Basic flash circuit without high voltage generation. For speedlights the photoflash capacitor is charged to about 300 volts. When a flash tube conducts the flash tube's resistance could be about 2 Ohms. So for a brief time you could have 150 Amps going through the circuit.


When the trigger circuit is closed by the camera's sync connection (hotshoe or PC circuit) the charge on the small capacitor in the primary side of the Trigger transformer steps up the voltage to about 5,000 volts on the secondary side. (green path) This voltage ionizes the gas in the flash tube making the gas conductive. With the gas conductive the charge on the photoflash capacitor discharges through the flash tube and produces light. (red path)

With the discharge through the photoflash capacitor voltage keeps dropping. At about 60 volts the flash tube extinguishes. When the flash tube starts to conduct it can not be turned off. It can only self extinguish when the voltage drops too low.


Simple flashes that have only a full power output operate this way. They may have some additional electronics to reduce the trigger voltage. Most studio strobes are like this as well.

The following was used in earlier Auto hotshoe flashes. A sensor was used to determine when there was enough light for an exposure and having sensed that point the remainder of the charge still on the photoflash capacitor was dumped to a Quench tube, throwing away the remainder of the charge.

The trigger circuit is triggered to generating the ionizing voltage causing the flash tube to conduct (red path). Once the exposure value was reached the Quench tube (also requiring a high voltage ionizing voltage) provided an easier path (closer electrodes) and diverted the current away from the flash tube. 


While the Quench Tube allowed exposure control it was not a very efficient circuit. The remaining charge on the photoflash capacitor was wasted. Every time the flash was fired the recharge time would always be as long as when a full power flash was called for.

A "thyristor flash" had an electronic switch circuit that would be able to interrupt the current through the flash tube. With a series switch in series with the flash tube the discharge of the capacitor could be interrupted and the unused charge still on the photoflash capacitor could remain there and not be wasted as was done with the quench tube circuit.

The Thyristor circuit required a more complex circuit. (a simplified version is shown below). The thyristor (Silicon Controlled Rectifier - SCR) was in some ways like a flash tube. Once triggered it would continue conducting until there was not enough current available to keep it turned on. You could trigger it but you could not shut it off. So a clever technique was used to momentarily divert the current from the main path SCR in order to allow it to turn off.


As with the other circuits above the process starts with the trigger signal generating the voltage to ionize the gas in the tube to make it conductive. A small signal was created to trigger the SCR in series with the flash tube. So with the flash tube turned on and SCR2 turned on this allowed the charge on the photoflash capacitor to follow the red path shown below.

Once the sensor picked up enough reflected light from the scene the quench signal turned on SCR3. This allowed the current to follow the blue shown below. This starved SCR2 in the main path and it turned off. But because of the small capacitor C in series between the bottom of the flash tube and the top of SCR3 that current path was only momentary until capacitor C got charged up. The blue path was choked off and the remainder of the unused charge on the photoflash capcitor remained there.

Thyristor circuits are now obsolete since Insulated Gate Bipolar Transistors IGBT became practical for flash circuits. Unlike the thyristor/SCR the IGBT has a switch input rather than a trigger input. A control signal is required to turn it on and keep it on. Once the control signal is removed the IGBT will turn off.


This is about as complicated as it gets. The trigger signal would come from the microprocessor in a current flash. The above Thyristor examples left out a number of parts to simplify the explanation.


Quench tube