Butters MEC 401 Torch staying live

[emmo1210]

Hi Guys,

Hopeing some of you more experienced people can help out here.
I have a single phase Butters MEC 401 MIG (now obsolete) and have an issue with the wire remaining live after i release the trigger.

As soon as i touch steel again it sparks and that is it until i press the trigger again (ie no longer live)

I can also say that the gas only flows if trigger is pressed.

I have notcied that a capacitor is loose inside the cabinet, think it attachs to the cabinet frame, not sure (pic below)

Looking at the contactor it appears to work properly, doesnt seem to be sticking on or anything.

Any suggestions on what could be at fault?

Thanks

 

[Paul.]

Is it a proper arc, or just a small spark, if the latter its most likely just a capacitor discharging when you short the wire to ground, some machines are fitted with a bleed resistor to empty the caps, its quite common and nothing to worry about, and not the first time its been asked either, "clicky"
Welcome to the forum

 

[Wozzaaah]

Does the wire continue to feed too?
If so then it sounds like it's in 4T, trigger on - start weld and release trigger, trigger on - cease.
If no wire feed but a proper arc then it's faulty.
Is there a 2T/4T or 'latch' switch on the front panel?

 

[emmo1210]

Hello Paul and Wozzaah,

Had done a search but didnt find anything to match.

Thanks for reply. It seems to be quite a discharge, when the wire touches earthed material the spark will spray everywhere and at times the wire will stick to the material which is annoying.

Definately not in 4t mode, wire only feeds with trigger pressed

Also didnt use to happen so think it must be a fault from that perspective.

Do you guys know what this capacitor is for, possible it is the discharge capacitor which has come loose?

Thanks
Emmett

 

[emmo1210]

Sorry, I was referring to the capacitor shown in the pic above.

 

[shenion]

That loose cap is probably for power-factor correction; it is an AC cap.

Look for some large (usually light blue or gray) caps with screw terminals. There should be a bleed resistor nearby; that has probably blown or come disconnected.

 

[emmo1210]

Hello Shenion,

Took another few pics of welder.

Think those are the caps you describe. Would the white component shown be a bleed resistor, feeds from white caps into relay, I removed from circuit and doesnt appear to have any affect to welding.

Unfortunately the writing has all but worn off the component

Thanks
Emmett

 

[shenion]

That does look like the bleed resistor. Does it show a resistance reading with a meter?

 

[emmo1210]

That does look like the bleed resistor. Does it show a resistance reading with a meter?

Tried meter, all settings show max resisitance. Is there any way i could measure the componet to know what replacement i need, one of the wires coming out of resistor is broken and irrepairable?

Thanks

 

[shenion]

Ok looks like 10 watt or so. There are no legible markings on it?

10-watt, 250 ohm should do.

It is not a critical value as it just discharges the caps. Higher resistance will take longer. I have 500ohm on my miller and it takes it about 2 seconds.

 

[emmo1210]

No marking at all, think they have worn off.

ok i have bought these:

http://www.ebay.co.uk/itm/160791518...eName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

at 10w 330 ohm, so will try one of these. I'll report back.

Many thanks for help

Emmett

 

[shenion]

They will work fine.

If it takes too long before the caps discharge, use the two in parallel.

 

[emmo1210]

Hi again,

Ok, so have tried out the 10W 330ohm resistors, firstly using one, then using 2 in parallel.

The result on using 1 resistor, took 1minute 34 seconds to discharge down to 1 volt (from about 30v)

Then using the 2 in parallel (660 ohm) it took 48 seconds to discharge to 1 volt.

Could you recommend a suitable resistor from these results? 2 seconds would be an acceptable wait

Would 7.5k or 10k ohm sound too much?

Thanks
Emmett

 

[shenion]

2 in parallel is 165 ohms. You want a lower resistance for faster discharge.

Must have been a 20W resistor (hard to tell from picture).

2 seconds would need to be about 7 ohms. That would need to be a 200 watt resistor to handle 40V (max OCV?).

two 20 watt, 100 ohm in parallel would discharge to 1V in about 15 seconds. On mine, i don't get any spark below 8V or so, should get there in a few seconds.

Power on resistors assume 40V max. I bet the existing one was under rated as it did not assume 100% duty cycle at full power. Probably one reason it burned out; they pushed the limits

See what you can find, in 10 watt, 180 ohm would max out the power (2 would give half the time you have now.) You could always use 4 of them.

Mine is a 100watt beast that is 10" long, I do plan on changing it to 111 ohm (has variable taps).

 

[emmo1210]

Yes i can find 10 watt 180 ohm resisitors ok.

Would the 20 watt 100 ohm resistor allow faster discharge?

 

[shenion]

Not as fast as 2 of the 10W/180 ohm in parallel (90 ohms)

You could probably match the original with 2, 100 ohm, 10 watt in parallel. Would not last if you weld over 32V.

Hard to be specific on discharge time as I don't know the max voltage and the size of the capacitorss.

 

[emmo1210]

Thanks for advice, I will try a few of the 10w 180 ohm and see how i get on. I'll let you know.

thanks
Emmett

 

[emmo1210]

I have 2x 10w 180o now fitted and its doing a good job, much better than it was before, just about 4 seconds until discharge.

Thanks Shenion and others for help on the matter.

Emmett