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According to Cree Inc Pulsed Over-Current Driving of Cree® XLamp® LEDs app note, driving LEDs with high current pulses can be performed as long as the following guidelines are respected:

Based on the 1-KHz pulse testing we have reviewed in this application note, Cree suggests the following guidelines for pulsed current operations:

  1. For duty cycles between 51-100%, do not exceed 100% of the maximum rated current;
  2. For duty cycles between 10-50%, do not exceed more than 200% of the maximum rated current;
  3. For duty cycles less than 10%, do not exceed more than 300% of the maximum rated current.

Although these guidelines may hold true for Cree's XLamp LEDs, are they applicable to smaller signal (5mm) LEDs?

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1 Answer 1

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The demise of LEDs occurs typically not due to overcurrent, so much as overheating due to the overcurrent.

Cree XLamp LEDs, as also some other high intensity LEDs (e.g. Piranha), are designed with special consideration for rapidly drawing heat away from the light-emitting semiconductor junction within the LED package. This allows for higher pulsed currents than designs with less effective heat extraction.

If the overcurrent pulse duration is short enough, and blanking duration for cooling off long enough, even the cheapest and most commonplace nameless LEDs can be driven with pretty extreme currents, well beyond specification.

The point to note is that duty cycle alone isn't enough to determine safe current limits, as even 1% duty cycle could be catastrophic if individual on-pulses are long enough to fry the LED, even before the blank pulse comes along to cool off the junction.

To determine how far an LED can be driven, experimentation up to the point of letting out the Magic Smoke on at least a few LEDs of a batch, would be the way to go.

I have successfully pulsed nameless 3 mm red 20 mA rated LEDs with as much as 100 mA, using on-pulse duration of 100 microseconds at 1% duty cycle, but there isn't really any practical reason to drive LEDs that hard, is there?

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  • \$\begingroup\$ Good answer! I've pulsed those little red ones pretty hard too, I'm surprised how much they can take. \$\endgroup\$
    – Garrett Fogerlie
    Commented Dec 16, 2012 at 12:29
  • \$\begingroup\$ There are reasons to drive LEDs that hard, like multiplexing. Let's say you've got 8-way multiplexing, so no individual LED is on longer than 1/8 of the time. You'd like to run 8x the full current thru them, but that's not going to be possible. What this says is the faster the multiplexing rate the better, but you have to assume you get diminishing returns after a while and can't get back to average 100% rated current even at high frequency. \$\endgroup\$
    – Olin Lathrop
    Commented Feb 2, 2013 at 23:37
  • \$\begingroup\$ Couldn't another reason be not using current limitation resistor (or a smaller one) to waste less power? \$\endgroup\$
    – clabacchio
    Commented Mar 18, 2013 at 13:14
  • \$\begingroup\$ @clabacchio That's a good reason, and one I'm seeing implemented in practice all over the place for the little LED toys and gimmicks: Save a resistor, use lower duty cycles, yet get a good visibility out of the cheapest possible LEDs. \$\endgroup\$
    – Anindo Ghosh
    Commented Mar 18, 2013 at 13:20
  • \$\begingroup\$ I think it's not just about saving the resistor (meaning the component), but the power dissipated on it. Personally I find the idea interesting... \$\endgroup\$
    – clabacchio
    Commented Mar 18, 2013 at 14:12

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