Why use resistors on LEDs?

Discussion in 'Electronics & Electrics' started by alvarez, Sep 14, 2006.

  1. Sadow

    Sadow Member

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    The reason you dont run the leds off the same current limiting resistor is due to thermal runaway. If one led gets hotter than the others, it will draw more current than the others, which makes it hotter, which draws more current, and thus a destructive cycle begins which can destroy all your leds.

    Having said that, i have run groups of leds off the same current limiting resistor before in a proto situation and not had any problem even with running them for many hours, but i certainly would not rely on the configuration for a permanent solution.
     
  2. LethalCorpse

    LethalCorpse Member

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    There's no problem running a heap of leds in series with one resistor - it's standard practice. Since they'll all get the same current through them, there's no danger of thermal runoff, provided you've engineered the circuit correctly and put the right resistor in. The calculations are the same as for a single led, just with increased Vled. You could look at a circuit with two identical leds in series, both with 3V, 30mA as a single LED with 6V 30mA.

    There is a big problem with using leds in parallel with only one resistor though. The leds will all have slightly different V/I curves, and hence will have slightly different voltage drops - could be as small as 0.01V. Only the led with the lowest voltage drop will conduct, as it will peg the voltage across the other leds to its own maximum (the other leds will still conduct at this voltage, but will conduct much less current, as small changes in voltage for a led result in big changes in current). It will therefore carry most of the current, and if you've set up the circuit to deliver the right current to ten LEDs, that's a lot of current for one led to handle. It'll go into thermal runoff, or just pop right away, and then the bulk of the current will go into the LED with the next lowest voltage. Rinse and repeat. Soon enough, the entire chain will blow. I've actually seen this happen on the bench.
     
  3. Sock-Munkie

    Sock-Munkie Member

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    Butcher9_9: Look closely at the wiring. I'm assuming the wires have been heatshrinked. Look for a tiny bulge near one of the ends near the LED. The resistors aren't very big and almost disappear under the heatshrink. I'm sure a business like PcCaseGear would have done a proper job. :)
     
  4. SomeGuy1234

    SomeGuy1234 Member

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    No idea, but LED's on truck trailers are designed to be interchangable between 12 and 24 volt systems with no modification, tends to be why companies with a fleet of vaired brand/models of prime movers get LED lights on their trailers
     
  5. Sadow

    Sadow Member

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    for this they probably use 2 banks of LED's, and depending on wether the supply voltage is 12v or 24v they are put in series or paralell. Determining wether they are to be in series or parallell would be a fairly simple circuit, few transistors some zener diodes, normal diodes etc.
     

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