Potentiometers vs Rheostats (or why a pot needs a ground connection)

  1. Hello everyone,

    Can you guys please explain why a potentiometer needs a ground (-) wire? or what does it do differently and what uses there are for it as opposed to a rheostat.

    Recently been playing around with automotive sensors like the TPS (Throttle Position Sensor) on a car and just can't understand why the manufacturer would put a potentiometer (3 wire sensor) over a simpler 2 wire sensor like the rheostat.
  2. jcsd
  3. trollcast

    trollcast 288
    Gold Member

    Both the potentiometer and the rheostat have a core made up of normally either a carbon film, a ceramic and metal mixture or a wire winding.

    In the rheostat one pin is connected to end of the core and the other is connected to a wiper (basically a small pin that scrubs up and down the core to complete the circuit). So the rheostat is therefore connected in series like an ordinary resistor and when you turn the knob this changes the core length and increases or decreasesing the resistance accordingly.

    The potentiometer is similar to the rheostat except in a potentiometer 2 of the pins are connected to either end of the core and the third pin is connected to the wiper. This then creates a potential divider that can be varied depending on the position of the wiper.

    So because of the potential divider 2 things happen, 1) the current can't be blocked completely as long as the 3 pins are connected up right and 2) this then allows you take a signal and reduce its amplitude by taking your connection from the wiper to the ground pin.

    Those couple of circuits should show how that works maybe a bit clearer.
  4. The words potentiometer and rheostat refer to 2 different functions.
    A potentiometer implies a potential divider designed to produce a varying voltage.
    A rheostat is a variable resistance connected in series to vary current.
  5. Danger

    Danger 9,663
    Gold Member

    Thanks, Trollcast. I've never known what a pot was before. Any circuit diagram that I've seen just confused me with that extra wire going to ground. I'm still not quite sure how to use one, but at least it makes sense now. Are both of those suitable for AC as well as DC operation?
  6. Some pots have enough bandwidth for low-frequency AC (like audio) but you have to check the datasheet.
  7. trollcast

    trollcast 288
    Gold Member

    You can use a potentiometer anywhere you would normally use a voltage divider, eg in a 555 timer circuit like this one:

  8. Danger

    Danger 9,663
    Gold Member

    I hope you realize that your diagram makes absolutely no sense to me. I know nothing, and I do mean nothing, about electronics. Everything that I've ever built or worked on was just straight DC or AC. I haven't a clue about resistors or transistors or any other kind of istors. Diodes look pretty if you hook the battery up the right way; otherwise they're just handicapped Legos.

    Oh, wait a sec... there is one istor that I've used. I once replaced the thermistor in a blow drier when my girlfriend of the moment was going to throw it out because it quit working. That's not something worthy of being placed in my resume.

    edit: By the bye, what the hell does that circuit do? I know that it's some kind of motor control, since there's a motor in the diagram, but beyond that I'm lost.
  9. trollcast

    trollcast 288
    Gold Member

    Its a circuit that implements a simple PWM controller for a dc motor to control its speed. It basically turns the motor off periodically for a certain amount of time but really really quickly (we're talking fractions of a second at a time) This overall has the effect of reducing the power sent to the motor so that the motor turns slower or faster depending on the percentage of time the motor is on and off.

  10. Danger

    Danger 9,663
    Gold Member

    Oh... cool. I've heard of pulse width modulation, but didn't know what it was or how it was applied. I'm starting to like this stuff.
  11. trollcast

    trollcast 288
    Gold Member

    Although normally if you had one you'd use a PIC microcontroller (Programmable Integrated Circuit) or a pwm driver instead of making the oscillator (I think thats the right term) circuit yourself from parts.

    Edit: And now we're completely off topic from the OP, sorry.
  12. Danger

    Danger 9,663
    Gold Member

    My apologies to OP as well; I got a little hung up in the thrill of the hunt. :biggrin:
  13. jim hardy

    jim hardy 5,460
    Science Advisor
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    2014 Award

    Both terms are old. They're not quite synonyms.

    It is instructive to look up their etymology.


    From the days before digital voltmeters, when you used a Wheatstone Bridge for accurate low voltage measurements . A potentiometer can be used for one half of a bridge to balanced an unknown voltage with a null detector. That is called a "Potentiometric" measurement, though that sounds like a redundancy.
    You need three connections on a potentiometer, one at each end of the resistor plus the moving one.

