Dent puller spot welder.

[Tiksom]

Afternoon all

I have a little eBay dent puller spot welder. I blew a varistor on the board and the company did sent me another spare board.
Problem is that the moment i connect the transformer to the board it trips the breakers.

I measured the primary winding on the coil and it measures 0.5ohm.... Isn`t that too low? Any body may have a clew what the resistance should be?
There is no short btw the primary and secondary widings.
Thanks for any help.

 

[MoreWellie]

I assume you have the weld tips separated

When you say primary I presume you mean the input side, the output side will have a low resistance as it is outputting a high current

Is the bench conductive that the board is sitting?

Are you connecting the transfer to the board whilst the unit is live? Otherwise it seems unlikely to trip the instant you connect it. You should always have the mains off (disconnected) when connecting internal wiring in any unit

I take it it is a breaker that is tripping and not an RCD which would trip if you inadvertantly connected neutral and earth

 

[500e]

You have ALL the insulating pads on the transformer ? you dont want any of the hold down washers touching the toroidial should be rubber between washer top and bottom or it can see the hold down as a coil that is short circuit
Sounds faulty, disconnect output if it still does it it is faulty
Would have thought a resistance ch should be 10.5 to 14 Ohms

 

[Tiksom]

It’s all properly isolated. I had the transformer on the bench with the same result. Can rewind the transformer but cannot see any obvious short on the primary winding. The secondary winding is also isolated from the primary with open contact all around.

 

[500e]

Transformer on non metal bench and no bolt through it connected direct to mains with no other connections?
If it is 0.5Ohms disconnected from board there is something wrong
Looks a tidy tool

 

[eddie49]

I measured the primary winding on the coil and it measures 0.5ohm.... Isn`t that too low? Any body may have a clew what the resistance should be?

Depending on the output power of this spot welder, the measured primary resistance value of 0.5 Ohm is about right. Typical values for a 140 Amp MIG welder at the highest power setting are 0.7 Ohm - see Post #14 in this thread:

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Does the timer control board run OK when powered, but when the transformer primary is also connected, the mains breaker trips, even without attempting to start a spot-welding sequence? If so, this implies that the control logic on the timer board has failed. It should not drive the transformer primary until you press a control button, and there should probably be a soft-start circuit, or at least power should not be applied until the zero-crossing-point of the AC waveform. It sounds like a TRIAC, or it's control circuit, has failed.

 

[Tiksom]

Depending on the output power of this spot welder, the measured primary resistance value of 0.5 Ohm is about right. Typical values for a 140 Amp MIG welder at the highest power setting are 0.7 Ohm - see Post #14 in this thread:

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www.mig-welding.co.uk

Does the timer control board run OK when powered, but when the transformer primary is also connected, the mains breaker trips, even without attempting to start a spot-welding sequence? If so, this implies that the control logic on the timer board has failed. It should not drive the transformer primary until you press a control button, and there should probably be a soft-start circuit, or at least power should not be applied until the zero-crossing-point of the AC waveform. It sounds like a TRIAC, or it's control circuit, has failed.

Yeah, without the transformer connected to the board everything is ok. The seller gave me brand new board as well, the first one blew the variation on the output. Same things happening when I remove the transformer from the case and put it on a rubber lined bench. So not sure what is going on. Next thing will be c rated breaker I think.

 

[eddie49]

Yes, but:

when the transformer primary is also connected, the mains breaker trips, even without attempting to start a spot-welding sequence? If so, this implies that the control logic on the timer board has failed. It should not drive the transformer primary until you press a control button

With the machine internals fully wired up, does the breaker trip as soon as you connect the machine to the mains and switch it on, without doing whatever process would normally set off a spot weld sequence ( e.g. pushing a button ) ?

 

[Tiksom]

No,it all powers up ok, you can go through the menu settings etc. it trips only when you pull the trigger to actually do the weld.

 

[Tiksom]

There are 2 SCRs on the output and varistor. Both, the old that I repaired, and the new behave the same way.

 

[eddie49]

That sounds like you are right, the mains breaker is not up to the job of handling the initial surge.

 

[Tiksom]

I did tried it again, and it tries to do the weld but pops second after. Tried on B20 and B32 breaker. I’ll try the C type tomorrow and let you know the outcome.

Thanks for now

 

[eddie49]

Although the resistance values quoted above for welder transformer primaries - 0.7, 0.5, even 0.14 Ohms - may seem low, they are the DC resistance. Due to the inductive reactance of the coil, the impedance to AC at 50Hz will be higher.

 

[Tiksom]

Hi All. I can confirm that 32amp C curve MCB solved the issue. There must be poor inrush current managment on this thing.
Thanks for all your help.