# Converting an AC welder into a DC welder

#### MrDEB

##### Well-Known Member
Found several schematics but the inductor is hard to locate but saw reference to a DIY inductor using a 1.75" cast iron pipe but locating cast iron pipe?
This is a 110v welder that outputs a max of 88 amps.
One schematic suggests an oil filled motor run capacitor as they are way less money but ?? to many if's. The schematic shows 20-40,000uf 250v motor run type cap. All I found are 5 - 25mfd caps. Maybe missing something here??
Any parts selection or ??
http://i36.tinypic.com/68y5pu.gif

http://shdesigns.org/Welding/Misc/AC2DC.jpg

#### HiTech

##### Well-Known Member
You better have some seriously high current rectifiers capable of handling heavy surges often associated with welding. The diodes you find in an automobile alternator will crap out fast despite their high amperage.

#### dr pepper

##### Well-Known Member
I've done various things with weld sets.
You need some diodes rated 150amps or more for a 88a welder, and if you use a choke then you need ones that'll take 200v.
I've made a makeshift choke from a microwave transformer for a vehicle mounted weld set, not to smooth the o/p, to maintain the arc, the back emf from the choke when the arc extinguishes causes a high voltage spike which helps to re-light the rod or tip.
There was some russian high current threaded stud diodes on ebay a while back.

#### MrDEB

##### Well-Known Member
Found on Ebay numerious 3 phase diode sets. Just use two so I am intending on purchasing two sets of 3 phase diodes rated at 150amps
http://www.ebay.com/itm/2pcs-MDS200...252?pt=LH_DefaultDomain_0&hash=item51a60b9294
The inductor is perhaps the most difficult to locate or maybe build. Need to look for a broken microwave oven but do the schematics look feasible?? and which one to use. I am assuming the motor run oil filled caps should work but ?

#### tcmtech

##### Banned
Given that it was originally a AC only output I am suspecting that it was a stick welder which is constant current?

You can convert it to DC just by using a full wave rectifier and a good sized inductor choke. For the inductor I would just take an old and fair sized battery charger and use its high current secondaries as the choke. Easy to work with that way.

Relating to capacitors for a welder that small most any capacitor with at least a 2X working rating over the welders OC voltage will be good enough.

#### Misterbenn

##### Active Member
What value inductance do you need?

#### MrDEB

##### Well-Known Member
Going by what I have seen a 2 mh should do the trick
Yes I have seen large diodes on Mouser but at a substantially higher price
This is a mig welder, a Century 80gl mig welder I purchased for $185 on Ebay. HD sells same for$265

#### tcmtech

##### Banned
I was not aware that they ever made an AC MIG unit. How did it weld?

#### dr pepper

##### Well-Known Member
That isotop bridge looks ok for light use maybe, yes just use 2 of the i/p terminals for 1 phase, dont forget doing this reduces the capacity by the square root of 3, or something like that.

Stick welders are constant current, but you can still achieve reasonable constant voltage with them, they usually have a magnetic shunt to reduce the o/p current, if you turn the amps all the way up and then use a variable inductor to regulate the current you can mig with them.

Have you thought about using a saturable core inductor to regulate the o/p?, then you can use a dc signal to control the current, I messed with this briefly, there was someone on the net talking about it, they had weld sets where I used to work that had saturable inductors not for current regulation for arc stabilisation, they were butters sets, now long gone.

The website for the geyser who built the saturable inductor was AAA welder, sadly it doesnt appear to be there any more.

#### MrDEB

##### Well-Known Member
The AC mig works ok but lots of splatter. After converting to DC I understand the penetration increases and no splatter. A much better weld as well.
Harbor Freight sells an AC mig unit as does many other welder companies. Most of the 110v units are all AC.
Trying to improve my welder using what other people have done but mostly have seen them using standard electrolytic caps not oil filled or motor run caps but shouldn't make a difference. I was curious about the two similar but different schematics and sourcing an inductor.
I have the diodes ordered as a 100mfd cap.
Need to order the four .1uf caps and the MOV

#### tcmtech

##### Banned
I used to be a service tech for our local Praxair welding supply store so I worked on too many 120 powered mini migs to count and I never saw an AC one.

Now for reference any common 100 - 120 amp Miller Lincoln or decent Century ran with diodes rated for about 10X their OC voltage and at least 1.5X their working amps. 300 - 600 volt 150 - 200 amp stud diodes were common. Capacitor wise they all used 40 - 50 volt electrolytics in the 30K - 60K uf range.

Here is the full Lincoln SP-100 Service manual with the complete schematics and detailed parts breakdowns!

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#### MrDEB

##### Well-Known Member
Here is a schematic of the Century 80GL welder. NO DC VOLTAGE
Kind wish I had spent the extra $150 for the DC unit. #### Attachments • 33.8 KB Views: 2,779 #### dr pepper ##### Well-Known Member Most Helpful Member Yes there are some ac migs, however ac isnt much good for migging, the ac ones I think are only made ac to save the cost of a rectifier. #### MrDEB ##### Well-Known Member I figure for$50 I will have a DC mig. I realized we have a 1 year old non working micro wave that is going to be salvaged shortly. Dang microwave was only one year old and while cooking it started to smell like something burning. When purchassed I was thinking an LG was supposed to be a good brand. Wrote the company but never heard anything back.
ITS SALVAGE TIME!! where did I put that hammer?

#### dr pepper

##### Well-Known Member
Theres an article on the mig welding forum where someone took a 50 dollar mig and added a rectifier and an o/p choke, modded the wirefeed and ended up with a half decent weld.

You microwave might be too new, later ones have a smpsu instead of a tranny, but have a look anyways.

Either use some good quality caps for smoothing or none at all, high esr on cheapo's will probably make them blow up.

#### dr pepper

##### Well-Known Member
Might be worth researching the value of spares or repairs on your miller they are desireable, ebay might fetch a few quid for it and buy you some new caps.