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.
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On Sun, 10 Oct 2010 03:19:26 +0000, DoN. Nichols wrote:

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
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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.
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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.
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On 10/9/2010 7:19 PM, DoN. Nichols wrote:

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
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    Thanks!
    My direct e-mail needs to be trimmed -- see first line of .sig below.
    Thanks again,         DoN.
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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
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    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.
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"DoN. Nichols" wrote:

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. :)
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    [ ... ]

    You definitely would not want to be dealing with that fellow. :-)

    Hmm ... unless the actual voltage was well above 5V I don't think that it would have been good as a charger -- except at very cold temperatures. 1.414 * 5V is 7.07V, *2 makes it 14.14, subtract 0.700V for the single diode, and you get 13.44 V. A typical room-temperature charge voltage is 2.4 V/cell, or 14.40V.
    However -- with that current capability -- you should have been able to *start* the car from the charger, and then leave it to the normal charging system to bring it up to full charge.
    Enjoy,         DoN.
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"DoN. Nichols" wrote:

Tell me! I had to use a hand truck to move it. The secondary was wound with 1/8" copper strap and it still had the copper buss bars from whatever it was used in. There were four paralleled transformers at one time. A ham radio type bought them surplus, and removed the secondary to use one as a choke in a transmitter power supply. I unwound one for the copper strapping, and to rewind for a 12V 200A power supply.

Or start charging a very discharged battery without overcharging it. :)
As I first described it, it was intended to start a large engine when the battery was dead, or even missing.
I had to start a 225 slant six in a co worker's Plymouth Duster in the '70s. No one had any jumper cables. My dad had borrowed mine, the boss was gone with the set from the shop, and the new guy didn't own a set. :(
I found six feet of 8 AWG aluminum clothesline wire in the back of the shop. I rolled the bumper of my '63 Catalina against his, and wedged the ends of the wire against the positive battery terminals of both vehicles. Luckily, it started the first try. The wire got so hot I had to drop it in those couple seconds. I told him to buy a set of jumper cables on payday, because I wouldn't do that again.
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    [ ... ]

    I suspect that the array was for heating the "boat" in a vacuum evaporation system -- depositing thin films of metal on (typically) non-conducting materials.
    There was likely originally a steering-wheel equipped variable autotransformer -- maybe General Radio Variac, maybe Superior Electric -- whatever their model name was -- to adjust the temperature of the boat, because the resistance and thermal mass of it varied.
    Sometimes it was a piece of thin metal, folded into a 'V' and crimped closed at the ends to go into the high current clamps. The metal to be evaporated was poured into the 'V'.
    Sometimes, it was a spiral of metal wire -- I think tungsten, but I never knew for sure, with the ends straight into the clamps. In this kind, the wire to be evaporated was wound around the spiral wire, and quickly melted to coat it when the temperature was cranked up. Needless to say, the spiral wire took less current than the folded "boat".
    The thickness of the plating was judged by a quartz frequency crystal with one side exposed to be plated. The thicker the coating the lower the frequency -- and it was beat against a reference frequency to determine the shift -- usually with the beat frequency fed to a speaker to give real-time judgement. Of course, the density of what was being coated affected how much the frequency would shift, so the process had to be calibrated with a measurement of the edge of a coating in a sample.

    Yes -- that too.

    O.K. That makes sense. I was wondering, because I would have expected you to know about the charging voltage requirements of lead-acid cells. :-)

    Overall poor planning on someone's part.

    And they used that stuff to wire houses for a while. :-)
    I presume that this was not winter, or you would have likely had good enough gloves on so you could have kept control of the hot wire -- at least for a while longer. :-)
    Good thing that one of the cars was not my MGA. That would not have worked for that. It was positive ground. Two 6V batteries -- one on either side of the driveshaft behind the seats and under an easily removed cover. An insulated jumper between the two batteries arcing over the driveshaft. They *claimed* that it was for better balance, but I think that they simply wanted to keep using the 6V batteries until their stock was depleted. A real pain to replace here in the US however. :-)

    Amen. Or make *him* hold the wire next time. :-)
    Oh yes, another story -- I noticed a lead-acid battery which we kept on the workbench for testing specific high-current loads was gone -- and then a few minutes later, a co-worker came in carrying it, and said "That is a really good battery! It started my car right away.". (Now, it had twelve filler caps, not the usual six. :-) I'm amazed that his light bulbs did not burn out. (It was an older car -- perhaps '67 or so, so there was not a bunch of sensitive electronics in there to fry. :-)
    Enjoy,         DoN.
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"DoN. Nichols" wrote:

It may have come froma large broadcast transmitter to power the finals. If so, the four transformers would have been used at less than 200A each for continious use. I do know that he bought the assembly at Mendelson's in Dayton. I've seen single UHF TV transmitter tubes that used 3 KW for the filament supply, a pait of 1.5V 1000 A filaments per water cooled power tetrode. thefilament voltage was adjusted & balanced by stretching a copper buss bar in each circuit.

Yes, I learned that around 12 years old. :)

I didn't know mine were missing. That's what happens when someone else has keys to your car. The boss serviced school clocks and took some of the extra tools when he was on the road for a couple weeks at a time. The tech was fresh out of tech school and still pinching pennies.

July or August. If it had been an Ohio winter, the engine would have been so stiff that teh aluminum would have vaporized before it started.

No next time. The aluminum arched and damaged the battery posts.

My dad had an old Ford station wagon in the early '60s that was a bitch to start. He burnt up three starters in a couple months. When the third one died, he replaced it with a six volt starter. After that it rarely failed to start. If it did, you had to wait a few minutes for the starter to cool off before a second try. :)
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On 10/10/2010 5:58 PM, DoN. Nichols wrote:

They're really not that big at all. I have a new one, holding in reserve, not sure if I'll need it down under or not. I was running an Eguro lathe with it, don't recall the amp capacity, but I'm pretty sure it'll handle 10-15 amps. It's buried out in the shed or I'd take a look. I can hold it in one hand....
Jon
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wrote:

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.
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    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.
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"DoN. Nichols" wrote:

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?
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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.
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"DoN. Nichols" wrote:

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. :)
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    [ ... ]

    [ ... ]

    Hmmm ... keep my eyes open come next hamfest (in February).
    Thanks,         DoN.
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