# Calculation of a Potentiometer

Discussion in 'General Electronics Discussion' started by Rui Pereira, Nov 2, 2016.

1. ### Rui Pereira

1
0
Nov 2, 2016
Hello all,
This may be a very basic question for most of you, but i don't know much about electronics and i start to feel desperate :-(
I would appreciate if anyone give me some guidance.
I want to build a small magnetic stirrer and i already have some parts:
1 - Power supply - 12V - 1A
2 - Fan - 12V - 0,25A
What type of potentiometer i need to use in order to have this stirrer working perfectly?
I was told i should use a 1K Potentiometer - is this correct?

Best regards,
R. Pereira

2. ### davennModerator

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Sep 5, 2009

you don't need a pot ...
the fan will happily draw its 250mA from the power supply without any problem

and what is a magnetic stirrer ?????

3. ### chopnhack

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Apr 28, 2014
I think he is hinting at using the pot to adjust the fan speed. A magnetic stirrer is used in labs to mix chemicals in glassware. A rotating magnet below a thin table surface is used to attract and spin another cylindrical shaped magnetic coated in a plastic that is dropped into a beaker.

davenn likes this.
4. ### Audioguru

3,132
698
Sep 24, 2016
A pot is used to control a speed control circuit then the circuit drives the fan. Your fan has a resistance of 12V/0.25A= 48 ohms. If you put a 48 ohms resistor in series with the fan it probably will not start running but if you spin it with your fingers to get it going it might run at half full speed. The resistor will heat with 6V squared/48 ohms= 0.75W.
if you connect your 1k low power pot to the motor then at 1k ohms nothing will happen. At half rotation it is 500 ohms and again nothing will happen. If you keep turning the pot down then it will get too hot and burn up before the fan runs. A 100 ohms 2W rheostat might survive but when it is turned to 100 ohms then the fan will not start running unless you get it spinning with your fingers. A speed control circuit will allow the fan to start running at any speed.

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5. ### davennModerator

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Sep 5, 2009
which probably wont work
it really needs a PWM drive for the motor
and then I suspect the small fan he is using .... maybe a computer cooling fan with the specs he has so far given ???
isn't going to have the torque to turn a magnet in a fluid

D

chopnhack likes this.

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Apr 28, 2014