Resistors getting hot?

Discussion in 'FAQs' started by tverskaya, Jan 19, 2007.

  1. tverskaya

    tverskaya Member

    I was toying around a bit, wiring up some LEDs and connecting them with the resistor that came with them to the transformer.

    Well, the LED works and I found out that LEDs are a nifty little way of determining the polarity, but I happened to burn my fingers when I touched the resistor - It was bloody hot!

    So is it normal for them to get rather hot or should I be careful with them, as I would rather not have it causing a fire? Might I be using the wrong resistor?
  2. CCT70

    CCT70 Member

    Wrong resistor if that LED is getting really hot.
  3. Gary S.

    Gary S. Senior Member

    The resistor should not be hot enough to burn your fingers. Can you give us any info on the components? Source voltage, Resistor ohms and watt ratings, any info on the LED?
  4. tverskaya

    tverskaya Member

    Source voltage: 12v, straight from the DC-side of the transformer.

    The LED is supposed to be for 12 volt operation...5mm bright and white...Don't know the details.
    The resistors seem to be gold-green-black-orange, but I can't be completely sure. I'm not literate in electronese.
  5. tverskaya

    tverskaya Member

    And it's only the resistor that gets hot - the LED stays cool.
  6. Gary S.

    Gary S. Senior Member

    Do you have the "current" rating of the LED? It should be on the package. Probably about 20 milli-amps but they do make LEDS with different current ratings, I have seen low-current LEDs with a current rating of only 2 milliamps. If you have this info, we can calculate what resistor you need.
  7. ezdays

    ezdays Out AZ way

    What's the value of the resistor? If you don't know, what's the color code reading from the end to the middle of the resistor? You probably have too low a value resistor causing too much current and heating it up.
  8. tverskaya

    tverskaya Member

    The package only says that they're white light emitting diodes. Might this imply that purchasing LEDs from Thailand isn't the best idea?

    Looked up the ebayseller's current, similar offerings, and chances are that they are the following: 3,5 - 3,8V, 20 mA, 60 mcd (I believe the resistor should take care of the voltage difference?)
  9. ezdays

    ezdays Out AZ way

    Now I'm confused. Do you have a resistor in series with the LED or not? They need to be separate components. The LED's won't work very long on 12 volts, so you must have something in series to limit the current.
  10. tverskaya

    tverskaya Member

    About reading the resistor - is there a direction and should the bands be divided evenly? (As in: Two in the middle thinner bit and one on each of the thicker bits at the ends?) It could also be: yellow, orange, black, gold.
  11. tverskaya

    tverskaya Member

    I have them in series.

    Transformer ---- resistor ---- long leg of the LED ---- LED ---- Short leg of the LED --- Transformer.
  12. Gary S.

    Gary S. Senior Member

    So using ohms law and assuming you have a 20mA LED:

    Resistance = voltage/current in amps

    R = 12/.02

    R = 600

    power = voltage x current

    P = 12 x .02

    P = .24

    so you need a 600 ohm, 1/4 watt resistor.

    This calculation isn't precisely correct because we are not taking into account the small voltage drop of the LED, but it is a workable solution. You could probably get by with as low as 480 ohms, and as high as 1000 ohms and it will still work. I wouldn't go less than 1/4 watt though. I suspect the ohm rating of your resistor is okay but the wattage rating of your resistor is too low.
  13. Jim Krause

    Jim Krause Active Member

    Resistance translates into heat, but you shouldn't be getting that much heat if you have the correct resistor.
  14. tverskaya

    tverskaya Member

    Thanks for solving this puzzle for me! :) A useful answer in little more than half an hour!

    Will be going to look for these next week, hope it'll work.
  15. Gary S.

    Gary S. Senior Member

    Let us know how things work out. Contributions to the data bank are always needed.
  16. ezdays

    ezdays Out AZ way

    Yellow=4, orange=3, black is the number of following zeros=0 and gold is the tolerance=5%. You have a 43 ohm resistor, far smaller than you need resulting in too much current and more heat dissapated across the resistor. You need at least a 1000 ohm resistor, (brown-black-red) and could go as high as 2200 ohms (red-red-red).
  17. Gary S.

    Gary S. Senior Member

  18. kf4jqd

    kf4jqd Active Member

    Here's a rule I use. For a 12vdc and a LED. I use a 1000 OHM (1K) resistor. I have NEVER heard a resistor getting hot over a LED!? I have seen them get hot on the final stages of a radio transmitter. When the currant bias is too high on the transitor!


    Stock up on some 1K resistors if you plan to use 12vdc. You can't go wrong!
  19. hiscopilot

    hiscopilot Member

    I like the "cookbook" electrical lessons I get here! Thanks everyone for giving your input and thanks for asking that question!!!!!!!!!
  20. tverskaya

    tverskaya Member

    Well, I was at the electronics shop today and given the same information they sold me something which apparently might be still wrong...should just have asked for 1000 ohms. When keeping the voltage low, around half power at most, the resistors don't heat up and the LEDs seem to burn bright enough. Bent some of the resistor's legs already, but might try swapping the rest. Blech.

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