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I keep burning up potentiometers on my spirograph

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CnJsdad

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I am new into this hobbie. I built a spirograph and I keep buring up the potentiometers. I am running 12 volt pc fans for the scanners(using a wall wart). The current on each fan is .16 amps. The pots I am using are 1K - 1meg ohm. Do I need a resistor in the circut? Also the speed of the fans are almost on and off. I have a hard time controlling the fan speed. All advice welcomed.
 





At 12V a 1K resistance (the pot turned all the way down) will only let 12mA through, that is 0.012 amps. 12mA @ 12V = 144mW. Your DC fan would need more than 10X the current the pots will let through to spin at full speed, I'm surprised they worked at all. How did you have your circuits set up? What is the wattage of the Pot you are using? I would guess your pots are really 1MOhm pots which have a resistance of ~0ohms when turned down fully. At this resistance, your fan will be supplied the full 160mA which at 12V is well more than the pot can handle runing through it's contacts. At 12volts a 160mA (0.16 A) current results in 1.92W (watts). Most pots I see are 1/4watt (250mW), the higher wattage ones are typically more expensive and aren't often found easily.

For full speed you would need a resistance of 75 Ohms, so use a 100 or 500 Ohm pot in series with a 68ohm resistor and a 10ohm resistor. Make sure the wattage ratings on all the components are as high as you can get if you can't get them around 2W.
 
The circuts are + power supply to side terminal of pot. Middle terminal to + motor. - side of motor to - power supply. The watts for the pots are .2W. From what I get on your reply, the pots I am using are inadequate. I have also been reading a little about PWM. Would it be worth looking into that or is that pretty expensive to do?
 
I have no experience with PWM, but that is how it is normally done in professional spirographs.


Yup, your pots are inadequate, they can only handle 1/10th the power your fan draws.
 
PWM circuits are not very expensive. Build one using a 555-timer type circuit like this. The timer and capacitors/resistors/diodes are cheap. You don't need that exact transistor, you can use other power transistors like this. Also consider buying a nice large rotary pot like this for the speed changing so that it is easy to change by hand and you can put a knob on it.

Note, I'm linking to Sparkfun parts only because they have some nice rotary pots and you can get all the parts you need in one place. The price per part can be more than if you buy larger quantities from other online part shops like Mouser or Digikey.
 
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There's another way to do this, instead of having the wiper of the pot drive the motor, you should have the wiper connected to the base of a transistor, collector connected to +V and the emitter connected to the motor. This will keep the current on the wiper at a minimum and give you better linear control of the motor. You'd want to use an NPN transistor, a darlington if you can.
 
You should use a transistor or mosfet to pass the current indeed - and one that is rated for the dissipation you are using.

It doesnt really have to be a darlington or anything fancy at these current levels.. if you have something with a hFE of 100-200 (say, a BD139) you can run a few 100 mA through the fans with only a few mA of base current.

You must, however, put a resistor in series with the potmeter: if you were to short the transistor base to the positive power supply this would otherwise still burn out the potmeter AND the transistor. I'd suggest something like a 20k potmeter and 470 ohm resistor to begin with at 12 volts.
 
You must, however, put a resistor in series with the potmeter: if you were to short the transistor base to the positive power supply this would otherwise still burn out the potmeter AND the transistor. I'd suggest something like a 20k potmeter and 470 ohm resistor to begin with at 12 volts.
Click to expand...

I wouldn't worry about a resistor between the base of the transistor and the wiper of the pot. The load on the emitter effectively limits current to the collector as well as the base, dependant on the gain of the transistor. A TIP122 or TIP120 from Radio Shack works well in this application.
 
They sell fairly cheap dc motor controller kits too. you may need one for each motor.

good thread- controlling speed and direction maskes all the difference in the world when it comes to spiro.
 
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