Simple CV power supply

Hey all,
I've been searching for a used mig machine to power my Ready Welder spool gun and I haven't come across much worth getting. Does anyone have a design,
or know the basics of a simple CV power supply? "Making my own" isn't new to me but I don't know where to start with the constant voltage thing. I've seen the schematics out there for home brew stick machines, but realize they won't work with my mig setup.
Thanks in advance,
Eide
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Why not a DC Stcik welder.
I usually run my Readywelder from my Inverter TIG/Stick welder.
I have run it from my Betamig 250 as well. It works fine on both.
The only real benefit to using a CV power source is you don't have to worry so much about arc length.
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I guess because I was told it wouldn't work. So as long as I worry a little about my arc length I'll be fine? Any other caveats?
Eide

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I am way out on a limb here, having very little experience with CV power sources, but in reading the schematic for a MillerMatic 35 I acquired, the only thing that stands out to me is that it has taps on the secondary side of the primary transformer for multiple voltages and a large bank of capacitors (about 54,000 uf) on the downstream side of the rectifier. And a stabilizer on the ground-leg, also on the downstream side of the rectfier.
The rest of the machine looks like a standard transformer stick welder schematic, except it doesn't have a large reactor for current control and no AC output.
Hope that helps...
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When calculating your amperage on the CC power source use the TIG rule of 1 amp per 0.001" of thickness of your base metal. That is for a single pass full penetration weld. It works pretty well. You don't need that much heat if you are doing a multi-pass weld.
If the CC powersource has an adjustment for the open-circuit-voltage , you can use this to drop the voltage a little for welding thinner materials with smaller wire with the readywelder.
On my Maxstar I was able to drop the voltage to weld 16 ga steel with 0.024" wire.
Readywelders are designed to run from anything, CC, CV or a couple of batteries.
You do have to learn to maintain a fairly constant arc length when using a CC power source, but that is pretty easy.

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Any idea what a Ready Welder costs?
If it works, it must cost a lot........
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Around $500, more or less, depending on precise model and vendor.
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I got mine on eBay for less than $400.
Eide

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I got mine for $450 new. Model 10250.
They go on eBay for more like $385.
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Cosntant current transformer usually has more windings than iron so the core saturates and limits the current. Constant voltage transformer has more iron, doesn't saturate, gives (more) constant voltage. Add heavy duty diode bridge circuit and some caps and away you go.
Eide wrote:

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Can you explain what you mean here? I'm guessing you mean the transformer itself is physically larger? More iron = fewer windings = the windings are a heavier gauge?
Any simple schematic you could throw my way?
Thanks, Eide

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CV supplies are conceptually easier than CC. You need a transformer with a low impedance, with a core that stays well out of magnetic saturation. This is the normal sort of transformer in the electrical world. The ones with adjustable sag used for CC welding are the exception (much harder to design).
The secondary of the transformer feeds a rectifier. This can be either a bridge circuit or a full wave circuit (the latter requires a center tapped transformer, the bridge circuit is normally used for welders). The concern here is to use diodes with adequate current capacity and adequate heatsinking. Again, this is the norm in the electrical world.
The rectifer feeds a filter. The job of the filter is to remove ripple from the pulsating DC produced by the rectifier. Filters can be made from a bank of *shunt* capacitors. These act like a storage tank for electricity. Current is drawn from them by the load (arc) when the rectified current drops, and current flows in to replentish them when current from the rectifier peaks. Thus they smooth out the delivery of electrical energy and maintain a *constant voltage* characteristic.
Filters can also be made from *series* inductors. The physics behind how a series inductor can help produce a constant voltage is a bit more esoteric than that of the shunt capacitor. I'll skip that here and simply say that an inductor resists changes in the instantaneous current flowing through it (but isn't sensitive to the absolute value of the current through it, as long as it stays out of saturation), so it alternately bucks and boosts the pulsating voltage coming from the rectifier in such a way that it levels the peaks and boosts the low points. The result is the same as for the shunt capacitor, a smooth constant voltage output.
Normally, filters are designed with both shunt capacitors and series inductors. The two elements compliment each other and work better together than separately. Now the *values* of capacitance and inductance used in a simple filter aren't too critical. The values have to be selected with both the 120 Hz ripple frequency and the desired current draw in mind. Bigger is generally better. There are methods to calculate these values for particular situations, but for the home builder, the simplest thing is to look at what a well designed commercial unit uses, and copy that.
Note, capacitors have a property called ESR or Equivalent Series Resistance. For power supply filtering use, you want a capacitor with the lowest practical ESR. Otherwise the capacitor will overheat and fail in high current supplies. ESR isn't as important in low current circuits, and it is cheaper to make a capacitor with a high ESR (they're much smaller too), so be wary, read the specs, when selecting a capacitor for a welding power supply.
High current inductors have large gauge wire and large heavy iron cores. Again, the same inductance can be produced with a small inductor using small gauge wire and small iron core, but it can't handle much current. That won't do for a welder power supply. There is a rule of thumb for sorting unknown inductors (and transformers too) which says 40 watts per cubic inch. In other words, measure the volume of the core and windings, multiply by 40, and that's the amount of power the inductor or transformer can handle. But reading the specifications is a more sure way of selecting the proper inductor.
That's all there is to a simple CV power supply designed for a single output voltage. To get adjustable voltage, the transformer will have switch selectable taps. Or, for newer designs, the rectifiers are replaced by SCRs whose duty cycle is varied by a control circuit to present the filter with a waveform which it will average down to the desired operating voltage. Filter design for this sort of supply is more challenging, component values become much more critical.
Gary
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I am half way to understanding this but I'm having a bit of trouble finding suppliers with bridge circuits that can handle the power.
I'm thinking of using two transformers to ease the stepping down of the power so I don't have to find such a huge transformer. (the ones I have are out of microwave ovens and one from a battery charger. I'm guessing as to the size of wire to do the turns, should I use the largest I can for the number of turns I need?
From 220 I was thinking of one transformer having a 5:2:3:4 ratio of turns (5 wraps on the primary and taps of 2,3,and 4 wraps on the secondary) feeding another transformer with a 5:1 ratio of turns. Am I way off base? Am I close?
Thanks to everyone who has written so far, this is a great place to hang out!
Eide


