Another wire feed mod

[Charred fingers]

I had a problem with the PCB on my Cosmo 130 and as I was thinking of doing a wire
feed conversion I decided to replace the entire board.
To do it I got a 15-24V variable output switchmode laptop PSU off
ebay, a 3 amp 18-24V PWM controller from Quasar electronics, and two
24V SPDT 16amp mini relays,a 2.5mm DC power socket, a 24V LED indicator
and a 10 amp terminal block from Rapidonline

I made up a board for the relays from a piece of plastic from a 20 litre
drum and used thin copper sheet from a scrap water cylinder to make the
tracking and copper wire to link into the terminal block

The relay board is spaced off the front case with nothing more than a
cut down H/D plastic pipe clip

The PWM uses the original hole and the plastic spacer washers from the
original board

The PSU is mounted on a stand which I folded from a piece of thin galv
sheet and riveted together

The 24V led is fixed in the original hole for the thermal cutout, you
do need to drill it out and I used the original knob for the PWM,
I shortened the shaft to match the original. The led is powered via
the output from the thermal cutout and is on as long as power is
going to the torch.

The photo's were taken before the finally wiring and don't show the wiring
of the main switch.
Briefly you remove the live feed to the original board completely, the neutral
and switched neutral go to the relay board. The 24V supply to the board is via the DC socket ,
the live out is split one goes to the PWM (note the PSU and PWM are powered up as long as the main switch is on)
the other goes to the torch via the thermal cutout. You remove the original return and thread a new wire up the torch
to carry the return to the relay board this is connected to the micro switch in the torch and is used to trigger both relays.
The power out from the PWM goes to the N/O contacts on second relay the feed to the motor is on the
common contacts . The neutral from the PWM goes to motor neutral and is linked to the N/C contacts on the relay.
Nearly forgot you remove the original motor neutral from the cluster near the transformer
The schematic as it stands now

Not a great pic anyone see any obvious errors.
Would like to thank everyone who has posted their mods which gave
me the imformation and the confidence to take this on.

 

[BChild]

Has it improved the welder?

What was the original PCB fault?

Nice post BTW

 

[Thinphil]

I think BTW must be too busy using his nicely modded welder to reply !
But it would be good to have him update us on the performance of the welder now ?

 

[BChild]

I think BTW must be too busy using his nicely modded welder to reply !

I'd like like to get to that stage

But it would be good to have him update us on the performance of the welder now ?

I'd really like to know what difference the PWM makes to the motor feed [clarkson mode]torques[/clarkson mode] at lower speed

 

[Charred fingers]

To answer a few questions the prob with the PCB was due to a blonde moment with a multimeter which took out the motor feed transistor.
I replaced that only to realise it had also taken out a resistor as well.I got halfway through an article on how to test resistors wired in parallel
when I realised my life was slowly ebbing away and decided to chuck the lot.

Electrically the welder is spot on very stable voltages, what it didn't cure is the crappy plastic gearbox which at low voltages and therefore low torque
had a tight spot. Solving this involved checking the lash between each tooth on each cog and with a flat needle file and taking a tiny fraction off each
side and off the tip of any tooth that was tight. Checking took ages but fortunately most of problem was with the roller drive cog.

Aword of warning for anyone doing the strap mod

Make sure the strap is cranked exactly right, if it imposes any lateral
load on roller retainer when you tighten the screws it pulls the outer bush
out of line and tighens the gearbox up

As for welding after tweaking the trim pot on the PWM to set the low end voltage I managed some pretty decent welds on a piece of scrap on the lowest setting using CO2 but the arc was still a bit unstable. So I connected the bottle of argon 5 that had been in the shed for a month, didnt use it before because when I built my welding trolley I made it to carry the pub CO2 bottle and the argon 5 bottle is a bigger diameter and wouldn't fit and the easiest way to kill a regulator is to stick it on an unrestrained bottle.So after lashing it to the workbench leg I had a go.

The result was a genuine EUREKA! moment, I can't believe how silky smooth this thing welds on even the lowest setting. I haven't tested it right through the power range, I need to get some heavier scrap from work.

Oh by the way cost of the electricals a shade over 58 quid, I may have lost the automatic voltage adjustment but i'm capable of turning a knob and at least it now works.