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Resistor used in A/C circuit?

  1. Sep 15, 2012 #1
    I thought that resistors, like the ones that I buy at RadioShack, could only be used on D/C circuits. I was looking up how to's on electromagnets and found a thread that mentioned using resistors with A/C power.
    The thread was on this forum but I can't find it now.
    I am guessing that the post was being very vague and that a power adapter must have been used in the circuit. Not just a resistor.
    A power adapter, like the ones for USB devices and pc power supplies aren't just transforming the voltage by downgrading it to a lower voltage, they are converting it to DC as well, right?
    So I'm guessing that that was forgotten to be mentioned.
    Or are there resistors that are used in A/C circuits? Well, I mean, true resistors, not lamps, etc.
  2. jcsd
  3. Sep 15, 2012 #2
    Basic role of resistor remains same in AC circuits as well - to limit the current flow.
    An easy example : I remember we used to have rheostat type of speed controllers for fans when I was a kid.

    The good old ac/dc adapters use transformers to step down ac voltage and then rectify it using diodes. They usually also contain a capacitor as ripple filter.

    Now in some mobile charger circuits, I am seeing a smaller adapter circuit which does not require transformers. Basically they use a large wattage resistor to drop the current from AC. then feed it to a bridge rectifier. the output voltage is controlled using a zener diode.
  4. Sep 15, 2012 #3
    Audio is also AC, of a sort.

    AC amplifiers are central to electronics, and they use every part you can think of.

    There are also AC power supplies such as inverters and UPS.

    I am a moderator for AllAboutElectronics.com, a teaching website for electronics. Look for Bill_Marsden .
  5. Sep 15, 2012 #4


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    resistors are passive components and do not care what kind of voltage is applied, just what the MAX voltage is (overdo it and they'll start to burn --- overdo it enough and they'll explode)
  6. Sep 16, 2012 #5
    That's a good example. I thought potentiometers were used in DC circuits only and rheostats were used in A/C circuits only. This a good example of parts that can't mingle between the two types of power?
  7. Sep 16, 2012 #6


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    And just what do you think is the difference between a "potentiomenter" and a "rheostat" and how do you see that making ANY difference in whether you put DC or AC across them?
  8. Sep 16, 2012 #7


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    In practice today, you'll find the term "rheostat" is used in high power applications, such as motors. A rheostat can be used for AC or DC motor controllers. A potentiometer is typically a very low power device. It, too, can be used for AC or DC.

    You'll find that most rheostats have only two terminals, essentially being a variable resistor, whereas the potentiometer with its sliding tap must have three terminals.

    (There is another altogether different electrical apparatus named a "potentiometer". )
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