Sciencemadness Discussion Board

Cheapest power supply for an electrochemical cell, no resistors neccessary

Melgar - 7-4-2017 at 11:17

Order one of these things for a dollar, on eBay:

http://www.ebay.com/itm/172264326792

Or make one yourself with alligator clips and an old cable. You probably have several of those USB-port phone chargers lying around, no? If so, check the amperage on them. These chargers are actually current-limited at their rated amperages, because they need to be able to charge phones without burning out, in cases where the phone can be charged at a higher amperage than they can output. Since battery charging is an electrolytic process, these cheap phone chargers are actually ideally-suited for electrolytic reactions at their rated current, provided the reaction requires 5V or less. Once you know the current, by reading the specs on the charger, just get your targets for the remaining design by working backwards from there.

I set up a bromate cell with a 1-amp phone charger and graphite electrodes, and the amperage measured across it was almost exactly 1 amp. The voltage was around 3.6 volts. Multiple chargers could also be set up in parallel, to increase the current.

It was surprising to learn that perhaps the cheapest, most ubiquitous type of power supply that's made today is actually current-limited, and won't burn out even from a short. I've certainly burnt out plenty of power supplies that were a lot more expensive than these things.

phlogiston - 7-4-2017 at 12:39

It is generally not a good idea to set up multiple chargers in parallel. Better use a single one that is rated for higher current.
Slight differences between chargers can result in small differences in voltage output. They will then start pumping current into each other, both trying to control the voltage to what each charger beliefs is right.

If they are current limited, that will probably not harm them immediately, but it is not an ideal situation.
If you have a load that sufficiently drops the output voltage to well below 5V (like your cell), the current will flow into the load as intended, but when the cell fails (open-circuit) for some reason (which is common when cells run for extended periods of time and connections begin to erode), the above scenario is likely to occur soon or late.

PirateDocBrown - 7-4-2017 at 14:10

On the microscale. one such charger might work. But here I sit, fiddling with one that goes to 20A, and realizing it's likely not enough.

Melgar - 7-4-2017 at 16:01

Quote: Originally posted by phlogiston  
If they are current limited, that will probably not harm them immediately, but it is not an ideal situation.
If you have a load that sufficiently drops the output voltage to well below 5V (like your cell), the current will flow into the load as intended, but when the cell fails (open-circuit) for some reason (which is common when cells run for extended periods of time and connections begin to erode), the above scenario is likely to occur soon or late.

They're limited by both voltage AND current, whichever limit gets hit first. I'm aware of the danger of doing that sort of thing with silicon voltage regulators, but phone chargers use a simpler circuit that doesn't pull down the voltage if it's too high. So if that happened, both chargers would just end up at the same state that they'd be in if you plugged them in with no load, no harm done.
Quote: Originally posted by PirateDocBrown  
On the microscale. one such charger might work. But here I sit, fiddling with one that goes to 20A, and realizing it's likely not enough.

When I was electrolyzing thoriated tungsten welding rods, I used one of these suckers:

https://www.amazon.com/Tanbaby-Universal-Regulated-Switching...

It's surprising how hot you can get 500 mL of solution with just 5 volts. The power supply held up just fine though, and I still have it. I was thinking of putting together an electroplating circuit with it at some point.