    It's just a variable resistor for controlling current, so you need only two connections.

    If you take a potentiometer and wire the moving connection to one end, you have made it into a rheostat.

    So the terms are not quite synonymous. They indicate different use and different connections.

    any help?

    old jim
  14. Ok I understand that a rheostat is a variable resistor and that a potentiometer is a variable voltage divider.
    Now what I don't understand (from an engineering prospective) is why would you use a potentiometer instead of a variable resistor for stuff like a volume control knob or a throttle position sensor on a car?
  15. Integral

    Integral 7,288
    Staff Emeritus
    Science Advisor
    Gold Member

    A variable resistor is the physical part. A variable resistor can be wired as a rheostat to control current or a potentiometer to control voltage. So the answer to you question lies in your application, do you need to control voltage or current? Most modern electronic devices strive to keep the current as low as possible, thus pots are used to control voltage while drawing as low a current as possible.

    The device we commonly call a potentiometer is a variable resistor designed for low current applications. If you wire it as a rheostat you will need to avoid exceeding the current limitations, especially at the low end, when the resistance is very low the resulting high current could easily damage the device.

    I have seen a rheostat designed to control theater lighting that was huge, about .5m in diameter and weighing 10's of kg. It used heavy copper wire and large copper contacts, all of this because they need to carry high currents.
  16. jim hardy

    jim hardy 5,460
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    2014 Award

    Do you understand term "Voltage Divider" ?

    Look at Trollcasts' picture:

    Example - look at second one, "Volume Control"
    Take a piece of graph paper and plot volts out versus rotation of the knob.
    Do you get a straight line?
    That's predictable.

    Now look at third one , variable resistor...
    Plot current versus rotation - observe it's nonlinear. That's because your numerator is fixed and your denominator approaches zero.
    Even if you add a fixed resistor in series, it's still nonlinear.
    That nonlinearity can be used to advantage if you work at it.
    But more often it's an inconvenience that you must work around.
    It can be handy for a nonlinear load like Integral's incandescent lamps.

    But a throttle position sensor probably wants a simple 1::1 relation between volts and rotation for straightforward programming.

    Likewise a volume control -- BUT -- the ear is logarithmic.
    So volume control potetntiometers are made with a logarithmic taper(of ohms versus rotation) to match our hearing.
    That's predictable.

    It used to be a drill in basic electronics class to derive the equations for output of a rheostat with various resistances in series. And for wheatstone bridges with varying degrees of unbalance.

    Mis-Using a log taper potentiometer as a rheostat one can develop interesting shaped curves.
    Use your algebra - that's what it is for !

    old jim
    Last edited: Apr 20, 2013
  17. So if I understand correctly, it's all about the output function:
    A rheostat in series will have an inverse (1/x) function.
    A potentiometer will have either a linear or logarithmic output function depending on taper.
    The reason the manufacturer chose a pot over a variable resistor is because it's easier to program for because the output is linear.

    Sadly I can't remember the math to calculate the potentiometer output.
    What I'll do instead is graph the output voltage of the pot in both the variable resistor and voltage divider modes using a multimeter. I'll make an effort to put it on here tomorrow for future reference.
  18. sophiecentaur

    sophiecentaur 14,712
    Science Advisor
    Gold Member

    It might be a good idea to look that up then. You can then plot a graph of how the two alternative setups work, for a range of load resistances and rheostat / potentiometer, as you vary the position of the slider / knob. A potential divider with a relatively low value of resistance will give a more or less linear voltage variation out when feeding a high resistance load but , once the load current is appreciable, this linearity is ruined (you have two parallel resistors in the 'bottom leg' of the potentiometer). If you want to vary the power into a motor or lamp, the required resistance for a linear potential divider will have to be low and that means there will be a lot of standing current and things will get hot and wasteful of power. There is also the point that you can never get zero volts out of a simple rheostat, whereas a potential divider wiper will actually be connected to 0V when at the bottom end of its scale.
    It's horses for courses and some simulated values will give you a feel for the subject. Try a 10k pot with a 100k load, a 1k load and a 100R load, to see what I mean.
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