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Resistor Questions

Discussion in 'Electronic Basics' started by Byron A Jeff, May 10, 2005.

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  1. Byron A Jeff

    Byron A Jeff Guest

    Yes. You can always use a higher wattage resistor at the same value.


    BAJ
     
  2. Chris

    Chris Guest

    1) No problem with buying a higher wattage than you need, as long as
    it fits in the circuit. Resistors are rated for maximum wattage -- how
    much wattage they can take before they get too hot and fuse, burn, or
    permanently change their resistance value. Any less than full power,
    and they'll just be cooler. Higher wattage resistors are usually
    physically bigger, though, so you should keep that in mind if space is
    limited.

    2) If your meter is OK, your 1.2 ohms might just be contact, wire and
    probe resistance. But you should check to see that it's not measuring
    on a higher range, because your two resistors almost certainly have
    different values. The one that says R39 is 0.39 ohms, and the one that
    says 39R is 39 ohms. Big difference, but if your ohmmeter is measuring
    megohms, they both look like a short circuit. Check things out
    carefully before you plug in the 39R one.

    Here's how the euro method of labelling resistors works:

    R39 0.39 ohms
    3R9 3.9 ohms
    39R 39 ohms
    390R 390 ohms
    3K9 3.9 Kohms
    39K 39 Kohms
    390K 390 Kohms
    3M9 3.9 Megohms

    and so on. If you see three digits, that usually means a 1 or 2
    percent resistor. For instance, 3K92 would mean a 3.92K 1% or 2%
    resistor. This isn't too difficult once you get the hang of it, and
    it's a lot harder to miss a "K" on a print than a decimal point,
    especially if it's a fourth generation Xerox.

    Hope this has been of help.

    Good luck
    Chris
     
  3. Macdonald

    Macdonald Guest

    Im a hobyest and I have a couple of questions.

    I was looking for a .39 ohm 1 Watt Resistor. I went to 3 places which all
    didnt have it. I guess I could mail order it but it seems kinda dumb for one
    resistor.

    So I bought a resistor that was 5 Watts .39 . at one place and then found a
    2 Watt .39 Resistor at the other. My first question is since the circuit
    calls for a 1 Watt resistor I suppose either one would work?
    Its a very simple circut.

    The second question relates to labeling of the resistors. The Bill for the
    one resistor says 39R and the actual lettering on the OTHER resistor (5Watt)
    resistor says R39.
    I tested them with a meter and they both say 1.2 ohms on the 200 scale. Im
    asuming that the meter just cant read the .39 so they really are .39.

    Why does it seem that 39 (with no decimal point ) can stand for .39 . This
    seems mis leading ??

    Can anyone clarify this.
     
  4. Macdonald

    Macdonald Guest

    1) No problem with buying a higher wattage than you need, as long as
    Thanks all again.

    I have never come across the R39 scheme before. In the past I have only delt
    with 1/4 watt Resistors and had never know about the "Euopean" method of
    listing resistors. I also have never had to have .00 Resistors either so
    that was a bit new.
    Normally I just looked at the color bands, which all up to this point had
    been 1 or above.

    Thanks. I learned something.
     
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