Reduce 12v to 9v or 5v
December 22, 2012 9:26 AM   Subscribe

How can I control/reduce the amount of power from a 12v battery?

I have a 12v battery that powers small fans in a camper. Depending on how cold it is, I'd like to be able to reduce power to the fans so they don't move as much air.

Fans are cheapo computer fans, so there's no controller that I'm aware of (actually I snipped off the extra wires, which are probably how they'd be controlled if used in a computer).

I'm looking for something cheap and easy since my technical skills are pretty limited. I can splice a wire, but that's about it.

I'm imagining something like a model railroad controller perhaps, but I don't even know if that will do the trick. Can I just walk into Radio Shack and get something that does this?

Thank you.
posted by powpow to Technology (6 answers total) 1 user marked this as a favorite
Might be better to add a solar charger.
posted by sammyo at 9:47 AM on December 22, 2012

The simplest would be a regular potentiometer (IE: variable resister) wired in series wiht the fan leads. The higher the series resistance the lower the fan speed. This is kind of wasteful though as the power not driving the fan gets burned in the pot.

Better and almost as simple is to use a single transistor. You'd be able to buy everything you need, probably, at radio shack and certainly at any electronics supply house. This will give you linear speed and control/conserve battery power. The author of my link has some Pulse Width driver circuits too if you want to get fancy.
posted by Mitheral at 10:17 AM on December 22, 2012

If your fan is actually a computer fan there are stand-along speed controllers. For example.
posted by Ookseer at 10:20 AM on December 22, 2012

If you are just looking for manual control, something like this should work:
Fan Speed Controller

Since you cut the connectors off, you'll have to use your splicing skills to get everything working.
posted by Talk To Me Goose at 10:21 AM on December 22, 2012

Excellent - those controllers look perfect, and cheap. Thanks again!
posted by powpow at 10:48 AM on December 22, 2012

Both a potentiometer and simple transistor controller are electrically equivalent. The transistor is just a variable resistor and the same power is dissipated in the transistor as in the potentiometer. There is no advantage to using a transistor rather than a pot except in high power applications, particularly since you still need a pot to control the transistor.

Either way, it isn't all that bad. When you increase the variable resistance in order to slow down the fan, you also reduce the current and that reduces the overall power used, both in the pot and the fan. (P = 12V * I) in all cases, regardless of how V is proportioned between the fan and resistor. Increasing R reduces I. Reducing I reduces overall P.

If you work through all the numbers, when the fan is adjusted to one-half speed, it produces one-half the air flow. The total power used by the system with a variable resistor is cut to about 35%. The theoretical power limit would be the cube root of one-half, that is, one-eight (12.5%) if you used a more efficient, but more complicated, pulse width modulator or switch mode voltage regulator.

Bottom line, running a fan at half speed using a potentiometer (or simple transistor) cuts power usage to one-third compared to a more complicated and expensive controller which cuts it to one-eight. Not a bad trade off for simplicity unless a desire for very low power consumption is the limiting factor.

All of the cheap fan controllers shown are just potentiometers and will work fine in most cases.
posted by JackFlash at 12:45 PM on December 22, 2012

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