Buck-boost transformer needed

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

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.."

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.

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.

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.

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.

Put a fan on the heatsink.

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.

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. :)

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.

[ ... ]

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.