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High frequency circuit board for TIG AC/DC welder, how does it work?

newcomer9747

Sep 11, 2022
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Sep 11, 2022
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So I disassembled the faulty (I think) HF unit from my Thermal Arc 300GTSW (Single & 3 phase selectable) tig welder and now trying to figure out how the thing works. I have attached pictures of the circuit board and a crappy hand drawn circuit diagram. Arc is weak and wouldn't ignite on AC mode, DC welding is fine although the arc won't transfer at times. I would appreciate if anyone is kind enough to explain the workings of this board. There is no replacement part so my only option is to repair it and to do that, I have to first understand how it works. All I know is this board is supposed to generate high frequency high voltage as an ignition for the torch arc transfer.

Also need help identifying that 151J part. What's it called, what it does and where can I find the datasheet for it? And why is diode D6 so long? It only has the number 79 on it, which I can't find on the internet. I assume T1 is the low to high voltage transformer? Why does it only have 3 terminals and why is it so small? All other info I searched on the internet on TIG HF starter circuit showed them to be pretty big and hefty like they weigh a pound or two. Is SSS a regular diode? It does appear to behave like a diode on my multimeter. Why is it bigger than the rest?

On a site note, is there a reason these units are fully potted in silicone rubber? Is it necessary for its operation or is it just an attempt by the manufacturer to obfuscate things?

Link to service manual can be found here:
 

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Last edited:

danadak

Feb 19, 2021
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Feb 19, 2021
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That power resistor seems to have a cracked solder joint on its right leg ?
Just below its lead....

1662893817163.png

I dont think the manual has a schematic of that particular board, unfortunately.

You might contact them to see if you can get that.

I think that transformer/inductor is part of a trigger circuit, hence small as it would
only be a low power trigger being developed. D6 I think is a HV diode that takes
the HV off the inductor and suppresses inductive kick in one direction, just a guess.....
It won't read as a normal diode on a multimeter, its Vforward much higher than a
regular diode.

Regards, Dana.
 
Last edited:

Bluejets

Oct 5, 2014
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Oct 5, 2014
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Couple of things....... your link shows a service manual with 130 pages in it so not intending to scan through all that BUT have you searched in there at all..??

151J part looks like a wire wound resistor (probably 150R value)

Transformers in high frequency ARE smaller, but I'm pretty certain it is just part of the trigger circuit for the adjacent thyristor.
Power transformer is likely elsewhere(not shown).

Diode I can't say but others may know, more than likely again because of the high frequency.

Potting, whatever the medium can have dual purpose but it usually is board protection.

Did you do any checks in the main power section?
I suspect it might be more obvious there.
 

newcomer9747

Sep 11, 2022
3
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Sep 11, 2022
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That's not a crack, just some scratch marks from the depotting. I did manage to ripped off one of the leads while removing the connectors...

@Bluejets. Yes I've gone through all 130+ pages of the manual, even performing almost all the tests, IGBTs, mains, output diodes, choke filter, output inductor, etc. Only thing wrong is I couldn't hear the HF unit when I jumper it (maybe due to the potting?), as required by the manual which states :
6.14 High Frequency Test
1. Place the Process Selector switch in the stick position.
2. Jumper pins 1 and 5 on the CN7 plug. Apply power to the unit.
3. If the high frequency buzz is heard coming from the HF Unit, the Main Circuit Board (PCB1) should be replaced. If no sound is heard coming from the HF Unit, the HF Unit should be replaced.


Although I should mention I clamped the torch lead and measured 260Khz, which is about what the HF unit should be operating at I think? If so, it is far above the audible spectrum, which got me suspicious if the test is even valid. Is the HF unit suppose to buzz out loud when it's on?
 

Bluejets

Oct 5, 2014
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What HF are you referring to, the weld frequency or...... the start HF.
The former shouldn't be anywhere near that high as far as I am aware and the start HF could be MHz.
 

newcomer9747

Sep 11, 2022
3
Joined
Sep 11, 2022
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The start HF. If I put the cup close to my ear I can hear a very low amplitude high pitch hum when I press the trigger. Still it wouldn't go full arc on AC, only weak sparks. It gets weaker the nearer the tip is to the workpiece.
 

kellys_eye

Jun 25, 2010
5,134
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Jun 25, 2010
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5,134
If it's arcing it's working. What's potentially wrong is the current monitoring side of the system. I'd suspect a diode in the current detection side (AC) that's suppose to rectify the monitoring current.
 
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