Pulsing LEDs

Discussion in 'Lighting' started by Swami, Aug 9, 2011.

  1. I heard talk of these years ago, but have yet to see one. The theory is simple:

    1. LEDs have the fastest on/off time (nanoseconds) of any light source.

    2. Pulsing does NOT shorten LED lifespan like it might wiht other light sources.

    3. It takes a plant so many milliseconds/microseconds to process a photon, so there is no benefit to a continuous light source from the aspect of photosynthesis.

    4. Finding the proper on/off time could save lots of power - probably as much as 70%.

    5. LEDs driven harder (more light ouput) have shorter lifespans due to heat.

    6. Pulsed LEDs have plenty of time to cool off when pulsed; therefore can be driven at maximum output with zero degradation in lifespan.

    7. Heatsinks are a large part of the cost of LEDs and could be shrunk 70% or more in size.

    In the interim, I have accidentally come up with my own pulsed LED. I got a poorly engineered Chinese LED driver (duh!). I think the LED draws maximum current (maximum brightness) which trips the overcurrent protection circuit in the driver which then resets and tries again.

    So what I have is an LED that turns on and off about once a second- which is nowehere near optimal. It is a 20w 630nm RED LED and it blinded me with four white paper towels over it during testing. Ouch!

    Because the unit did not work as advertised, I got it for free and figured I might as well use it rather than scrap it. It has been running for several days with no problem - and unsurprisingly runs very cool.
  2. Try it and let us know.
  3. Your claims of 70% power saved and 70% or more in reduced heat sink sizes is grossly over-estimated. In and off every one second is too long of a delay. You need to get into the kHz or even MHz frequency range.

    You can pulse them at certain frequencies at full output when you allow enough time for the LED Tj to cool off. Testing this with a flat temp probe between the LED and heat sink should work fine.

    Read some actual literature on the subject from a reputable LED manufacturer to get you started.
  4. As an engineer, I am well aware that my situation is not taking advantage of pulsing an LED, nor are there likely to be any real benefits. It was just an anecdotal lead in to a real discussion.

    I just see this as a field of possibility wherein the properties of LEDs could take them far from where we are now using current technology.
  5. LED's pulsed for say 200% of their 'normal' current at a 50% duty cycle appear brighter than 100% current for 100% duty cycle even though the maths says that it's all the same average current. Provided the refresh rate is sufficiently high. Your eye retains an image for ~ 1/60th second. If you go slower than this, the image fades and you can perceive the flicker. If you go faster, the decay rate is such that it can appear brighter. This may have been the reason for the choice of 60 cps AC power line.

    The attack time is significantly shorter than the release (or decay) time, making the eye tend to be a peak sensing device rather than an RMS detector. The human eye is responsive to the peak value of illumination. It is assumed here that the pulses have a repitition rate greater than 24 pulses per second. Otherwise, the perceived intensity is also a function of the repetition rate. Above about 40 pulses per second this perceived increase in brightness levels out and is no longer a factor. this varies greatly between men and women, eye (and hair!) color (melanin content), age, and heritage.

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