Cooling a Small Transformer

[Bacon46]

I am using a automotive battery charger/starter to operate a chlorate cell. It has three amperage settings, 2, 12, and 75 amps for starting a car. The 75 amp setting will only run in 30 second bursts, then the thermal protection kicks in and the charger shuts down completely.

I would like to be able to cool the transformer enough to get it to push 75 amps continuously. I have a small (3.5 cf) refrigerator / freezer that I am not using. I am considering placing the transformer in the freezer section and cooling it that way but I’m not sure if it will overwork the refrigerator. The cell is running outdoors. Today’s ambient temperature where the cell is located is 104F in the shade, and it’s not always in the shade.

I UTFSE but was unable to find anything on this subject. All I was able to find in a Google search was cooling methods for larger transformers or systems that had to be built into the transformer during the manufacturing process.

Does anyone have any ideas?

I have attached an image of the transformer so you can see what I am working with. I added the heatsink and fan to keep it running in the 12 amp mode.

[chemdude1999]

The pic is still pending approval, but I have a suggestion or two.

If you don't mind tearing into your old refrigerator, you could take the compressor out and run the coolant through a small water/glycol bath with your transformer setting in it (or a container in it). This would reduce the thermal loading on the compressor by reducing the volume it has to cool. It would be like a poor man's cooling bath.

Actually, before doing the above, I would see if the whole setup could be placed in the fridge. This would be easier and it might work just as will. Just some thoughts.

[monkeyboy]

Haven't seen the pic yet.
Are you sure it doesn't use a capacitor to briefly boost the power?

I would look around for an old water cooler. Around here you can pick them up for $20-25. Remove the water jug, if it came with one. Remove all the goodies inside the water pot. The water pot is almost always made from stainless steel, with the evaporator wrapped around it. Put some plastic stand offs in the bottom to keep the transformer from arcing to the pot. Fill it up with mineral oil(Mineral oil is non-conductive). Give it about 4-6 hours to bring it down to temperature, should be 34-36F. Should do a pretty good job of keeping the running transformer cool. But you might want to increase the size of both the secondary & primary leads, as close to the transformer as possible. 12 or 14ga for the primary & 6 or 8ga for the secondary.

Since you aren't using water, you might want to look for the thermostat & crank it all the way down. You might be able to hit low 20s...
Usually inside the back of the box.

I built one with a bigger compressor & condensor & an insulated lid. Filled the pot with 20% brine & used propane as a refrigerant (whole other discussion). It's stable at about 12F. Will cool a can of your favorite beverage to 35F in 1 minute:D

[nbk2000]

Submerge it in a metal bucket of clean motor oil, with a copper coil in the bucket through which cold water flows, at such a rate as to keep the oil below the thermal cut-off temperature of the power-supply. :)

[Charles Owlen Picket]

I'm glad you enclosed the picture as I think thats a Sears charger which shouldn't pulse at 75amps and then shut down. If I am correct (& often I am not...so take this w/ a grain of salt) the 75amp setting should pulse continually, especially with a load. It's for marine batteries and charging some of them is a long chore.....so - that charger should keep pumping 75 but it small doses. But it should not overheat & shut down.

Methinks you have a wee issue with the circuitry therein. And cooling it; no matter the efficiency will just get the thing to 'die harder".

[ETCS (Ret)]

Bacon46,

Unfortunately, the battery charger you are using wasn't designed to output 75 amps for prolonged periods - only in relatively short bursts. For high current output both the transformer and the diode rectifier bridge will tend to overheat severely. They're somewhat small for the application you have in mind.

It would be better to find a cheap electric welder transformer at a place like Harbor Freight Tools and adapt it to your needs. Those are designed for hefty current outputs although at a higher voltage than you need for your electrolytic cell. It may be necessary to modify the secondary winding of the transformer in order to obtain the lower voltage that you need.

It is also possible to re-wind the secondary of a damaged Microwave Oven Transformer (replacing the high voltage secondary, which tends to burn out, with a very heavy wire low voltage number of turns) to obtain the voltage and current you need. Heavy duty bridge rectifiers are now quite inexpensive thanks to the computer power supply industry.

Another good choice is to find a high wattage (300 watts or better) AT style computer power suppy and use either the 3.3 Volt output or the 5.0 Volt output to drive an electrolytic cell. Some of those power supplies are able to deliver very impressive current levels for long periods of time as long as their internals are clean and not packed with dust accumulation as tends to build up inside the computer. Be sure to open the supply and clean it out well with a small brush and a vacuum cleaner.

[Bacon46]

Thanks for all the input

I have most of what I need on hand to give all of these ideas a try.

I may combine all of the input and rewind a microwave transformer, place it in a bucket of mineral oil and sacrifice my little refrigerators compressor, coils etc to cool the oil.

All I need is the mineral and the wire for rewinding the microwave transformer.

And my wife thinks I collect junk!:D

[Jacks Complete]

You might be able to pick up a high powered transformer fairly cheap if you look around on places like ebay. I used to have one that stepped mains at 240V down to 3.3V, converting 10A to about 700A!

A cheap arc welder will shift down for 40+ amps at fairly low voltages. More expensive ones will have fans to cool the coils and the like, but very few are rated for outputting at a 100% duty cycle. You will need to watch that!

Basically, you could add a cooling fan system to what you currently have, and see how it runs. If it melts, you drew too much current and didn't cool it enough! Try not to fry yourself, either.

[Unsunghero]

Most of the marine chargers that have 2, 12, and 75 amps on them are NOT designed for prolonged 75 amp pulses. The 75 amp pulse is designed to start the vehicle (be it boat or car) so that the alternator can charge it.

Just giving a little fact to back up ETSC, as a matter of fact I'm pretty sure it says so on the charger. I left mine in Florida (><) or I would check.