SIP Weldmate 140 PCB Repair

I've got two sip weld mate 140 that I've been trying to have a bit of a look at

OK, where are you at with them? What is the fault manifestation? First thing to check is that you are getting line through to those line caps.

Moved these into a thread of its own under SIP

I'm getting diddly squat no fans no 12v, the resister that's part of the soft start slug with the relay on 230v I. Was open circuit so linked it out. Still nothing

OK, so you need to apply some volts to the input where the mains would go in (you can even just apply a few volts DC from a bench power supply for the purposes of this test) and see if you are getting any dc volts on those 3 big black capacitors.

Once we have that determined we can go to the next stage.

madkayaker said:

I'm getting diddly squat no fans no 12v, the resister that's part of the soft start slug with the relay on 230v I. Was open circuit so linked it out. Still nothing

Click to expand...

Ok will try that at lunch time and report back

This is like a detective story, gather the clues, make the deductions and find the villain.

The soft start resistor is there for a purpose. When the big capacitors are discharged if you connect them to the mains through a rectifier, momentarily they present a short circuit, so you get a massive current surge, which can blow fuses and damage components, the switch, the rectifier or the capacitors. Shorting it out with a bit of wire and then connecting to the mains isn't a great idea. Much better to replace it with another resistor. 220R 7W seems like a good guess if the old one is unrecognisable. Some sets use a thermistor in that position, for some reason.

You can power the welder through a 100W light bulb or say 30V from a current limited bench supply if you have one. Don't try to weld with it powered through a light bulb, but the fan, relay and control electronics will get enough juice to work. It limits the current and therefore the damage, if you have a serious fault.

The 12V for the fan, the relay and the control electronics is usually derived from the 330V DC across the big caps. There's a switching chip which chops it through a little transformer to produce the 12V. When the welder is powered on, the caps charge slowly (half a second) through the resistor. When they are near full charge, the 12V appears and powers the fan and the relay, which shorts out the resistor.

If there's a fault whereby the relay doesn't close, if you try to weld with it in that state, you burn out the resistor.

I don't understand this.

If there's a fault in the relay, you would burn out the resistor. Replacing the resistor with a bit of wire should give you 330V on the big caps and the 12V supply, powering the fan at least if everything survives the current surge.

If there's a fault causing there to be no 12V supply, you should still have 330V across the big caps, with the resistor shorted out.

As said before, you should really have 330V across the big caps and sorting that out first is the way to go.

I think you may have cascading faults.

Come on MK, give us the next installment.

Some sets use a thermistor in that position, for some reason.

As a thermistor heats up its resistance decreases, so it presents a high resistance at first to suppress the surge, heats up and so allows full operating current to pass with lower voltage drop than would be the case with a resistor.

They used them in the heater circuit of valve TV sets, the heaters were all in series and then in series with dropper resistors. This presented a low resistance when cold so a thermistor was added in series to limit the surge current.

But since, I think in all cases of welding inverters, there's a relay to short out the resistor/thermistor and since some sets use a thermistor and some a resistor, I can't really see the point of the thermistor which is much dearer than a resistor.

Valve TVs always had the thermistor in circuit, so presenting a low resistance when hot and none of them had a relay to short it out as far as I'm aware. Anyway, there's quite a difference between running a chain of valve heaters through a thermistor (80W or something like that, and a constant load) and a welder, about 3KW when welding and 40W when it's idling.

I suppose you want a soft start, but you want it as quick as possible so people aren't trying to weld with the set powered through the soft start resistor/thermistor and the thermistor might be a bit quicker.

Never got it fired up lunch time will try it Monday. I've had mains on it and got nothing past where mains connected to pcb

But I didn't try at the caps but at the two mini transformers to try to find the 12v

There'll be at least two little transformers.

One will be the current feedback transformer usually connected between the collector of an IGBT and the primary of the step down transformer which produces the welding current. The secondary goes to the controller card.

The other will be the transformer which steps down the 330V to LT for the controller, fan and relay. This will probably be running at 100KHz plus, so you may not see much on the secondary of that with a DMM. This is the sort of area where a scope comes in, although it needn't be a very good one. Almost any old scope will do. Better to check the diode which rectifies the output and check the windings of the transformer for continuity. The diode will be a fast recovery number, so can't be replaced with a 1N4007 etc.

Should be easy enough to tell the transformers apart with a bit of circuit tracing.

If the LT is working, the fan should come on and the relay close. However, the LT won't work with no HT to feed it.

I suppose you might have an ordinary 240-12V 50Hz step down transformer to produce the LT, but that's not the way they usually work it. It should be easy enough to spot if they have.

There may be another little transformer to drive the IGBTs. This will have a primary driven by the controller card and two secondaries, one for each IGBT. Not all designs have one, some use optoisolators and other arrangements.

You may have dry joints or cracked circuit lands contributing to the problem. Not always easy to spot.

As Welderfixer said, the first step is to find why that bank of big caps isn't charging to 330V which should be easy enough to do with continuity checks from the plug through the mains lead on both sides, through the switch, through the mains filter, check the big bridge rectifier and then onto the caps.

You haven't even got to power it up with the mains to do it. It's wise to check every time that the big caps are discharged. With everything running properly, when the welder is disconnected from the mains, they discharge in a few seconds by keeping the fan and the relay powered. Under fault conditions they could store enough charge to do you in for a surprisingly long time.

It's important to be systematic because this could be something daft like a blown fuse, dead switch or a duff plug or mains lead. You can waste a lot of time looking for obscure faults and missing the bleeding obvious.

Very true about easy to miss simple things but its not a blown fuse or dodgy lead as it currently has neither! Will report back findings on the caps on Monday

PTVor has described everything really well, except that these welders typically employ PFC (power factor correction) which means the voltage on those black caps is likely to be closer to 380Vdc or so (uses a boost circuit).

Good discussion on the resistor vs thermistor thing. The main reason they have moved away from thermistors in modern switch mode psu's is that you always have to 'waste' a few watts in them to keep the resistance low which hurts efficiency. Also, with a thermistor, if you switch the mains off and back on again very quickly (like in a brief black out) the surge current can be very high as the thermistor is stil hot.

But this is all getting too complex for where you are at. You have to look at the failure mode here. Generally speaking the only thing that blows that soft start resistor is if one of those 3 big line caps is short circuit, or (more likely) the main PFC FET / IGBT is short or one of the main converter FET's is short. The problem is that when these devices fail they tend to get a lot of volts on the gate which also kills the drive components.

Sometimes (depending on the design) the power devices are driven from a small drive transformer which does tend to help protect things.