power control

Variac?

Karl Townsend fired this volley in news: snipped-for-privacy@4ax.com:

You can control the output power a LITTLE, if it's a transformer-based unit, but not a lot. Transformers don't do a very good job running at load with greatly-reduced voltages on their primaries.

If it's switcher-based, good luck! Ain't gonna happen.

Unless you're just blasting holes in thin sheet metal, timing should be enough. If you are, the best solution is a multi-tapped unit.

LLoyd

Pete and you gave me an idea...

Its basically a 1.5 KVA step down transformer. I can do most of the control with a timer.

What if I got a 2:1 transformer to run for "LOW" mode and not use it in "HIGH". If I look, I might find one with several taps. there are lots of tansformers in the old semi trailer full of junk out back.

My application is spot weld AK47 rails to receiver flats. Many folks report trouble with the 220 volt spot welder burning a hole. I'm sure the timer will help, but its just too much power.

Karl

Shouldn't that be better TIG plug welded?

I've got a few 2KW variacs kicking around, there were originally a ganged, motor driven three phase Lutron dimmer.

I have just this in a large three phase unit. I've often wondered if you can use just one of the three coils and have a single phase transformer. Will this work?

karl

If you can find a transformer with a bunch of taps, like 50, 60 70 80 and 90% of input, that would likely be perfect. 2:1 may be too much reduction, it will reduce power to the weld by 4:1

Jon

Pete, would you like to send one to a good home?

karl

The weld transformer will not be happy with the kind of power coming out of a dimmer, I'm afraid. And there will likely be some interference between the timing adjustment and the power output pulses from the dimmer or speed control, as both tend to variable pulse widths, and there is no provision for synchronizing them.

You might be able to put a *big* Variac/Powerstat (variable autotransformer) between the timer and the weld transformer. I've seen some 240V 20A ones -- but never lifted one. :-)

I have one with a digital timer for adjusting the pulse width, and a multi-tap weld trandformer with something like a dozen switch positions feeding the taps on the transformer. As I acquired it, it was set up for 208 VAC, but I found a spare tap at the bottom end of the transformer which allowed making it happy with 240 VAC instead. This is the *right* way to do it, but difficult as a retrofit to the HF device.

Good Luck, DoN.

That would be one less to trip over. I'll check to see if it will fit in a small PMFRB.

Right. If you cut the voltage in half, the current will also drop in half so the power will be 1/4th. Easy to see the result. Just plug your 220 volt spot welder into 120 volts.

You could use a buck boost transformer. Even make one from a Microwave Oven Transformer.

Dan

What I've got:

120V input, might need two of them ganged back together.

I've got 220 here. I assume two will work?

Why don't you email me your price and shipping to MN karltownsendembarqmail.com

karl

Why would the timer just 'help'? If it gets too hot at 1 second, turn it down to 500 milliseconds. If that's too much, try

200 milliseconds. Or 20 milliseconds.

Timer range can get you a LOT MORE control than a measly two:one transformer ratio can.

whit3rd fired this volley in news:214112ab-b0e7-408b- snipped-for-privacy@googlegroups.com:

I agree 100%. Unless the electrodes were "hot" when clamped and retracted (which they should never be), time equals energy delivered.

LLoyd

It may get you more control, but may not be as good. My experience is that if you are making many spot welds that are identical on clean metal, then you can use a timer to good advantage after you go through some work to find the best time. Or better yet is to use a capacitor discharge spot welder which will give you good control.

But if you are only making a relatively few spot welds and the time is several seconds, you can observe the pressure on the weld and know when to stop the welding. No material wasted finding the optimum time.

Dan

I have one of those 120 Vac spot welders from HF (), and found it too powerful for many things. I toned it down by running it from a Variac, and can use the welder to spot weld two sheets of 0.002" thick stainless steel foil together.

The welder is just a big transformer, completely free of electronics.

Oddly enough, it's made in Armenia, not China.

Joe Gwinn

transformer-based

Two would work. They were originally configured as a motorized three phase bank for lighting control from the days when auditoriums were lit with a zillion 300W mogul base incandescents. You just need to re-gang them with some threaded rod to tie them together and something like a

5/8 shaft and knob.

2x USPS Medium PMFRB @ $11.35

As long as I have one left it should cover any odd low line voltage testing I might need to do and that's two less to trip over.

Based on the other replies I'm not sure the spot welder will benefit since weld time seems to be the controlling factor. Do any commercial spot welders have any sort of amperage control or just weld time control?

With a couple of caveats for US power systems.

1) Wire both Powerstats (these are that brand, not Variac, which was General Radio's brand) with the CCW end of the winding connected to neutral, and the CW ends of each to the two hot lines

2) Wire the load to the two wipers.

With this wiring, your load will remain centered around ground, and both sides will increase together.

Oh yes -- also be sure to turn both to fully CCW before you tighten the setscrews for the common shaft to the knob.

Hmm ... another thing to watch out for. At least the General Radio Variacs used shafts covered by a black plastic (Bakelite, perhaps, given when they were designed) and there was no attempt to insulate the center of the rotor plate from the shaft, so you would have a short between the two output sides. I forget whether Superior Electric (the maker of the Powerstat) did the same or not. Check it out for insulation if you are going to use a bare metal shaft. Perhaps a Delrin shaft would be a better bet.

With other wirings, you would likely burn out one of the variable autotransformers (Powerstats, Variacs) and/or have more hazardous voltages.

Also, fuse both hots -- ideally with a shared circuit breaker.

Good Luck, DoN.