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Registered Member #97
Joined: Thu Feb 09 2006, 05:40PM
Location:
Posts: 61
My goal was to trigger a camera flash with a mosfet. The camera flash is from a disposable camera. To trigger the flash I used a mosfet as a switch, but the problem is it doesn't work. I used a push button switch and a 555 to trigger the mosfet. Is the problem turn on time? Resistance? Something else? On this site what do they mean by to flash?
...well sometimes the search function reveals exactly what I was searching for, except that the problem was'nt solved...
After killing my last disposable camera flash unit, I rebuild it and made it much more robust. (µC controlled voltage, safety discharge after too long idle etc...) This way, I thought I wouldn't have to get a new one or even buy a a 'real' flash.
But now my selfmade one (flash triggering part copied from original) needs to be triggered by the µC's logic level output. I tried with several transistors and last with a triac output optocoupler but neither works. I guess my methods trigger too slow as for a high enough di/dt in the triggering coil to produce the needed voltage spike... Has anyone an idea to solve this problem?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Please show us a schematic (even a photo of hand-drawn one). A triac or SCR, optocoupled or not, should work because the turn-on action is regenerative. Does it turn on and discharge the -trigger- capacitor when it's supposed to? (Pay attention to the amount that your voltmeter loads the circuit).
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
If you interchange the trigger capacitor and the triac, then the trigger function should work just the same (except reversal of HV pulse polarity). Which way was it in original unit from camera?
Then the switch or triac or SCR would have a main terminal on the "charger negative" net. That could be common with your µC ground net (with care about which segments carry the trigger and main flash pulse currents).
Now I have this setup (attached). Unfortunately it doesn't work, too. Notice I changed the polarity in the schematic to match my actual circuit. I also tested if the triac is discharging the trigger cap at higher voltages. With the internal resistance of my dmm these don't go higher than ~100V. Up to 90V the discharge took place, then I guess the triac died, it isn't even discharging at low voltages any more. Guess I'll get a 'normal' flash :/ But thanks alot so far!
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Camera flash trigger problem
