Chinese zx7 250 increased ocv

[Paul Hewer]

Just posting this as I thought it was interesting. I'm not recommending the machine itself or what I have done to it but I thought it was something worth sharing. If only so someone can tell me why what I did had the effect it has had.

So it's one of those 35 quid mini chinese welders. More of a toy and a conversation piece than a welder. However I had a few broken welders lying around and thought I'd swap over the cheap no name capacitor in the unit I had for a better quality one out of my parts bin.. So the one in it was 670uf 400v and I noticed there was space on the board for 2 smaller caps instead of one. So I added 2 decent 470uf 400v in parallel instead of the one 670uf. Put back together and all works fine. Infact it seemed to start rods much better. So out of interest I checked the ocv and was a bit surprised it was now 88v whereas before the change it was 60v. So I've ran 6 or 7 rods on it and it hasn't exploded (at least not yet) but it's amazingly better to weld with than it was before. Yes it's still only 110amp or so despite being labeled 250amp, but now it's much more useful and easier to use.it'll probably die prematurely but I'm not that bothered if it does.

 

[Richiew]

Crack out the 6010s!

 

[R Kraft]

The original Jasic ZX7200, are what all of the small welders are patterned after.
The Jasic would put out an easy 200A.
It had 4-470uf capacitors.
The newer Chinese copies have far smaller capacitors, that are usually mislabeled as large capacitors.
The last one I checked would barely put out 110A.

 

[Paul Hewer]

I haven't got a clamp meter so can't check if the new caps have had an effect on the amperage as well as the ocv.

 

[Tom O]

i had to replace the capacitor bank in my Millermatic 135 I forget what the new bank of six was but it was the next size up they said it would make it weld smoother.

 

[eddie49]

The capacitor(s) is/are after the bridge rectifier, which changes the incoming 230v AC mains to DC. Any value of capacitance should raise the DC voltage to the peak of the AC sine wave, which is 230 x 1.414 = 325v DC.
I guess that once the IGBTs/FETs start humming away at 20 or 50 kHz and you begin to pull 100A out of the secondary of the inverter's ferrite transformer, the supply voltage available across the smaller capacitor values will drop away. Seems that the current feedback circuit cannot make up for this by driving the transistors harder.....

 

[Paul Hewer]

The capacitor(s) is/are after the bridge rectifier, which changes the incoming 230v AC mains to DC. Any value of capacitance should raise the DC voltage to the peak of the AC sine wave, which is 230 x 1.414 = 325v DC.
I guess that once the IGBTs/FETs start humming away at 20 or 50 kHz and you begin to pull 100A out of the secondary of the inverter's ferrite transformer, the supply voltage available across the smaller capacitor values will drop away. Seems that the current feedback circuit cannot make up for this by driving the transistors harder.....

So on this basis is it likely to blow up? :-)

 

[Paul Hewer]

I suppose what I should say is. Is it likely to blow up any quicker than it would of normally?

 

[R Kraft]

Kind of a mixed bag.
The circuit is being driven harder but has less power to work with.

I would guess the biggest problem would be , the main capacitors becoming un-formed.
The oxide plate coatings in the Chinese capacitors seems to break down from being stored.
You plug one in that has been idle for an extended time and it fails.
The circuit in the welders is almost identical to the plasma cutters.

 

[R Kraft]

The original Jasic copies from China had 4 470uf capacitors, for a total of 1880uf.
They would run at 120A at 100% duty cycle, and 200-220 Amp max.

The new Chinese copies have 3 capacitors that claim to be 820uf, but they are only 150uf, for a total of 450uf.
100% duty cycle is only 70A, max amps is about 130A@ 25% duty cycle.
The top capacitor is what it should have.