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HV flash trigger
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Please be positive and constructive with your questions and comments.
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erniehatt
- Posts: 96
- Joined: Tue Oct 06, 2009 5:08 pm
HV flash trigger
Hi I am wondering if this circuit would be suitable to use with a modern digital camera. Ernie
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uoip
- Posts: 127
- Joined: Wed Sep 23, 2009 5:48 pm
Re: HV flash trigger
I think so.
Some background for those who know electronics but don't know specifics about how camera flashes work:
Historically, cameras fired flash units by shorting together two metal contacts when the shutter was fully open. The flash unit itself was responsible for putting a voltage across the contacts, and sensing when the contacts went from open circuit to closed circuit. This scheme dates back to the days when cameras didn't often have their own batteries, but were strictly mechanical devices with springs, gears, and stuff, recording their images on film.
Some electronic flash units from the 1970s or so would put a high voltage across the contacts, sometimes upwards of 170 volts.
Cameras and flash units have since evolved, and modern versions of both often have lots of computerized electronics. But the basic protocol for sending the "fire" signal to a flash is unchanged. The flash puts a voltage on the contacts, and senses when the camera shorts the contacts out.
Some cameras use sturdy SCRs to signal when it's time for a flash to fire, but others use much less robust semiconductors, that will be damaged by the high voltage of certain flash units. The tolerance of cameras for high voltage varies widely; some cameras will handle 250V or so without damage, but others will be damaged by more than 10V or so. Most modern flash units use something in the neighborhood of 5V on the trigger circuit.
The idea of that circuit is to take the voltage from a high voltage flash unit, and send only about 5.6V (that's the zener diode voltage) to the camera. Further, the circuit is designed so that, when the camera shorts out its terminals, the circuit will fire the SCR and short out the terminals that the flash sees, thus firing the flash.
Some background for those who know electronics but don't know specifics about how camera flashes work:
Historically, cameras fired flash units by shorting together two metal contacts when the shutter was fully open. The flash unit itself was responsible for putting a voltage across the contacts, and sensing when the contacts went from open circuit to closed circuit. This scheme dates back to the days when cameras didn't often have their own batteries, but were strictly mechanical devices with springs, gears, and stuff, recording their images on film.
Some electronic flash units from the 1970s or so would put a high voltage across the contacts, sometimes upwards of 170 volts.
Cameras and flash units have since evolved, and modern versions of both often have lots of computerized electronics. But the basic protocol for sending the "fire" signal to a flash is unchanged. The flash puts a voltage on the contacts, and senses when the camera shorts the contacts out.
Some cameras use sturdy SCRs to signal when it's time for a flash to fire, but others use much less robust semiconductors, that will be damaged by the high voltage of certain flash units. The tolerance of cameras for high voltage varies widely; some cameras will handle 250V or so without damage, but others will be damaged by more than 10V or so. Most modern flash units use something in the neighborhood of 5V on the trigger circuit.
The idea of that circuit is to take the voltage from a high voltage flash unit, and send only about 5.6V (that's the zener diode voltage) to the camera. Further, the circuit is designed so that, when the camera shorts out its terminals, the circuit will fire the SCR and short out the terminals that the flash sees, thus firing the flash.
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erniehatt
- Posts: 96
- Joined: Tue Oct 06, 2009 5:08 pm
Re: HV flash trigger
Thanks, uiop for the info,
my reason for asking whether it is suitable was , I believe the circuit was originally designed for a Triac, but an SCR has been substituted and was wondering if this was OK, it is to be used in an Air Gap flash, which shows 300 volts at its terminals. Ernie
my reason for asking whether it is suitable was , I believe the circuit was originally designed for a Triac, but an SCR has been substituted and was wondering if this was OK, it is to be used in an Air Gap flash, which shows 300 volts at its terminals. Ernie
Please be positive and constructive with your questions and comments.