Getting a very low setting on a Cheap MIG (any Electrical Engineers here?)

[HarryD]

I bought a cheap MIG welder to help do minor repairs but the low setting is not low enough. Per the manual I have a 60 amp and an 80 amp setting.

I looked at the wiring diagram (see below) and it looks like I can create an even lower setting by adding a DPDT Switch and minor bit of rewiring (see further below).

Can one of the EE's here tell me if I am nuts?

Thanks!!! SmileWavy

Current wiring setup:

Proposed Modification:

 

[madkayaker]

can't see it making any difference. But then I am only an electrician

 

[Robotstar5]

If the max/min legend on S2 is the correct way round it means you need A/B + C/D winding for min current (60A) and A/B winding on it's own for max I (80A) so to drop the current any more you would need to add a winding not switch out the A/B winding.
I may be wrong it's been a long time since college!

 

[madkayaker]

I wondered if the switch was labeled right way round. If the voltage was increased by the two sets of windings then the current will decrease. Might be looking down wrong end of periscope again.

 

[HarryD]

If the max/min legend on S2 is the correct way round it means you need A/B + C/D winding for min current (60A) and A/B winding on it's own for max I (80A) so to drop the current any more you would need to add a winding not switch out the A/B winding.
I may be wrong it's been a long time since college!

Now you know why am asking.

I am thinking that the two primary windings are of different sizes which is why I get 80 and 60 amps.

When do I get the low setting? When both windings are tied together (Current goes A to D) or when just one winding is selected (current goes A to B)?

I may be too smart for my own good (My Chemical Engineering degree just gets in the way....)

 

[shenion]

The low setting is both windings in series.

You do not want to tie the windings together, it will burn up. One is about 120V, the other is probably about 30V.

The only way to reduce the output is to add windings to the primary.

The only other way is to use something like a variac. I used one on my Clarke 130EN when doing alumimum, helped a little. Does need to be a big variac. Mine was a 2.4kw one. But that would cost more than your cheap welder.

Flog it on ebay and get something decent.

 

[HarryD]

The low setting is both windings in series.

You do not want to tie the windings together, it will burn up. One is about 120V, the other is probably about 30V.

The only way to reduce the output is to add windings to the primary.

The only other way is to use something like a variac. I used one on my Clarke 130EN when doing alumimum, helped a little. Does need to be a big variac. Mine was a 2.4kw one. But that would cost more than your cheap welder.

Flog it on ebay and get something decent.

sigh...

I suspected as much but thought I would try the poor boy approach....

 

[R Kraft]

When the current goes from A to D you get 60 amps - @19.5v. OC
When the current goes from A to B you get 80 amps - @21.5v. OC
That would mean that the C-D winding "only" , would have very few turns on it.
It would probably burn out, also if it did last, it would make the output amps/volts higher.
Robert

 

[shenion]

Don't tell me, Harbor Freight Chicago Eletric junk?

 

[HarryD]

Don't tell me, Harbor Freight Chicago Eletric junk?

OK, I will not. But for $40 it is what it is.

 

[Master-of-none]

Variable resistor

Scary thoughts time!
To get a lower current you could add a 'suitable' variable resistor in the output circuit, obviously able to handle a large current.
Here's some old technology that might work!
Saltwater dimmer

 

[madkayaker]

Scary but worked

 

[shenion]

That won't work as MIG is constant voltage, the resistance will allow the voltage to vary.

 

[bill_gj]

Shenion - you're the electrically educated man here - what if you inserted a suitably heavy-duty lamp dimmer-type circuit into the full primary?

The turns ratio on the transformer is then unaltered, keeping the output voltage up, but wouldn't the output current be variable?

 

[shenion]

Lamp dimmers are not for inductive loads.

Something like a welder will caus ethem to become very unstable.

 

[bill_gj]

What about something akin to SCR control of an inductive load?

I think I saw something somewhere on a site about a home made TIG welder where the power to a transformer primary was controlled using SCRs - can't think where that was....

 

[shenion]

What about something akin to SCR control of an inductive load?

I think I saw something somewhere on a site about a home made TIG welder where the power to a transformer primary was controlled using SCRs - can't think where that was....

That requires zero-cross detection and accurate timing.

Light dimmers use SCR's (actually Triacs.) They become istable with inductive loads as they can't maintain an accurate pulse timing to fire the SCR.

BTW, one of the SIP welders use the SCR to control voltage.

 

[bill_gj]

This was the one I was thinking of:
http://www.dansworkshop.com/electricity-and-electronics/homebuilt-arc-welder.htm