Lincoln Electric LE31MP welder not turning on

[tibimakai]

Re: Lincoln Electric LE31MP welder not turning on

Good idea, thanks.

[tibimakai]

Re: Lincoln Electric LE31MP welder not turning on

I did not find anything that way either.
It's kind of scarry to work on this, with all those 3 fans spinning at max rpm.
That 27k big resistor's value start increasing quite fast, so I can't keep it on only for a few seconds at a time.
If I'm looking at a UC3842 diagram, pin #7 is the Vcc, which is 14.5V for me and the output #6 shows 5.5V.
At which pin can I check if it's oscillating at the right frequency? At pin #4 I'm getting 55.99Hz and it seems like it is on and off.

[petehall347]

Re: Lincoln Electric LE31MP welder not turning on

Depending on the output power, switched mode power supplies (SMPS) operate in the frequency range from about 10 kHz up to the MHz range.
maybe a shorted diode letting line frequency through

[tibimakai]

Re: Lincoln Electric LE31MP welder not turning on

It may very well be, what is happening here, but which one?

[petehall347]

Re: Lincoln Electric LE31MP welder not turning on

what chip is it ? need the datasheet .

[tibimakai]

Re: Lincoln Electric LE31MP welder not turning on

Uc3842

[petehall347]

Re: Lincoln Electric LE31MP welder not turning on

https://www.ti.com/product/UC3842
plenty to read through .
i skip read it and it appears frequency is set with a resistor and capacitor so that needs looking at .

[tibimakai]

Re: Lincoln Electric LE31MP welder not turning on

Sorry about not getting back sooner.
This IC needs 16V to start oscillating, and I'm getting only 15.5V.
Most likely that is the issue.
The question is, which component causing this drop.
There are 3x56K resistors in series after the diode bridge and from there it is already 15.5V.
I'm attaching a generic schematic of the UC3842 IC.
Before pin #7 there is a 56K resistor. On my welder I have 3x56K resistors.

[sam_sam_sam]

Re: Lincoln Electric LE31MP welder not turning on

“I'm attaching a generic schematic of the UC3842 IC.”

If you can find the right diagram for the welder that you have would help

What else does not match the welder circuit that you

There components that generate the low voltage to start the circuit I would agree with you that something is keeping it from getting the high voltage to start

Take a picture of the board in question so we can see what type of circuit configuration that you have

[tibimakai]

Re: Lincoln Electric LE31MP welder not turning on

Unfortunately I can't find any schematic for this welder.
I have posted pictures already.