Pot or variable resistor - does it matter?

Discussion in 'General Electronics Chat' started by nuggetz, Sep 28, 2009.

  1. nuggetz

    Thread Starter New Member

    Sep 28, 2009
    15
    0
    I was planning on using a wall wart as a power supply for a simple circuit. I was hoping to use a LM317T to vary the voltage and use my Pot as R2 to adjust. Problem is I don't know if what I have is a pot or a variable resistor. It has 3 prongs on it and the resistance I measure between the 2nd and 3rd pins is 10K.

    If I'm insane, can someone maybe tell me what would work better? Wall wart outputs 24volts.

    I haven't even thought about using capacitors with this as I'm not really building a full fledged power supply, just something that I can use to drive a computer fan on my desk to circulate some air and maybe just screw around with. What do you think?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The 10k Ohm thingie you have is most likely a potentiometer.
    Usually, potentiometers have the wiper terminal located between the two opposing ends of a fixed resistance.

    The way a potentiometer is normally used is one end goes to +V or Vcc, the other end to ground, -V, or Vee (whichever), and the wiper will have a voltage potential that can be adjusted anywhere between the higher and lower voltages by turning the shaft or knob of the pot.

    A variable resistor is also called a rheostat. It just has two terminals. A potentiometer can be used as a rheostat by connecting the wiper to one of the end terminals.

    24v is kind of high for your project, as most computer fans run at 12v. If you use a linear regulator like an LM317 to step the voltage down to 12v, half the power will be wasted in the regulator as heat.

    Better to start off with a 14v to 16v supply.

    Even better would be to experiment with PWM. Bill Marsden has some schematics kicking around for that.
     
  3. nuggetz

    Thread Starter New Member

    Sep 28, 2009
    15
    0
    Thanks a lot. After posting this I noticed that the wall wart was 24V AC! :) Anyway, you're explanation helps. The only part that I didn't quite get was when you said "'A potentiometer can be used as a rheostat by connecting the wiper to one of the end terminals" By wiper, I assume you mean the middle pin? And by end terminial you mean either the first or 3rd pin?

    In terms of using my Pot as a Pot, what is the middle pin used for? I played with an Arduino sketch in which I connected the middle pin to one of the Analog In ports on the microcontroller to read the values. Is that all that pin does? Sorry for the whacky questions but I'm trying to learn more about this.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That could work, but you will have to use diodes or a rectifier bridge to convert it to rippled DC, and a large capacitor to filter the rippled DC to "smooth" DC voltage. But when you rectify AC to DC, you wind up with a higher voltage output. 24VAC will become nearly 34VDC after rectification and filtering.

    It's not real easy when it's a new idea.
    Yes.
    You have a multimeter, right? Set it to read in the 10k to 20k Ohm range.
    Measure the resistance across the two end terminals, and move the shaft. You should see that the resistance doesn't change much, if at all.

    However, if you measure from the middle pin to one of the end pins, you'll see the resistance change.
    If you switch the test probe from one end to the other, you'll also see the resistance change, but in the opposite direction.

     
Loading...