Most new digital cameras with a hot shoe use a thyristor capacitor switching circuit. They are typically spec'd to tolerate a 6V to 12V trigger voltage from the flash unit depending on the make/model of camera. Therefore, it's strongly recommended to avoid directly attaching an older flash unit if you don't know it's trigger voltage is in the range acceptable to the camera. If in your unawareness you attached a higher voltage flash, it may or may not permanently damage the camera's switching circuit, depending on how high the voltage and if used repeatedly. However, the end result is that it can burn out the thyristor capacitor. I've heard that some camera's can handle a higher trigger voltage if you use the PC connector socket on the camera meant for studio flash direct connection, if it has one, instead of triggering via the hot shoe.
If you want to use an old flash unit in the hot shoe, the best approach is to purchase a
Wein Safe Sync module that attaches between the flash's hot foot and the camera's hot shoe. The Safe Sync has an optical isolation switching circuit built inside of it to buffer the camera's switching circuit from the high voltage of the flash. I've used one for years on several older Sunpak flash units and it works great with Pentax and Canon cameras.
It also has a PC socket to use with studio flash or flash units requiring a PC cord connection and gives you the same "safe" optical isolation that way too.
For some weird reason, they don't seem to work consistently with Nikon digital cameras. (I guess there's the Nikon way and then everyone else...maybe they just want you to use their SB series flashes only...anyway, Nikons are not as robust as most other cameras in that regard)
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