Strobing a high power LED with a constant current power supply?

Discussion in 'General Electronics Chat' started by Razor Concepts, Jul 10, 2014.

  1. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
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    Hello, I noticed on eBay there are lots of low-cost high power constant current LED drivers.

    Say I want to build a high power LED strobe light. I could potentially wire a FET on the output of the constant current driver, and switch the FET rapidly to create a strobe effect on the high power LED. However, I'm worried that the constant current driver will get "confused" by switching - If it doesn't react fast enough, could it send too high of a current into the LED? Thanks!
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What would be the maximum strobe frequency?
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You can use an emitter follower CC driver for this task, it's very reliable and can run at high speeds.

    I drive a NPN transistor base at 0v and 4.5v (from a PIC digital output pin) then connect it's emitter to ground via the current sense resistor (will see approx 3.9v on resistor) and the LED goes between PSU + (+12v?) and the collector.

    The current is set by the value of the emitter resistor. I = 3.9v/R

    The circuit is good up to MHz, assuming your transistor is fast enough. It's a great way to pulse high current LEDs for IR comms etc.
     
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  4. wayneh

    Expert

    Sep 9, 2010
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    Elegantly simple. Seems like this would allow you to avoid buying a constant-current driver as the OP was asking about, and just use any old power supply.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    Some of the cheaper LED drivers are nothing more than a blocking oscillator - the flyback pulse is energy limited by how many lines of flux will fit in the inductor core.

    There is a way to bias a separate feedback winding from a C/R network to produce discrete pulses.
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I would find one with dimmer capability. Then drive the pwm pin. That way the current comes up instead of needing to go down. Or at least limit the current to something safe for the led at the maximum output voltage of the driver until it has time to recover.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yep, and it's inherently safe because any overcurrent situation causes the transistor to bias off. Making it immune to HF overcurrent spikes etc.

    I can't claim originality, it's something I learned from looking at commercial designs when i worked in repair.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    A big part of wisdom is learning to stand on the shoulders of giants.
     
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