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Potentiometer R-ranges

Discussion in 'General Electronics Discussion' started by fatalmk1, Aug 31, 2010.

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  1. fatalmk1


    Aug 31, 2010
    hi guys, i need some help finding a pot with the right resistance range. im working with an automotive circuit, so somewhere around 12v, and im trying to vary the speed of a cooling fan. im using a toggle switch and a relay that switches output from one lead to another when the coil is energized. so basically, with the switch off current runs through the normally closed lead of the relay, through a resistor, to the fan, and to ground. with the switch on, current runs straight from the relay to the fan.

    ok, so the max current draw of the fan is around 6.8A, so resistance is 1.77 ohms...if i want the fan to run at 1/4 speed, i would need to increase resistance to 7.1 ohms, if i want it at 3/4 speed id need 2.4 ohms, etc...

    id like to use a potentiometer in place of the fixed resistor, so that with the switch off i can run the fan, ideally, from 1/8 speed (14.1 ohms) to 7/8 speed (2.0 ohms), with the switch on, full speed..actually, if i can find the right pot, i may use one on each path so that i can set both switch positions to whatever speed i want..

    now, maybe i just dont know how to read the specs, but all the pots ive found are rated at like 10k ohms.....the fan will barely run with that much resistance...anybody know where i can find a pot with the resistance i need?

    thanks guys
  2. shrtrnd


    Jan 15, 2010
    It'd have to be a physically large pot to handle the amperage. And that won't be cheap in low resistances. Maybe somebody will give you a place to find what you're
    looking for in a later post.. I don't know how much experience you've got in electronics. If it was me, I'd be looking at a three position rotary switch, and solder the (cheaper) resistors between the rotary switch contacts (like an old style wafer switch). And switch to different speeds, instead of using a pot to do it. Just a second option for you to consider. Good luck getting a response you can work with, if you go with a pot.
  3. Militoy


    Aug 24, 2010
    I agree with shrtrnd's assessment. This kind of control will work – but maybe isn’t the most efficient, compact or economical way to control fan speed. Let’s say you just wanted to set a particular speed, using a fixed resistor. The calc would be simple – 6.8A measured draw at full speed, measured at 12V. So, maybe figure around 1.75 ohms for the motor – and close to 7.75 amps with the engine running, and 13.6V supplied by the alternator. At ¼ speed, total current drops to around 1.94A – total resistance of 7 ohms, minus the original 1.75 of the motor – so you’re adding around 5 ohms, picking a standard value. Dissipation in the resistor will be the square of the current through it, multiplied by the resistance (2.0^2 x 5) = 20W. So, a 25 to 40W power resistor at 5 ohms would be about right – depending on if it’s a heat sink type, and how hot you want it to get.

    Using a rheostat (variable power resistor) though – things get a bit more difficult. Since you will want full speed sometimes, the sweep contact needs to be rated for full current, or 7.75A. Using the same max resistance of 5 ohms (lets not complicate it by wanting below ¼ speed), you’ll need a rheostat that’s rated for around 300W – that’s (7.75A^2 x 5 ohms). You’ll never dissipate that much – so you can probably find an Ohmite part with an oversized contact brush – and get away with maybe a 100W part – but those are still kind of pricy.

    An alternate solution would be to switch in a pair of fixed resistors. Pick 2 values that will give you around 5 ohms in series, around 2-3 ohms selecting one of the pair, and a bit over an ohm in parallel. This would give you 4 selectable speeds (including no resistor – full speed). The Dale or Ohmite aluminum body power resistors can be mounted to a heat sink – and are readily available in various power sizes.

    Edit - I didn't mention, that the dual switched resistor isn't exactly an original idea. I have an old '49 Willys jeep, with a Sears Roebuck bakelite aftermarket heater from the 1950's. The fan control switches 2 resistors in and out. Simple - and still works.
    Last edited: Aug 31, 2010
  4. NickS


    Apr 6, 2010
    A switch and fixed resistors really is the way to go. But if you don't mind turning off power to change the settings perhaps this beast could suit your needs(I used a pair of them to test a 150W power supply). It operates by sliding the wiper manually down the coil so you would have to pre-measure and mark your key spots

    Good luck
  5. Resqueline


    Jul 31, 2009
    The 5-way speed controls for car heater fans have usually consisted of three different resistor wire coils connected in series and placed inside the heater air duct for cooling.
    The same principle can of course be applied to a cooling fan, like people have suggested here. It's cheap, reliable, simple, small, and is usually good enough.
    A more modern approach though is to use a PWM fan controller (that won't heat up). Those can be had as ready-made modules, kits, or completely diy (diagram only).
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