Buck-boost transformer needed

All,
I just got a spot welder which wants 208 VAC, and I have 240 VAC
instead. I checked for taps on the transformers to switch for 240 VAC,
and that appears to not be present.
But I am more worried about the electronics of the controller.
The power supply is on the logic board with a heat-sinked TO-3 regulator
chip, so I'm not sure how able it would be to accept overvoltage without
cooking. If the power supply were separate, I would pull it out and hook
up a switching power supply which could produce the 5 VDC anywhere from
90 VAC to 250 VAC -- but digging into the board to split off the power
at the right point is a bit of a trick.
So -- ideally I need a transformer capable of producing a
secondary voltage of 32 VAC (28 would be close enough) from 240 VAC
input with a 15A current capability on the secondary.
Or -- I guess -- an autotransformer which could handle the 15A
load.
Or a big 15A 240 VAC Variac.
I figure that the shipping costs would be least on the
buck-boost transformer (less iron though probably almost as much
copper).
The spot welder will handle 16 Ga steel without going to the top
tap on the transformer.
Thanks,
DoN.
Reply to
DoN. Nichols
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I'm guessing you have just the ordinary household center-tapped 240, and not 3-phase. 208 is kind of a "side effect" of 3-phase power, but you already knew that.
If it was up to me, and I didn't want to hack anything, I'd of course shop the heavy iron surplus shops (do they even have them anymore?) a 240V 15A variac would be the ne plus ultra, with an eye on price and shipping - my next choice would be the bucker, but again, if your friendly local neighborhood heavy iron surplus guy doesn't have one, either way would cost an arm and a leg.
There might be some modern newfangled electronic solution, but I've been kinda out of the loop on that since, well, variac and buck/boost days. ;-)
Good Luck! Rich
Reply to
Rich Grise
240 is only 15% uo from 208, so you might wonder how close to the edge the original design was. Any chance you could get a part number off the TO-3 regulator? What kind of voltage is going into it, and what kind of current does it have to supply? You may well have more than enough headroom to just accomodate the 240V. Also, you could add a dropping resistor or some diodes in fromt of the regulator to be absolutely sure, ore even a zener across its input.
Reply to
rangerssuck
Saw one at a yard sale this morning, I can run by in the morning and see if it's still there... (and double check the amp rating)
Jon
Reply to
Jon Anderson
No trick at all, easy peasy. Just pull the TO-3 regulator and feed +5 from a switcher into where the output of the TO-3 regulator connected.
Reply to
Don Foreman
Have you got a dead UPS laying around that uses a pair of 12V battries in series? Or an old 12V RV power supply made to power the water pump & 12V lights when you're parked?
Reply to
Michael A. Terrell
564 > =A0 =A0 =A0 =A0 =A0 (too) near Washington D.C. |
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=A0 =A0 =A0 =A0 =A0 =A0--- Black Holes are where God is dividing by zero =
Reply to
co_farmer
Those regulators will take 6V to 35V input, and for efficiency would typically have 8V on the input; your 15 percent higher voltage won't exceed its limits unless that input terminal is near 30V.
So, it's OK for voltage. Power dissipation will rise, but there's simpler solutions than buck transformers to that (put a power resistor, or diode, in series with the regulator input to lose the extra volt). The regulator has internal thermal shutdown, so you will see it power-down if the heat is too much for its heatsink.
Reply to
whit3rd
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Right! If I had three phase, I would be happy in several ways.
I don't know any around here (Northern VA, near Washington DC) -- which does not mean that they don't exist -- just shows the limits of what I know. :-) The places which I would have gone for that back in the 1950s and 1960s are all gone. :-(
And my memory of one set up in an audio facility in an embassy in Central America -- and a younger kid of one of the diplomats seeing the small steering-wheel shaped knob for the Variac used to being the power to the right level (it was a 240V one center tapped so it could boost at need) -- and spinning it fully clockwise. I had to replace a selenium rectifier in a Magnecorder as a result.
I would want a way to lock the knob -- or take it off after adjusting to the proper voltage.
Agreed.
A VFD can be tuned to produce lower voltages at need -- and waste two phases. But I'm not sure that the electronics cabinet would take to the PWM voltage gracefully. (Yes -- I *do* have a VFD capable of handling it -- actually up to 30A -- but I just don't trust it for that task. Two smaller transformers inside the electronics box -- again without taps for 240 VAC.
Thanks, DoN.
Reply to
DoN. Nichols
Thanks!
My direct e-mail needs to be trimmed -- see first line of .sig below.
Thanks again, DoN.
Reply to
DoN. Nichols
Yes -- if it is *only* using the +5 and not some other voltages as well. I'll have to trace it out. I guess I can power the electronics with a variac on one side of the 240 and the other side directly connected, until I can verify what voltages are where. Most of it is 74?? TTL, so that is just 5 V, but I'm not sure about the transistors driving the big SCRs. A bit of time measuring will save burning things out.
Thanks, DoN.
Reply to
DoN. Nichols
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Nope for both. I've got a couple of 12V powered UPS in service, and one 48V one (Four cells in parallel). But the computers need those more than the spot welder does.
Good ideas, though.
Thanks, DoN.
Reply to
DoN. Nichols
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However -- I'm close to as far away as I could be. :-) So shipping would be a killer.
Thanks, DoN.
Reply to
DoN. Nichols
Yes -- I suspect that they designed it to cover the full range, because they make similar welders for 240 VAC with supposedly the same controller. But I wanted to be sure before zapping something which would result in my replacing a dozen TTL chips and who knows what else.
*That* I did not know. That makes life easier.
Now all I need to worry about is saturating the welding transformer at the highest current setting. There is one more tap, and I think that I can move every switch position down one, isolating the one with the fewest turns and bringing in the open one which I suspect is the maximum number of turns.
Thanks, DoN.
Reply to
DoN. Nichols
Hey Don,
I just drove over, and it's not there. They remembered the transformer, but we couldn't find it anywhere. Sad thing is, they don't remember anyone buying it, so someone probably walked off with it. One of the hazards of holding a large sale with only two people...
Jon
Reply to
Jon Anderson
I made a 30A battery charger out of a RV supply. I set the charge rate with a variac.
I have dozens of junk UPS. Another seems to show up at least every other month. Everything from 300 VA to rack mount server units. Just keep an eye out for them. They have a bunch of usable parts, if you build your own power supplies. :)
Reply to
Michael A. Terrell
Well ... thanks for trying, anyway.
If someone casually walked off with a transformer of that size, I don't think I would want to be the one to stop him. :-)
Enjoy, DoN.
Reply to
DoN. Nichols
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Hmmm ... keep my eyes open come next hamfest (in February).
Thanks, DoN.
Reply to
DoN. Nichols
564 > =A0 =A0 =A0 =A0 =A0 (too) near Washington D.C. |
formatting link
=A0 =A0 =A0 =A0 =A0 =A0--- Black Holes are where God is dividing by zero =
Reply to
rangerssuck
I had a pair of 5V 400A transformers disappear a few years ago, and they were bolted to a steel plate. I was going to make a 12V battery charger out of them. Wire the secondaries in series, Add a single high current diode and put a contactor in series with the primaries. Connect the output across a dead battery. Turn the ignition switch and push the button to power the contactor. Of course, you wouldn't do that on a modern vehicle. :)
Reply to
Michael A. Terrell

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