[robski]

Just to remind folk that my background like Foxy Bob is in electronics. I was an electronics engineer for 25 years and basically lost interest 10 years ago when the majority of equipment became uneconomic to repair. By then most designs were a one chip solution and the manufactures were not going to release any information on its inner workings.

To start with I’d thought I would give you a layman’s guide to the workings of an auto thyristor flashgun. As a word of warning I strongly recommend that you don’t mess with these devices unless you have a healthy respect of high voltages with a large current (250 – 400 Volts DC generated from those tiny AA batteries)

The Bulb is a device called a Xenon tube. It has 3 connections. Applied to 2 of them is the High Voltage (ranging from 250V to 400V dependant on the power rating of the Xenon tube used). This high voltage charge is generated by a voltage converter circuit (the thing you hear whistling) and is stored in a Capacitor.

The camera uses a switch to fire the flash. When the switch operates it sends the trigger Voltage through a step-up transformer to generate high voltage pulse (4KV to 10KV dependant to the Xenon tube used). This high voltage pulse is applied to an exciter electrode on the Xenon tube to ionize the xenon gas inside the tube. The xenon gas suddenly becomes a low resistance and the energy storage capacitor discharges through the tube resulting in a short duration brilliant white light.

Old cameras used a mechanical switch to fire the flash. On flashguns from this era the trigger voltage was tapped off the high voltage supply and was in the order of 100 – 200 Volts. This meant a small step-up transformer could be used to generate the exciter pulse. Flashguns designed for electronic cameras have a much lower trigger voltage (5 – 15 Volts) and use an opto-coupler (electrically isolate) to connect to the former exciter pulse generating circuit.

OK that’s what a cheap and cheerful flash unit does. Then some bright spark came up with away of controlling the duration of the flash with a device called a thyristor. Basically a thyristor is a special kind of electronic switch used to interrupt the current flow through the tube and thereby turn the flash off. On the old auto type flashguns a sensor fitted to the gun provided the turn off signal to the thyristor. On modern TTL metering systems the camera provides the turn off signal when enough exposure has been received.

The above gives us the understanding that the brightness of the flash is not controlled but the duration.

Now we come around to the optical slave trigger device. It is fitted with a sensor sensitive to light. Most appear to use a Cadmium Sulfide photocells (CsS) used in the old cheap and cheerful handheld light meters. There is a whole bunch of theory on light sensors which is important in Gigabit fiber optic links but I won’t bore you with it here. The key thing about this theory is a big sensor has a strong output signal but is less sensitive to very short pulses and the converse is true for a small sensor.
I think the pre-flash is still long enough for the sensor to respond to it well enough to fire the slave.

Now to look at slave flash units that copes with pre-flash. In Saphire’s thread on wedding photography she has such a device. I think Metz also has a unit but it is expensive. These units expect and make use of the pre-flash. In a nutshell the electronics uses the first short flash to generate an enabling signal which is held for a period of say 0.5 second. By which time the main flash has occurred and because its path has been enabled by the pre-flash it goes through to trigger the slave.