there must be a reason why they set the voltage low.
There is a lot of hearsay and paranoia on the internet. The most weird is the commonly repeated advice that Canon cameras can handle 250v on the pc sync socket but only 6v on the hot shoe.
In my opinion, this is nonsense. The two will either be connected together or use a similar circuit. No designer would put a component rated at 250v on the pc sync but decide to find a lower rated component for the hotshoe circuit.
I think the reality is that Canon are worried that someone might slide a charged flash onto the hotshoe and have the centre contact temporarily touch one of the other pins which handle TTL control and which will be more susceptible to damage.
The idea of a designer purposefully choosing a trigger component which could be damaged by anything over 6v is clearly nonsense when a 400v is the normal rating for a thyristor and they are available for very little money, especially when bought in bulk (perhaps 8p each). It's actually very difficult to find a thyristor rated lower than 250v and it certainly wouldn't be done to reduce costs.
Even my cheap Cactus radio triggers have a 400v rated thyristor in them so there is no reason not to fit them to every camera.
Before digital and even after AF and other camera electronics, no one gave a second thought to connect any flash to any camera. Everyone just assumed they would work together (other than propriety TTL) and they did.
Surely the flash doesnt send much if any power back into the camera?
The amount of power is miniscule. Whilst it is possible and common for the voltage to be the same as the main high voltage flash supply, the sync voltage is derived via a very high resistance charging a very small capacitor. It is only this very small, low energy charge which is discharged into the trigger transformer by the camera.
The trigger transformer works a bit like the HT coil in a car and creates a very small high voltage pulse which ionises the xenon gas in the tube which causes it to conduct and draw current from the high voltage supply. The tube is connected directly to the high voltage supply* so the high level of current flowing through the tube does not pass through the camera.
(* there is usually a thyristor in series to stop the tube conducting when the TTL circuit has decided that it has received enough light)
Steve.
