Welder not turning on - Learning

[Olles4]

oh shit but i have 2.6V on curent sense pin3

[CapLeaker]

Hmm… that’s not good.

[Olles4]

Hmm… that's not good.

i used 1001 (1KOHM) Resistor for R209, should i try higher resistance ?

BTW im measuring on Gate or Surce of the mosfet only 2.4V

[CapLeaker]

Looking at the circuits I got here, they all used 1k Ohm. You should have more voltage than that on the mosfet. So there is still something wrong somewhere.

[Olles4]

Looking at the circuits I got here, they all used 1k Ohm. You should have more voltage than that on the mosfet. So there is still something wrong somewhere.

okay but how should i deal with the over Voltage on Isense ?

[Olles4]

im using a K9J90E mosfet the old one was K3878
C201 is 103 2KV (10nF)
D201 new UF4007-E3/54 old BYV26E
R201 98.8kOhm

R205 and R206 should be 1Ohm am i right ?

[Olles4]

I found something R205 and R206 are open

C204 15nF is a good value?

[CapLeaker]

I found something R205 and R206 are open

C204 15nF is a good value?

I can’t tell what R205 and R206 are in the schematic, they aren’t ther. Don’t change these capacitors, specifically capacitance values.

[Olles4]

R205 and R206
oh I see, it’s R1 and R2 on the schematic.
by colour code it should be 1ohm +/-5% 2W(by size)
What do you think ?

The problem is i took down cap204 to measure it and unfortunately I broke it.. but on board I measured and it gave mi something between 15-18nF

[CapLeaker]

These two resistors would have to be very low ohm readings. I read brown, black, silver and red = 0.1 ohm, 2% to me and are 5w carbon film resistors.
i think c204 is the same as c208.

[Olles4]

These two resistors would have to be very low ohm readings. I read brown, black, silver and red = 0.1 ohm, 2% to me and are 5w carbon film resistors.

its brown, black, gold, red it hard to see on the photo

[CapLeaker]

If the third band is gold, it 1 ohm. To me it looked like silver to me. But 1 ohm is plausible too and much easier to get.

[Olles4]

Okay tomorrow I will change it and it will run I’m pretty sure!
Just one thing is bothering me, c204 is a for current waveform spike suppression if I’m right, what is changing if you changes the capacitor value ?

[Olles4]

These two resistors would have to be very low ohm readings. I read brown, black, silver and red = 0.1 ohm, 2% to me and are 5w carbon film resistors.
i think c204 is the same as c208.

Why do you think it's same?
I mean I totally trusts you 😄 just want to know

[CapLeaker]

The size of the SMD cap is the same. Knowing that and part of the circuit, you can figure out a “range” on what voltage / capacitance the SMD capacitor was. Just download a SMD cap spec sheet from a good manufacturer and you will notice that besides the length and width, the height will change due to rating. So check the thickness of the bad SMD cap with the other one. On the pic they look the same.
Dont trust me too much, as I only can see whatever pictures you provide or info you give. Changing these 1 Ohm resistors for 0.1 Ohm won’t make a difference.

C204 is a part of a “rc filter” that suppresses transients (spikes). It is a small value cap something like 0.0001uF (that is 1000pF). Could be even a bit lower at 100pf…
Like I said, this cap function is only there to suppress “transients or noise” on the sense pin. Does it have to be there? Yes, but at the same time it isn’t extremely critical either.

[sam_sam_sam]

The size of the SMD cap is the same. Knowing that and part of the circuit, you can figure out a “range” on what voltage / capacitance the SMD capacitor was. Just download a SMD cap spec sheet from a good manufacturer and you will notice that besides the length and width, the height will change due to rating. So check the thickness of the bad SMD cap with the other one. On the pic they look the same

This information is good to know about SMD capacitors and how to figure there value

[Olles4]

The size of the SMD cap is the same. Knowing that and part of the circuit, you can figure out a “range” on what voltage / capacitance the SMD capacitor was. Just download a SMD cap spec sheet from a good manufacturer and you will notice that besides the length and width, the height will change due to rating. So check the thickness of the bad SMD cap with the other one. On the pic they look the same.
Dont trust me too much, as I only can see whatever pictures you provide or info you give. Changing these 1 Ohm resistors for 0.1 Ohm won’t make a difference.

C204 is a part of a “rc filter” that suppresses transients (spikes). It is a small value cap something like 0.0001uF (that is 1000pF). Could be even a bit lower at 100pf…
Like I said, this cap function is only there to suppress “transients or noise” on the sense pin. Does it have to be there? Yes, but at the same time it isn’t extremely critical either.

Thank you thats a valuable lesson!
I measured C208 and it’s 9.42nF so my guess is 10nF for both C208 and C204

[Olles4]

Update:
I bought 1Ohm 1W +/-5% resistors and its not working it looks like the resistance is too big, if i short the resistor to GND screw-hole it runs. Any thoughts?

Btw from my perspective C204 looks like C205 by the size. C208 is way thinner.
C205 is 97nF (measuring de-soldered) what do you think ?

[CapLeaker]

The resistors are fine. Not sure right now what to think on why it isn't triggering as it supposed to. I do know that there are a few caps where we are guessing, since the size gives usually a choice of 3 or 4 capacitance values assuming they are 50V rated. Close, but yet so far…

Sometimes on these Chinese welders and plasma cutters there is a part of one circuit on one board and the other part on a different board. That happened to me on a plasma cutter I repaired a while back.

[Olles4]

Alright i used 10nF capacitor and bought new resistors and it's running.

What kind of thermal paste should I use between components and heatsinks ?