drop battery voltage

[hydrojohn]

Hi folks, I need to drop a 12 volt battery down to 2.2 volts. The load will be 20 - 25 amps not sure yet but it will be consistent. I've been reading about using a series voltage dropping resistor and I am out for easy, I don't mind power spill as it's a one off test scenario (single cell electrolysis for respirator). Is this the easiest way to do this?

 

[James]

I would use a variable resistor, I expect you will need to adjust the resistance regularly as the electrolyte changes conductivity and the electrodes wear or tarnish.

 

[DPG]

It would be a very hefty variable resistor to handle 25 Amps.

 

[telectrix]

It would be a very hefty variable resistor to handle 25 Amps.

make toast in it, or design and build a suitable electronic witchcraft circuit. things like SCRs and Zener diodes are still available even after the demise of Maplins.

 

[hydrojohn]

I would use a variable resistor, I expect you will need to adjust the resistance regularly as the electrolyte changes conductivity and the electrodes wear or tarnish.

Thanks I figured i'd have to look into adjusting the voltage around the 1.7 to 2.2 mark any way. It's a PEM elestrolyzer and so doesn't go through the same aggressive electro chemical conditions as a normal hho unit. The reaction takes place on the membrane/catalyst layer. Further it's just distilled water

 

[DPG]

Thanks I figured i'd have to look into adjusting the voltage around the 1.7 to 2.2 mark any way. It's a PEM elestrolyzer and so doesn't go through the same aggressive electro chemical conditions as a normal hho unit. The reaction takes place on the membrane/catalyst layer. Further it's just distilled water

Sounds interesting. Just remember whatever you use to regulate/reduce the voltage needs to be of suitable power rating.

 

[hydrojohn]

Sounds interesting. Just remember whatever you use to regulate/reduce the voltage needs to be of suitable power rating.

Yeah It's got to carry 20 - 25 amps..
[automerge]1601826593[/automerge]

Yeah It's got to carry 20 - 25 amps..

Probably going to be hard to find

 

[stewartrossinv]

Dropping 10V at 25 Amps would be 250 Watts power dissipation - a lot of heat to cope with. The inefficiency of this approach will also exhaust your 12V battery - they are not designed for continuous current output of 25A (starting peaks are much higher but for short duration). I wonder how long a typical 40-70 Amp-hour battery will last?

Wirewound or thick-film 1 ohm resistors that you could put in parallel of about 250W rating are phenomenally expensive - like this thick-film one from Farnell at £78 - https://uk.farnell.com/vishay/rps0250dh1r00jbza3/resistor-250w-1r/dp/1141565.

Some form of switch-mode power supply delivering this is the only way I can see it would work in the short term, but I've not found anything suitable for 2.2V at 25A.

This reminds me of the power supplies for the first generation of British colour TVs in the 70s - the Thorn 2000 chassis (I think) used large dropper resistors to derive some higher-current supplies for line scan circuits before switched-mode techniques using thyristors became commonplace. The large dropper resistors regularly failed as a result of the heat dissipated, which kind of defeated their purpose.

-Stewart

 

[DPG]

Some kind person may design you a switch-mode supply with the right parameters. In fact there will be circuit suggestions on the web which you may be able to tweak. Obviously this would need a good knowledge of electronics though .

 

[Julie.]

It may be better to review your choice of battery.

Some batteries expose each cell, so it may be possible to use one cell at a time ~ 2V or so, or use a couple of cells and a correspondingly smaller resistance - you could even use a simple bipolar transistor with an adjustable voltage regulator if you are only dropping a volt or two 25-50W could be easier to disapate than 250W with a standard heatsink.

However even going to a 6V battery will significantly reduce the resistance and power lost