In a commercial optical trigger, for triggering flash units remotely using the on camera flash, there are very high resistors two 30Mohm in series. Why such a circuit would need resistors so big? This trigger works well with old flash units that have an high sync voltage (up to, and sometimes in excess of, 300V when the flash fires) or newer flash units usable with digital cameras that stay in the range of 2-6V (but also slave triggers built from the simplest circuits that have only a photodiode and an SCR work with both if the SCR can take an high voltage)
In series with a photodiode they would be too big, as a divider they would leave nothing to the circuit to trigger the strobe… What am I missing? I cant undertand their function or benefit… (and i dont have one at hand to break apart)
I attached two pictures, the first shows two slaves, same brand, same circuit, one is builtin in an hotshoe adapter the other has a plug at the back for connection to other adapters, cables or directly to the flash unit. The circuit has a photodiode, SCR, 4.7K resistor, 400V 0.047 capacitor, and two 30Mohm resistors that can be seen soldered in series! I suppose the 4.7k resistor and the capacitor are the RC to filter ambient light. The resistors are very visible in the closeup picture of the peanut slave. Triggering happens when the SCR shorts the negative rail of the flash unit and the positive center pin of the flash unit.
Best Answer
It's measuring the voltage on the high voltage supply, so any loading will drain the flash battery.
300VDC through 30M\$\Omega\$ is actually quite a large current (\$10\mu\$A), and would represent several mA being continuously sucked from the flash battery when the HV converter is operating.