Arc welder buzz box - convert to DC

[GeorgeB]

A standard transformer-based arc welder nearly always produces AC output, whether it be the cheapest buzz box or the more highly regarded Oxford type oil immersed arc welders. I'm going to try adding rectification to my arc welders, to produce a DC output. The resultant DC output should have at least two uses:

1. DC arc welding of steel. Many people say it's smoother than AC.

2. TIG welding of steel, when AC is said not to work, and I'm sure that's right.

Plus use as a battery charger, perhaps. It should be quite easy to get the voltage of about 48 volts down to about 14 volts for the purpose of charging a car battery.

This should not be difficult. On another thread, I suggested I could simply add a 50p bridge rectifier, rated at about 35A or 50A. With such a small rectifier, I'd need to keep the arc welder on it's lowest setting for DC applications but that's OK, as I'm mainly interested in DC for arc and TIG welding of thin sheet steel and other small scale work, which needs only low power.

thesnooer responded as follows:

i cant see a small bridge rectifier working for welding even if the amperage is within you spec as the heat generated will probably destroy it.
you would be better off buying 4 diodes that can be mounted on a heatsink and make your own rectifier with a bigger tolerance

I agree it's desirable to use higher-rated diodes or a larger bridge rectifier but why should a 35A or 50A bridge rectifier overheat, if the arc welder is kept turned down to 30 or 35A? I can add a large heat sink and fan.

 

[rtbcomp]

The surge might blow a small rectifier.

I got four stud-mounted diodes (not sure of the spec, they came from a 30 Kw inverter) and fixed them to a heatsink and welded OK with them.

For the unwary, stud diodes have one of the connections to the body so fix each one to its own heatsink to be on the safe side. The heatsinks should be mounted on insulating posts.

 

[GeorgeB]

The surge might blow a small rectifier.

The rectifier is described as having high surge current capability.

The peak forward current is 400A. It seems unlikely that my cheap buzz box can produce an output (surge or otherwise) of 400A when on its lowest setting.

 

[awemawson]

400a surge is VERY easily achieved with even a small welding transformer as by it's nature it is inductive. You'll need diodes with a peak handling of probably in excess of 1000a even though the average only needs to be 50 or so. The other parameter you need to consider is the PIV (peak inverse voltage) rating. When you are welding, and stop, you get a high voltage spike generated as the current is interrupted, and it can be in the order of thousands of volts briefly.

 

[GeorgeB]

400a surge is VERY easily achieved with even a small welding transformer as by it's nature it is inductive. You'll need diodes with a peak handling of probably in excess of 1000a even though the average only needs to be 50 or so. The other parameter you need to consider is the PIV (peak inverse voltage) rating. When you are welding, and stop, you get a high voltage spike generated as the current is interrupted, and it can be in the order of thousands of volts briefly.

I concede you're probably both right. Thank you for your help. Option two might be to use the bridge rectifier plate assembly from a MIG welder. Is there any likelihood of danger to that, when used on an arc welder, rather than with a MIG welder?

Also, is it as simple as adding a rectifier to get DC output for arc or TIG welding or do you also need to add a choke or anything else?

 

[SirDick Diodenob esquire]

A mig rectifier will probably be ok , have seen used ones on ebay for £5 upwards for a 100 amp and higher rated . I have been using the output from a MIG to assist start engines , even worked on a tractor when my purpose built boost starter got fried , an old Migmate and old Nu Tool unit worked well , better in fact as battery chargers/starters than they are welders . Fit a switch to disable the motor and for convenience a separate dinse socket for the live output and make sure you have it on the lowest volt setting 16 to 18v or less will be ok which needs to drop about 4 volts on load , there must be a technical reason why this feature is not given on many welders ? Also tried a better quality MIG unit ( Thermal Arc ) and that worked as well but there is a danger of frying the starter motor as the better stabilised output dosnt drop down as much as a cheap one on load and I suppose there is the possibilty of also frying the welder and car electrics depending on the starter current so do this only as a last resort .

 

[shenion]

There is more than just adding a rectifier. There are large spikes generated that may e thousands of volts (especially when breaking the arc.) The start surge can be large.

Some sort oy over-voltage protection is needed. Either capacitors or a MOV across the input and output.

Adding a rectifier reduces the effectiveness of the main transformers inductance. So an additional reactor may be added. Smoothing caps can also be used to help stabilize the arc. Without both, it will probably not be much smoother than AC.

 

[GeorgeB]

Some sort oy over-voltage protection is needed. Either capacitors or a MOV across the input and output.

Thank you for your help. Do you mean the small ceramic capacitors typically found on the rectifier arrays fitted to a MIG welder, or extra special capacitors due to the move from a MIG welder to an arc welder? I know that smoothing electrolytics are another factor, but quite separate.

 

[shenion]

Yes, the 0.1-0.5uf caps across the diodes help protect them from spikes but they also reduce the amount of electrical noise radiated back through the power line.

There have been posts here complaining of lights flickering when using cheap MIG's (Wolf was one.) The MOV also protects the diodes, better welders will have those also.

Large electrolytics are used more on MIG welders. I find they don't work as well on arc as it limits the speed the voltage can change to changing arc currents.

 

[bill_gj]

Scott - any change of drawing a pic of what and where?
- what caps and where
- what MOV and where

I've been collecting parts for a rectifier to add to my AC arc welder and haven't been looking for MOVs or small caps across the diodes.

 

[shenion]

T1 is existing weld transformer
C1-4 are ceramic disc, 200V or greater. Typically 0.1 to 0.5UF. These are to suppress noise being radiated back on power line through transformer.
C5 can be a 0.1uf or higher, I used a motor run capacitor of 160uf with a 0.1uf ceramic in parallel. I have a 4700uf computer grade electrolytic that will be used on the next version. bigger is better but should be less than 470uf if L1 is not used.
The MOV should be placed close to the rectifier bridge and have a voltage of less that the lowest capacitor or rectifier rated voltage.
L1 is optional, usually 20 to 30 turns of heavy wire around an iron core (I used 1.75" cast iron pipe in a steel frame.)

Here is a thread on another arc welder converted to DC.

 

[GeorgeB]

shenion - many thanks for the circuit and notes. It could prove invaluable and I shall certainly be giving it a try.