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auto thyristor?

Brian Willis , Mar 27, 1998; 09:02 a.m.

I recently inherited a Vivitar 285 flash without an instruction manual. I am not much of a flash user, because I mostly shoot outdoors, but was thinking that this unit might be handy for occasional fill in situations or the odd birthday party. I played around with the flash last night on an old manual body (without film) and it seems to work ok. However I can not figure out exactly what an "auto thyristor" does (I assume it has something to do with varying flash intensity, but can someone clue me in). Luckily my N90s was at work, because this morning after reading the threads on Vivitar flashes I realize there may be a voltage incompatibility using older flashes on newer cameras. Would a flash isolator (like Nikon NS-15) protect a N90s from damage. The last thing I want to do is trash an expensive camera with a cheep flash.


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Brian Willis , Mar 27, 1998; 09:33 a.m.

Whoops, sorry, I found an older thread detailing the Vivitar 283 auto thyristor settings. I should have looked longer before I posted a question. I am still conceded about voltage protection; is a flash isolater the thing for this?

Glen Johnson , Mar 27, 1998; 10:23 a.m.

I don't know if this was covered in the other post or not, but what the autothyristor feature allowed was fast recycling. Instead of dumping the energy to the tube, and then shunting it to a resistor when the exposure was right, the autothyrister circuit diverted the excess energy back to a storage device, so the capacitor didn't have to start from gorund zero after every flash. For close subjects, recycle times were dramatically reduced.

The flash isolator sounds like a good idea.

Gerry Siegel (Honolulu) , Mar 27, 1998; 04:45 p.m.

I am using the Nikon AS-15, the hot shoe- to -PC cord connector that was recommended elsewhere in net for portable Vivitars. I am still really unclear, with the goshawful little I know of electronics, how this coupler acts to "isolate" the flash circuitry from the camera electronics. Does it have some semiconductors, resistors or whatever inside? I opened this question in connection with trigger voltages from Vivitar flashes, which I was told vary by model and may indeed be high enough to damage synch circuits.(haven't done any lab tests on these voltages, nor been able to reach Vivitar yet.) It would be comforting to know for sure though that a flash insulator(the AS-15 and its kin) indeed insulates the flash electronically. Thanks to whomever if you care to respond. G.S.

Dan Brown , Mar 27, 1998; 04:50 p.m.

I am going to hazard to offer a little engineering talk here. Glen, feel free to correct me if I error.

A flash functions by storing an electric charge in a capacitor. When the flash is fired, the charge is transferred from the capacitor into the xenon flash tube creating light. The charge transfers very quickly and the flow of current is rather high. So high that a simple transistor can't be used because the high current would destroy its delicate silicon junction.

Originally, solid state devices called an SCR's (silicon controlled rectifiers) were used to fire the flash. This device had an input, called a gate which turned it on, allowing the high current pulsh to flow (without destroying it and at reasonable parts cost). The problem was (and is) that there is no way to turn the SCR off. What this means is that the flash always fires at full power, like a flash bulb.

The interim solution to this problem was to add a second SCR which could be turned on (gated) during the transfer of the charge to the xenon tube through the first SCR, but diverting the remainder of the charge to another place (Glen mentioned a resistor). In this way, the first SCR was fired to start the flash and the second to end it (i.e. auto or TTL control). However, all of the charge from the capacitor was used up for every shot, and the batteries would have to fully charge the capacitor after every shot. This made recycle times long and battery life short.

The thyristor vastly improves this because if can not only be turned on, but can also be turned off. When it is turned off, the charge simply stops flowing, and whaterver is unused remains in the capacitor. Then, the battery only needs to "top-off" the capacitor. If only a little of the charge is used, such as for a close-up photo, then the recycle time is much shorter, and battery life is improved.

This is why thyristor flashes are much better, and practically the only kind manufactured today. Thyristors are good.

Andrew Booth , Mar 27, 1998; 07:41 p.m.

Dan, It's my understanding that a thyristor is essentially a generic term for SCRs and triacs (bi directional SCRs). All these devices share the property that once triggered, they don't switch off until the current stops flowing through them. The auto-thyristor circuitry therefore dumps the electrical energy to another storage device when the subject has recieved enough illumination by activating a second SCR, rather than turning off the first SCR.

Tim Dunn , Mar 28, 1998; 04:29 p.m.

If you have the money, I would recommend getting the flash isolator. I just measured my Vivitar 285 and it puts out a constant 8V across the two terminals (I haven't looked inside to see if the flash circuit is isolated or not).

The standard way (read old) for setting off a flash is to short the two conductors (the pin and the shoe). Since older cameras used to just attach the flash sync contacts to the mechanical shutter, it was OK for the flash people to connect up their capacitor to that line. When the camera went off, all the energy in that capacitor would run through the camera and to the flash tube.

In today's new cameras, the shutter is timed electronically, so the flash has to be set off using a transistor or a solid state relay. Thus, it is essential that the current which runs across the chip does not cause the silicon to break down (either creating a permenant short or open). Running a large flash capacitor across the chip will certainly destroy it.

What a flash isolater should do is electrically isolating the two components (probably optically) and use the small amount of current that runs across the terminals to set off the flash. (I'm not sure about the exact specs for the NS-15)

Perhaps someone can give some insight on the Vivitar 283 and Nikon flash circuits, as I recall it is similar to the Vivitar 285 and is still sold.

Tim Dunn , Mar 28, 1998; 05:01 p.m.

Ok... looked inside, no obvious form of isolation circuitry, so I'd be careful. (there is another thread on this BTW)

Dave Etchells , Apr 01, 1998; 04:09 p.m.

This is probably overly pedantic, but yes, "thyristor" is generic for both SCRs and Triacs. Yes, both remain conducting until the current drops to zero, then they revert to the "off" state. The "auto-thyristor" flashes use something called a "gate turnoff SCR," in which the current flow can be interrupted by an appropriate voltage pulse to the gate or trigger terminal. This stops the flash, leaving any unused charge on the capacitor, so the next recharge cycle will be much faster, and consume less battery energy. A side-effect is that the minimum-exposure flashes from these are *very* short: Lots of fun to take pictures of flying bullets bursting balloons, etc.

Gerry Siegel (Honolulu) , Apr 02, 1998; 01:48 p.m.

< The last thing I want to do is trash an expensive camera with a cheep(sic) flash>. I hasten to note, Brian, that the model 285HV ( takes a broader range of HV power sources than the earlier 285) is a fine portable flash, still on the market and used by surprising number of professionals who hate lugging a Norman power pack. The question of voltage damage to the flash circuitry seems to be an open one still. I mean, the use of a hot shoe- to- flash connector seems desirable, but noone really knows for sure if this prevents the current from non "dedicated" (love that word, has such marketing appeal) flashguns will damage camera synch circuits. Personally, I think it's as likely as the aluminum wiring in my house causing a fire, although it is theoretically possible. Hmm, what's that smell?

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