Wire-Wound Resistors / So Hot ?

Discussion in 'The Projects Forum' started by Leccy-Lee, Aug 21, 2008.

  1. Leccy-Lee

    Thread Starter Member

    Aug 2, 2008
    22
    1
    I am using some wire-wound resistors (large white blocks) just so i can experiment with some LEDS. There rated at 5w, and by my maths the circuit is putting 3.1w out these resistors. I realise there may get warm, but there so hot i couldn't touch them!

    Is that normal on wire-wound resistors well within there ratings ?
     
  2. pradeeba

    Active Member

    Feb 27, 2008
    104
    0
    Did u measure the current flow in that resistor?
     
  3. Leccy-Lee

    Thread Starter Member

    Aug 2, 2008
    22
    1
    I left my Meter at work today sadly!
    But using a voltmeter and ohms law it should be about 340mA
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You have to calculate the power dissipated by the resistors to be less than 5 W and not the power drawn by the leds to be less than 5 W. Measure the voltage across the resistor and find the power dissipated by it by this formula:

    P=(V^2)/R

    where

    P=power in watts
    V=voltage in volts
    R=resistance in Ohms
     
  5. Leccy-Lee

    Thread Starter Member

    Aug 2, 2008
    22
    1
    Thanks for reply, i already calculated that as 3.1w above. So everything seems ok, just wondered if they usually meant to get quite so hot?
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    maybe they require a heatsink to be able to dissipate 5 W
     
  7. Leccy-Lee

    Thread Starter Member

    Aug 2, 2008
    22
    1
    Ah well, i have run them for over an hour, and although they smell warm and hot to touch, nothings cooked yet!

    I will endeavour to make the final design more efficient anyhow, just experimenting right now.
     
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Its not good to run the resistors like that. You degrade their life time. Its better to mount them on a heatsink or fun cool them. Some resistors are manufactured in an alluminium case and can be mounted on a heatsink
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Those sand resistors can run hot safely, but it's best if they get mounted in the air - not in contact with the PCB - for extra cooling air flow. The extended lead length also insures the solder joints won't degrade.

    There is another type of wirewound made by Ohmite that has the resistor inside an aluminum case for heat dissipation. Just screw them to a heat sink.
     
  10. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    You should check the specification sheet for the resistors. They may require a
    heatsink or cooling to get to 5W. I know that 25W and 50W resistors in the metal
    shell derate to half power without cooling.

    (* jcl *)
     
  11. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    Wouldn't hurt to check the actual resistance either, you may have one that's not properly marked, or one that's way out of tolerance..
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It's always a good idea to calculate resistor power dissipation beforehand, and then double your result to get the minimum wattage rating to use for your resistors (and any other component.) Operating components near their ratings will adversely affect their service life due to the thermal stresses. Heating/cooling cycles eventually result in metal fatigue. The more severe the thermal cycle, the more rapid the failure.

    Since you calculated 3.1W, your minimum power rating is 6.2W. Select a resistor (or combinations of resistors) that will have the capacity to dissipate >= 6.2W of power. There is always a tradeoff between cost, space available and MTBF.
     
  13. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    If the resistor is dissipating 3W it will be too hot to touch but this is OK. Too hot to touch is not really all that hot and is nothing for a wirewound resistor.

    Don't worry about it.
     
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