Buck-boost transformer needed

"DoN. Nichols" wrote:


Good luck. I haven't made it to a hamfest in years. Nothing worthwhile around here. Of course I was spoiled by growing up near Dayton, Ohio. :(
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If you were a bit closer to the Left Coast, I would give you one or
two or three.
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    [ ... ]

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    However -- I'm close to as far away as I could be. :-) So shipping would be a killer.
    Thanks,         DoN.
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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.
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    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.
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But wait...Did I miss the part where you told us the part number of
the TO-3 device on the heatsink? While a 7805 is, in fact, thermally
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wrote:

Put a fan on the heatsink.
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    A good idea -- and I've got some spare CPU fans which are about the right size. (The heatsink is not much bigger than the TO-3 case the regulator lives in.)
    I spent some time tracing out the high-current box, and found that life was easier than I expected. I was expecting to have to unsolder every tap from the selector switch and move it down one position, dropping one winding off the switch, and adding in the spare wire.
    However, I hooked a resistance soldering transformer to the welding electrode side of the transformer, and started probing voltages relative to the common wire from the primary. Here is what I discovered was the real configuration (neglecting the wiring going off to the electronics box):
1)----------3 || 2)----------3 || 3)----------3 || 4)----------3 || ########## 5)----------3 || ##########---------------------- Moving electrode 6)----------3 || ## 7)----------3 || ## 8)----------3 || ## 9)----------3 || ## 10)---------3 || ##########--(spare) 11)---------3 || ########## 3 || ## 3 || ## 3 || ## 3 || ## 3 || ##########---------------------- Fixed electrode 3 || ########## 3 || 3 || COM)--------3 || 3 || SPARE)------3 ||
    So -- I simply need to disconnect the one marked (COM) and replace it with the one marked (SPARE). Since this means that I will have the old (COM) stripped, and I am reluctant to cut it any shorter, I plan to crimp on fully-insulated female spade disconnects to both (COM) and (SPARE) and replace the connection on the terminal strip with a short wire leading to a male spade disconnect so I can switch between 208V and 240V at will.
    Tonight, I will do that, and power it up to see what the voltage applied to the regulator actually is -- and how warm it gets over time. It may be that I can feel comfortable just leaving it as is.
    Thanks,         DoN.
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    All done. The regulator sees only 9.5 V at 240 AC input, and does not seem to be overheating at all.
    I just spot welded two zinc-plated fender washers (0.070" thick, so a bit thicker than 16 Ga) and it worked fine. Tap switch was at 6, and it goes up to 11 (Spinal Tap, anyone?) so I've got a lot more range to play with for thicker materials. I don't yet know what the maximum is, and may never know.
    Thanks all,         DoN.
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An autotransformer buck design for this won't be huge, as it doesn't need to be a typical 240 primary/208 secondary step-down transformer.
A step-down design is only needed if the application requires isolation, which most welders perform (internally) by design.
The A-T autotransformer design will be 240/22V, for the application you have. See paragraph starting with "The more economical alternative.." http://findarticles.com/p/articles/mi_qa3726/is_201002/ai_n52371827 /
Common power transformer pri/sec-types can be wired as buck (or boost) by wiring them in the A-T fashion, but like variacs, an A-T will not provide isolation.
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"DoN. Nichols" < snipped-for-privacy@d-and-d.com> wrote in message
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    This would be a combination of a variac for adjustment plus a separate transformer for the buck function.

    Right!
    O.K. I was thinking of a secondary voltage based on 240VAC being the local supply, thus calling for 32 V to be a true match, but a 24V secondary should work well enough. That is going to require a 360 VA rating which should not be that heavy. And with the 24V secondary, I could actually do without the autotransformer. Worst case would be 216 VAC output -- and dropping to 220 VAC input would give a low of 198 VAC. All pretty safe with the spot welder.

    Of course. The main transformer in the welder is certainly isolating primary from secondary. I can *see* most of the secondary -- several layers of copper sheet perhaps 1/16" thick wound a nearly full two turns with no interconnection to the primary. The primary has plenty of layers of insulation -- but the secondary is bare. :-)
    Thanks,         DoN.
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