the sake of it. I am stumped, what can I be missing? It feels like there is a voltage but no or negligible current, if you see what I mean, but yes, there should be at least a tiny spark but there is absolutely nothing.

I am going to test the voltage between the tip and the clamp whilst trying to get a spark, maybe the voltage drops or disappears when the tip is grounded.

 

[Welderfix UK]

Yep, agree with both comments. With an ac reading of 34V you should be seeing something in the order of 45Vdc so something does not stack up - need to check those rectifiers.

If you have a high resistance in series with the connection you would see the voltage but not be able to take any current so as Mike says, make sure connections are clean and try welding on the earth clamp.

 

[rtbcomp]

That's interesting, the DC should be higher than the AC? How does that happen?

Basically the AC voltage reading is an average, the actual AC peaks at voltage greater than the average, which is reflected in the DC reading after the rectifier.

If they are paired (ie you have 8 diodes) if one of a pair is shorted the pair itself will appear shorted. If one is open circuit it will seem OK from a diode test, but all the current will be diverted through the good diode, which will very quickly turn into a bad diode.

 

[zxz1]

Well it seems that I've fixed it. The earth clamp is a jump lead clamp the previous owner had fitted. This only worked on one side, not the side that was connected to the work piece or the side I was touching the tip to! Doh! It was only when I touched the end of the cable itself that I got a spark. I've now replaced the loose self tapper attaching the copper teeth to the clamp with a nut and bolt and scraped off the plastic on the other side of the clamp and replaced the loose rivet with a nut and bolt.

There is now a good spark and with very quick go without gas I seem to get a bead.

I do not know why it was not working before though. Maybe running a soldering iron over the PCB has fixed a dry joint.

Thanks for all your help, I am very impressed with the speed and quality of your replies