Sizing a potentiometer

When the fan gets 6v at 75mA, the resistor drops 6v 75mA (12v psu). R is then

75mA x 6v = 0.45 watts, pretty much any pot can handle that.

NT

I don't know how the fan would behave, PC fans have brushless solid state DC motors they might need almost the full 12 v to run the electronics. Also 0.15A will be the average, they might draw their current in short heavy pulses.

However 50 ohms would drop 7.5volts at 0.15 A.

Might be cheaper to get a 50 ohm and a 25 ohm resistor and try it with either one in circuit, or both in parallel (= 17 ohms) and see how it goes.

Derek

They're often run in PCs at reduced voltage. Minimum is generally

Not sure they'll have the ideal torque profile for a stirrer.

The "proper" way to speed control these is using PWM. The OP may be able to get a PWM controller for a case fan and just run everything off 12v as its supposed to. The problem with using a reduced supply voltage is that the torque falls off drastically. When used as a fan this *appears* to be ok, but the airflow is actually far less than the speed would suggest. Using the fan as a stirrer is more likely to just stall it. Note that PWM can run the fan much slower (although it is much noisier) than a dropper resistor.

Is this any good to you?

In message , Derek Geldard writes

Prolly better to knock up a cheap and nasty controller using a voltage regulator IC with the pot controlling the voltage

Thanks guys...By all accounts there are lots of these in operation without any major issues such as stalling etc...I'm just keen to understand how to size in the pot given our local suppliers don't have a huge range to choose from. They start at 500ohms and up..I can source a 200ohm one online.

If you want to control fan speed why not use one of these?

Think you'll find most common ones are 1/4 watt.

If you can get hold of Wireless World magazine for (ISTR) September

NT

You can get a fair bit more torque from motors run at low speed by using a 2nd external motor to fan air thru the 1st motor. The 2nd one stays at full speed.

NT

What a memory ;)

These are not simple motors, but contain a driver circuit to generate rotating magnetic fields. It's response to a PWM supply is not well defined, and you'll get different fans behaving differently.

Ah, yes I remember it well.

The problem was trying to overcome stiction so that a model train would pull away smoothly as opposed to sullenly sitting there, then hareing off without warning and coming off the tracks at the first bend which is what you got with a traditional (Hornby) rheostatic controller.

Derek

I built one, and it worked very well, although at 'minimum' speed there was still a tiny pulse from the controller, which made the train creep very slowly at a 'station', especially if it didn't have many wagons/coaches.

I was wondering about the disipation as well 1/2 watt does feel a bit high to me. Also bear in mind the rated disipation would be across the whole track not just a fraction of it.

Everyday Electronics did one sometime in the 1970's.

Nowadays, you can buy a chip which does it all for you. A couple of years ago, I designed a circuit to drive a motorised potentiometer and used the A3953 which is a full bridge PWM motor controller. (Only need full-bridge if you want to have reverse control too.)

However, as I said in another post, PC fans are not simple motors, and they aren't designed for PWM control and certainly not reversing. (I did permanently reverse one once, by carefully disassembling and swapping around the motor field winding connections.)

Surely most can simply be mounted the other way round?