Quote Originally Posted by Steve Smith View Post
It would be interesting to find out what the component is in these and look up the data sheet for it.

A normal flash trigger circuit consists of a a high value resistor (about 1M) connected from the HV supply to a low value capacitor (about 10nF). All the trigger has to do is discharge the small capacitor into the trigger transformer. The amount of energy involved is very small.
I would say this was the case in the stone age. Around bronze age flashes became more intelligent and started talking to cameras in more advanced terms than "fire flash" and "quench flash", and that was the time when 5V (or 3.3V) signals became the norm. If you hook one of these stone age flashes with their 300V trigger voltage to my EOS 3 (early medieval age) it will burst in flames. So will my RZ67 (late Renaissance) and the Yongnuo RF602.

Note that the ability to handle 300V does not start and end with the actual switch, you need to make 100% sure that all insulation from that switch all the way to the hot shoe is rated for this voltage. Crammed as modern SLR cameras are, it comes as little surprise that only few modern camera models can handle high trigger voltage.