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Old Strobe, New Strobe…

At the risk of going way over nearly everyone’s head, I write this post.

I have recently acquired my fourth Sunpak 611 strobe (see photo below). It is an old potato-masher style strobe that uses C-cell batteries or an AC power supply that essentially doubles and rectifies the wall voltage. These strobes pack a pretty bright whollop, having a guide number of 160.

One of the four I bought is not fully functional in that the variable power ratio does not work, whether from the outboard thyristor or the power setting switch on the back. From examining schematics of similar units, it would seem that the part of the circuit that shuts off the discharge is not working. It could be a bad capacitor or SCR, I’m not sure, and I’m somewhat reluctant to open it up because of the high voltage involved.

Despite that fear of deadly high voltage, I am tempted to gut the original controller and build a new one using an application note from Fairchild Semiconductor that discusses making making a modern strobe controller using an IGBT (insulated gate bipolar transistor) device. IGBTs are simpler to use in strobe circuits because they don’t dump the full capacitor charge once turned on like an SCR (silicon controlled rectifier) does. (SCRs have long been used in strobes since forever.)

In a strobe controller, an IGBT can be turned on and then turned off again anytime during the discharge cycle. This also means that rather than power control, flash timing control can be achieved.  Very fast flash times can be obtained which can be used to stop the motion of water droplets and such.

So, rather than repairing the incompletely operational flash, I may just rebuild it some day…

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Posted in diy by david on October 24th, 2011 at 8:34 pm.

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