Is a Vivitar 2800 Auto Thyristor Flash safe for my Nikon D5100 SLR?

I bought a Nikon D5100 a week ago and my father has this old camera a decade ago (it was a Yashica with a Vivitar 2800 flash) and I tried to plug it in with my D5100. Everything was fine when I used it. Thank goodness, it didn't fry up the mechanics.

My only concern is I don't want to buy a Nikon Speedlight flash since it will cost me a fortune to have one. Although I'm planning to set it up with a hot shoe adapter from eBay, will it be safe to use on the long run?

Hope you could help me on this one.

Well… There are concerns.

First, do NOT try to use any "smart" flash on your camera other than one made for it. A "smart" flash would be one that has more than one contact on the bottom of the hot shoe. (I'm not counting the "ground" contacts in the flanges at the sides of the shoe.)

I believe the "auto thyristor" flashes were not "smart". Their "auto" mechanism worked solely with a light sensor on the flash body. The only communication it has with the camera is that the camera closes a circuit between the hot shoe "flanges" and the pin in the very center of the hot shoe. Fine so far.

The other concern is that SOME older flash units put fairly high voltages on that center pin. Back when the camera had literally a simple mechanical switch that closed that circuit, this was not a problem, but it could be a problem with more modern electronic cameras that use an electronic circuit.

I do not believe your Vivitar 2800 is one of those, since its "auto" nature means that the external trigger contacts go through some "intermediate" circuitry before getting to the actual flashtube trigger coil.

If you have a digital multimeter you can test this. Simply charge up the flash and then test the voltage between that center pin and the outside shoe contacts. Or, just see the link below.
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Old flash? Single electrical contact at the bottom? Yes. Use flash in Manual or Auto mode with the camera is Manual mode.

http://keerok-photography.blogspot.com/2012/08/how-to-use-generic-flash.html

My son uses cheap old Vivitar flashes with his D50.

If it works then why not? If the mechanics are gonna mess up, it's gonna be the flash… Unless you drop your gears.

Long run? It'd be safe but I'm sure you will get bored of the functions of your flash soon. Start saving up for a Nikon speedlight. Trust me, it will be worth it!