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One generally buys 4 diodes and puts your own bridge together at this sort of power level.
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www.digikey.com lists 88 different diodes capable of handling 300A welding current. Four of them in bridge configuration is what you want. (Digikey lists diodes up to 2300 amps, but you don't need anything that big.)

Not even. If you series cascade two transformers that way, each of them has to be capable of handling full welding power. Since that means each one will have to be large, it is not a good plan. Use one. Transformers are typically designed for about 1 turn per volt. So the primary should be about 240 turns of #8, and the secondary should be ribbon wound with about 30 turns of ribbon with a current carrying capacity equivalent to #2, tapped appropriately. (Ribbon is used because winding #2 on the core is difficult.)
Note, the exact turns per volt for a particular core design can vary a bit from the 1 turn per volt rule of thumb. Assuming you'll be winding your own secondary, but using the existing primary, wind 10 turns of light wire and measure the voltage, then use that to calculate how many turns of heavy ribbon you'll need to reach a max welding voltage of 33 volts. Figure out your taps from there.
Note that a possible source of a transformer core would be an old utility pole transformer. You can extract the core from the oil cooling container, it'll be much smaller that way, and you don't need oil cooling for a reasonably sized welder. One winding will already be 240 volts, the other will be 2300 volts. Remove that one, and replace it with your new secondary.
You can sometimes scrounge old pole transformers from your utility. But be cautious that they aren't trying to palm one off on you that uses PCB oil. If the EPA found out about that, it could cost you dearly. A 10 KVA rated pole transformer should work Ok as a 5 KVA welding transformer when stripped out of its oil cooled enclosure.
Another source of transformer cores would be the dry transformers used in large buildings to step down power for subpanels. These can often be obtained salvage during a building renovation or demolition. They don't use cooling oil, so that eliminates the hazards and mess of dealing with a pole transformer. 5 KVA is a common size.
Gary
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In general, it is safest to avoid oil filled transformers or capacitors of unknown content, due to the prevalence of PCBs in these items. I'm not sure how difficult they are to test for, but I do know that they are bad enough news that simply avoiding them is a good bet. Even without PCBs, transformer oil is messy.
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wrote:

I agree that they're messy. But any transformer with a date code less than 20 years old should be PCB free. Power companies are *supposed* to attach a permanent label to any older transformers which do contain PCBs.
Gary
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Thanks for the great instructions and pointers. I looked at digikey.com already but I'm sure I was looking in the wrong spot, I'll try again. Now I'll start looking for a transformer right away and let everyone know how things go.
Eide


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Thanks all, for the discussions! Who knows if I'll get it right, but then what fun would it be?
Eide

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