Old flash units - both studio and hotshoe-mount - used pretty high voltages between the camera and the flash - often from 25 to 250 volts. This is because the flash units were fired by simple switches - electrical contacts.
Modern cameras, however, rely on electronic circuitry rather than electric switches. This allows for more flexibility and the possibility for computerization, but the circuits can't withstand high trigger circuit voltages (anything above 6 volts, in the case of EOS cameras, according to Canon) and can be damaged by units with high trigger voltages.
Note that this 6 volt limit does not necessarily apply to PC sockets. Canon states that its 1D digital camera, for example, is capable of withstanding trigger voltages of up to 250 volts when firing flash units with its PC socket. The 6 volt limit applies to the camera hotshoe only. Unfortunately Canon doesn't always state what trigger voltage the PC sockets on all of its PC-socket-equipped cameras can withstand, so if this information is not supplied in the manual you should probably contact Canon.
Anyway. If you intend to connect an old flash to your EOS camera's hotshoe be absolutely sure that its trigger voltage does not exceed 6 volts. You can measure this with a voltmeter. Various accessories, such as the Wein Safe-Sync hotshoe unit, can be used to protect the camera from these high voltages if you want to use such a flash. Even safer are optical triggers, since there are no physical connections between the camera and flash unit at all.
Note that the damage to the camera can apparently be subtle and cumulative -simply hooking up the flash and seeing if it works is no guarantee that the high voltage isn't slowly damaging your camera's flash circuit. (of course, Canon is probably being a bit conservative with its 6 volt limit, so you probably aren't taking a huge risk if the voltage of your flash unit is a tiny bit over) Note also that the power supply used by the flash is irrelevant - it has no bearing on the trigger voltage. Many Canon Speedlite flash units, for example, can use high voltage battery packs but they still have low trigger voltages. And portable battery-powered flash units may require 6 volts in battery power but nonetheless may step up the trigger voltage considerably.
An additional problem is that some older flash units have reversed polarity. EOS cameras all have a negative ground and a positive centre pin on the hotshoe itself, though some pro models have polarity-detecting PC connectors that can work with either type of flash unit.
Finally, some flash units have all-metal hotshoes. This can be a problem if they inadvertently short out any of the four small data contacts on EOS cameras. If you have such a camera you could cover up the contacts with electrical tape or use a PC cord adapter so the flash unit doesn't plug directly into the camera's hotshoe mount at all. The same applies if your flash unit has a really large central contact. EOS cameras have fairly small hotshoe central contacts with four tiny data contacts below it. If your flash unit's hotshoe contact is so large that it shorts out any of the data contacts you may damage your camera.
The old Canon EOS FAQ also has a great deal of information on the subject of trigger voltages, and Kevin Bjorke maintains a comprehensive table of trigger voltages for various flash units.
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