help with transformer wiring

[Eightpot]

I'm going to have a go at fixing the power to the motor drive on my Sip mig welder -

the mini pcb mounted transformer which powered the motor circuit died a couple of years back, so as a quick fix I just connected two wires to the motor control, dangled them out of the welder and connected them to a 12v battery - not sure it's a good idea to sit next to a car battery in a shower of welding sparks though, so time to fix it proper.

I've picked up a decent sized 240v>12v transformer, but am a bit wary on the wiring - I'm presuming the thinner guage red/black are 12v +ve/-ve which I connect to the motor drive circuit, and I'm also presuming I would attach the thick orange to the 240v live and the blue to the neutral terminal in the welder - can anyone confirm if this is correct??

Bit wary on this as I tried wiring up a smaller transformer I picked up from Maplin a few months ago, but it had more wires and more colours - my initial attempts at deciphering the wiring using a 12v car battery and a voltmeter ended abruptly when I inadvertently built a very successful tazer device and deployed it up my own arm

[Bowie69]

Without out being able to look at it closely it's rather difficult to say....

Testing is a reasonable way to do it... the output winding should be lower resistance than the input.

The input winding will have loads of coils of copper on it, the output far fewer and thicker.

HTH

[MrKev]

With a 12V - 240V transformer, the 240V primary has 20 times the voltage, and therefore about 1/20th the current of the secondary. I would therefore expect the secondary to have the heavier gauge wire. If you go in backwards, you'll either blow up the transformer, or output 20x the input voltage (4800V - well on the way to making your next tazer....)

What Bowie69 says is also true, depends how good / cheap your multimeter is though.

What is definitely not true, is your original post says "12v +ve and -ve". A transformer will be giving AC voltage out, to feed a DC motor, or do the same job as your car battery, you'll need a rectifier, and if you're doing it properly, a voltage regulator too, though the wire speed control board on your welder may have this on it - depends how you connected your car battery up.

Kevin

[LiamC]

Don't forget that the transformer will give you an AC output, presumably there is a rectifier etc on board the pcb that was used by the old transformer? Just make sure the new transformer is connected before the rectifier.

[LiamC]

.........We seem to have posted at the same time

[pete3000]

http://www.mig-welding.co.uk/forum/showthread.php?t=11532

http://hobbymechatronics.com/projects/28-electronics/26-modifying-the-sip-migmate-150-dp-gasgasless-mig-welder

hope this covers your sip mig, lots of things to check. Like the motor, is it 12v or 24v.

Full credit to the original posters in the links.

[Eightpot]

Thanks for the replies - I did some electronics at college, but that was 25 years ago and was obviously skiving off on the day we covered transformers and AC power

So yes, there is a small board with the transformer/rectifier etc, which feeds into a relay which operates the motor. I've attached my car battery to the relay. Was presuming the motor was 12v, and I certainly get a decent wire speed using the car battery but will investigate further - hope it's not 24v.

So presuming I've actually got the correct transformer, all I want to do is remove the wires to my car battery, remove the old dead transformer, and connect this one in its place - still confused over which wires to connect though - thanks for the links, looks like there might be some good info in there, I'll have a thorough read through..