150,000 Flashes Later

A project log for Edgerton, A High-Speed LED Flash

Affordable photography tool used to capture images of bullets with no apparent motion blur

tyler-gerritsenTyler Gerritsen 05/29/2019 at 02:250 Comments

Strobing the LED's at several times their rated voltage is one thing, but a good flash needs to have a useful lifespan.  I began testing the lifespan of the unit a couple weeks ago.  Testing was done using only one LED (too expensive to risk multiple LED's) strobing at 1 hZ.  I started at 70 V, cycling 10,000 times at 1 microseconds and 5,000 times at 4 microseconds.  The LED was aimed at an 18% grey card 20 cm away and a DSLR photographed the card every 20th flash.  After the LED cycled 15,000 times I incremented the voltage +5 V and started again.  At 95 V, the LED showed no sign of quitting so I increased the flash rate to 2 hZ and photographed every 50th flash.

Well, it turns out that the LED could handle all the way up to 125 V - and I stopped the test as the circuit board wasn't designed for more than that!  The photos were batch analyzed with ImageJ and no fluctuations in light output was observed.  Above is a photo of the LED with 150,000 flash cycles vs an LED that didn't go through the torture test (0.3 seconds vs >0.001 seconds total illumination time).  Interestingly a single element died at some point, but it still conducted current and light output was barely affected.

Afterward I disconnected part of the board and began strobing at 20 hZ.  I continuously increased voltage until the LED showed damage.  It happened somewhere around 200 V (unfortunately I had an issue with the voltage measurements and am not sure exactly what the voltage was).  

I peeled the potting material off the LED and put it under a microscope (photo above).  The little wire on the left side connects the centre slot-shaped LED element to the next LED in the series (these LED's are actually several parallel sets of elements in series).  The wire on the right was fried right off the the edge of the element!

Given the results of this testing, I've increased the driving voltage from 75 V to 110 V.