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Resistance other than from resistor

  1. Mar 5, 2010 #1
    Hi experts,

    i am wondering if i want to have a few ohms of resistance but does not have any resistor and i use coil a wire that produce a resistance of a few ohms, how different is this 'resistance' compared to the resistance from a resistor of the same ohm value ? I know that a coil produces inductance but as a resistor, how do the 2 types compare?

    sincerely
    Ramone
    :smile:
     
  2. jcsd
  3. Mar 5, 2010 #2
  4. Mar 5, 2010 #3

    marcusl

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    If you are using DC currents, then there is no difference. The inductance will have an effect on AC.
     
  5. Mar 6, 2010 #4
    Hello Bob S and marcusl,
    thank you very much for your advice.

    You are great!
    :)
     
  6. Mar 6, 2010 #5

    sophiecentaur

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    "ordinary' coils of wire have very low inductance when considering mains AC frequency and are probably the cheapest way of achieving an Ohm or so. There is just the possible problem of overheating, when currents of more than an Amp or so are concerned. (Look at the warnings on drums carrying mains extension lead.)
     
  7. Mar 6, 2010 #6
    Erm, you can get 1 ohm resistors for a few cents, and they'll be smaller, more robust, will give more repeatable results, and will have known ratings for power, inductance, etc. A spool of wire with equivalent resistance will cost several dollars and be quite unwieldy in comparison. Also, a coil of wire with 1 ohm DC resistance can easily have inductive reactance on the order of an ohm at 60 Hz, especially at larger wire gauges needed for higher currents...it's just a resistor at DC, but you can not assume it will be so for AC. Worse, the inductance can drastically increase due to ferromagnetic materials nearby, and the coil will be quite sensitive to electromagnetic fields.

    Lengths of wire can be useful when very low resistances are needed. 24 AWG copper wire has about 1 milliohm per 12 mm, and you can easily trim it to adjust the resistance. 12 meters of 24 AWG wire for a 1 ohm resistor is a bit less useful. It'll work if you need a DC resistance right now, but it is certainly not cheaper than an actual resistor.
     
  8. Mar 6, 2010 #7

    sophiecentaur

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    1W resistors are very cheap but higher power resistors get a bit pricey, I seem to remember. Bell wire is cheap as chips and is infinitely adjustable..
    But it depends on the application. An off the shelf job is the best long-term solution, of course.
    btw, how does the wire gauge affect the inductance, cjameshuff? And, in any case, if you use both conductors of a pair, you can more or less cancel it with the 'there and back' path - join the two at the far end end and connect to live and neutral wires (bi-filar, I think they call it).
     
  9. Mar 6, 2010 #8
    You can get a 25 watt 1 ohm wirewound power resistor for $1.09 from Mouser. That's 25 amps and certainly in an enclosed space, you will need 10 gauge wire...20 AWG bell wire's not going to do the job. 250 meters of 6 gauge wire, or 125 meters of 2-conductor cable. A 500 foot (150 meter) spool of such cable costs $438.68 from Mouser.

    It's only remotely feasible to use a spool of copper wire for low-power applications, there's certainly no advantage for high powers. The insulation's not designed for the kind of heat a power resistor can dissipate, the spool's not designed to dissipate it, and the low resistance of the copper makes the problem worse by requiring layer upon layer of wire wound on the spool. The possibility of insulation weakening (especially cheap thermoplastic bell wire insulation) and turns shorting together makes it foolhardy for high powers. And even for low powers, 20 gauge 2-conductor bell wire will take 15 meters to do the job, which will cost considerably more than that single power resistor.


    Larger gauge wire has lower resistance per unit length, requiring greater lengths, more turns, and often a larger spool, which all means greater inductance. Tying the wires of a pair together at one end and using both will halve the length needed and mostly eliminate the inductance, though you are then left with the capacitance of the pair...the insulation of typical wires available as pairs is thick enough to keep this low, but will also make the whole thing even clumsier to work with.
     
  10. Mar 7, 2010 #9

    sophiecentaur

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    That about clinches it. You are right. I should have done the actual sums rather than using my memory.:-)
     
  11. Mar 7, 2010 #10
    Well, now I feel foolish...that's 5 amps. 20 gauge "bell wire" will technically work, though it'll still be more expensive than the ~$1 resistor. That $438.68 spool of wire still compares poorly to a $125 1 kW 1 ohm resistor, though. And again, the combination of possible thermoplastic insulation, the mechanical stresses of the winding on the spool, and poor heat dissipation still make it a bad idea, especially if you're using it to do something like a constant current supply...as the turns short out and the resistance drops, it'll just increase current to keep dissipating 25 watts in the spool.
     
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