Electronics > Beginners

Help making an camera flash trigger

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mribble:
The http://www.CameraAxe.com  has flash triggers using an optoisolator (http://www.mouser.com/ProductDetail/Lite-On/LTV-847/?qs=sGAEpiMZZMteimceiIVCBx3Y662pemTNEFNr76lpuig%3d) .  This works great for modern low voltage flashes, but older high voltage flashes exceed the component's 25V max.

The standard solution online for high voltage flashes is to use an SCR to trigger flashes.  I've tested this one and it works for high voltage flashes (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-3321-1-ND).  The problem with SCRs is that modern low voltage flashes never drop their current to zero so the SCR never turns off.  This means you can trigger a modern flash once and then you need to disconnect it from the circuit before you can trigger it again.

I would make a circuit that would trigger both low voltage and high volt flashes.  I thought a high voltage transistor would be the solution.  First I tried a standard 2N2222 NPN transistor and that worked fine with low voltage flashes.  I thought all I'd need is a high voltage transistor so I tried this one http://www.mouser.com/ProductDetail/Supertex/LND150N3-G/?qs=sGAEpiMZZMv4eh0jmGe02%2ftg6hfkwCDeGhYm1QIW7Ls%3d.  This didn't work.  It would trigger the flash (ie transistor was switched on) when I had the gate hooked attached to ground or 5V.  I thought attaching the gate to ground would turn the transistor off, but it doesn't seem to do that.  I admit there are a lot of things on that datasheet I don't understand...

Could someone help me understand what is going on here?  If you have another idea how to trigger low and high voltage flashes that would also be appricated.  I thought it would be cool if I could use the voltage from the flash to power my circuit, but I couldn't get that working either so I've decided this will need a battery.

Zero999:
If it's a high voltage flasher, it's a good idea to have isolation so if I went for an SCR I would've used a pulse transformer or opto isolator at the gate.

How about just using a relay?

mribble:
Hero, I agree that you should have isolation.  Since I'm using it with the Camera Axe it will have the 8000 V of isolation through an opto isolator I linked above.  In theory this isn't needed, but I agree it's a good safety mechanism.

This is used for very high speed photography so relays are too slow.  I need something with less than a 1 us delay.

Zero999:
A solid state relay?

Still might not be able to get down to 1µs though.

mribble:
Solid state relays are interesting idea, but when I looked at them for the voltages I needed their on times ranged from 10s to 100s of microseconds so I don't think they are fast enough unless someone knows about a really fast one.  I kind of doubt that though since for really fast things like this people don't usually use relays.

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