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05-28-2013, 08:55 AM   #1
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Thyristor

What is the difference between a thyristor and a flash or are they the same thing ? Sorry to sound so ignorant folks.

05-28-2013, 09:27 AM   #2
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A thyristor flash is a type of automatic flash that pre-dates TTL. There is a sensor (thyristor) on the flash that can set the power of the strobe.

What is a Thyristor flash? - Yahoo! Answers
05-28-2013, 10:16 AM   #3
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Flash units that have a thyristor will usually be labelled "auto", "thyristror", or "auto thyristor". And as boriscleto said, you will be able to see a small electronic eye on the front of the flash unit, where it faces your subject.

Such flash units are quite easy to use on a modern camera, provided the trigger voltage of the flash unit is not high.
05-28-2013, 11:19 AM   #4
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QuoteOriginally posted by Tanzer Quote
, provided the trigger voltage of the flash unit is not high.
Yes, be careful with old flashes on a DSLR, as they can damage your camera. Read more about the trigger voltage, or use disposable wireless triggers

05-28-2013, 12:05 PM - 1 Like   #5
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What is a thyristor?

Note, for all those who have asked and responded,

a thyristor is a semiconductor device developed in the late 1960's, which is able to block voltage in the off state, and which can be turned on by a single pulse of current into it's gate terminal. Once on it is self sustaining, i.e. it will remain conducting until the current goes to zero, at which point it is turned off.

For the real electroncis types, a thyristor is a 4 layer semiconductor, (P-N-P-N) and usually modelled by a PNP and NPN transistor, cascaded such that the collector of the NPN and base of the PNP are common, as are the Base of the NPN and the collector of the PNP. The Cathode is the emitter of the NPN, the gate is the base of the NPN, the anode is the emitter of the PNP. If sketched out it is easy to see that this device can latch up and be a self sustaining switch once on. The beauty of thyristors are that they are extremely rugged and reliable devices, capable of supporting high voltages, and very high pulse currents (like those in a flash tube) without damage.


Camera flashes use thyristors for controlling the light in the xenon arc tube (Flash tube) of the unit, by controlling the time the arc is present.

A camera flash will have 2 thyristors,
- a main thyristor, which is triggered through the central hot shoe contact to start the flash, and
- a second, which is triggered either by the sensor built into the flash (on an auto flash) or by a sensor built into the camera (TTL and now P-TTL), or by a simple timer circuit (manual) in order to force the current to zero in the main thyristor by firing a resonant circuit,

The gate of the main thyristor can be fed either by a low voltage circuit, or from the main capacitor at very high voltage through a resistor. This latter type of drive is what was used in old mechanical cameras through contacts on the shutter, and it is this type of circuit that can damage newer cameras.

Last edited by Lowell Goudge; 05-28-2013 at 12:33 PM.
05-28-2013, 12:52 PM   #6
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Thanks Lowell, I'm now skeptical about a few of the flashes i will be getting and some I'm fairly confident will be ok. Also I need to order a safe sync hotshoe and will be using some wireless triggers as Na Horuk suggested. Now for the research to begin on getting the right stuff, unless you or someone out there, can help me with confidence on what I need. My budget is very limited or I would not have purchased the equipment as I did from the auction on the other hand I want everything to work without causing damage. Once again, Thank You all.
05-28-2013, 01:55 PM   #7
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QuoteOriginally posted by marquis1955 Quote
Thanks Lowell, I'm now skeptical about a few of the flashes i will be getting and some I'm fairly confident will be ok. Also I need to order a safe sync hotshoe and will be using some wireless triggers as Na Horuk suggested. Now for the research to begin on getting the right stuff, unless you or someone out there, can help me with confidence on what I need. My budget is very limited or I would not have purchased the equipment as I did from the auction on the other hand I want everything to work without causing damage. Once again, Thank You all.
i am not sure where, but somewhere there is a list of flashes and their trigger voltages and whether they are safe or not.

It might be in the accessories forum.
05-28-2013, 02:00 PM   #8
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QuoteOriginally posted by Lowell Goudge Quote
i am not sure where, but somewhere there is a list of flashes and their trigger voltages and whether they are safe or not.

It might be in the accessories forum.
This list is old, but the voltages don't change...

Photo Strobe Trigger Voltages

Look for older Pentax flashes, especially the SA & T versions. The AF 280T is one of the best old flashes out there. Don't let the average price in the review database, I think the $550 in the first review is a mistake.

Flashes (Genuine Pentax - Legacy) - Pentax Camera Accessory Review Database


Last edited by boriscleto; 05-28-2013 at 02:08 PM.
05-28-2013, 02:09 PM   #9
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QuoteOriginally posted by boriscleto Quote
This list is old, but the voltages don't change...

Photo Strobe Trigger Voltages

Look for older Pentax flashes, especially the SA & T versions. The AF 280T is one of the best old flashes out there.
Or if you want a handle or potato masher style, the AF400T will blister paint at twenty feet.
05-29-2013, 07:15 PM   #10
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QuoteOriginally posted by Lowell Goudge Quote
i am not sure where, but somewhere there is a list of flashes and their trigger voltages and whether they are safe or not.

It might be in the accessories forum.
Or...if you have a DC voltmeter, simply measure the trigger voltage.


Steve
05-30-2013, 10:45 AM   #11
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QuoteOriginally posted by stevebrot Quote
Or...if you have a DC voltmeter, simply measure the trigger voltage.


Steve
The only reason I did not mention this was that there are issues depending on the quality of the volt meter, and its design, which can lead to big mistakes.

I posted the results of my sunpak auto zoom 3000 flash with different volt meters and scales, and can show you, as a function of what meter and what scale you use, the 200volt trigger voltage could read as low as 1.5 volts on a cheap volt meter.
05-30-2013, 11:04 AM   #12
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QuoteOriginally posted by Lowell Goudge Quote
The only reason I did not mention this was that there are issues depending on the quality of the volt meter, and its design, which can lead to big mistakes.

I posted the results of my sunpak auto zoom 3000 flash with different volt meters and scales, and can show you, as a function of what meter and what scale you use, the 200volt trigger voltage could read as low as 1.5 volts on a cheap volt meter.
This is probably why digital VOMs are preferable. The old style needle meters, unless they are virtually professional quality, and expensive, probably have a low enough resistance to drop the voltage they're trying to measure. I'd imagine that the old vacuum tube VOMs would do the job, but they're ancient and costly. Any hams out there with tech chops on this sort of thing?

I'm getting old to remember vacuum tube VOMs......
05-30-2013, 11:48 AM   #13
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QuoteOriginally posted by grhazelton Quote
This is probably why digital VOMs are preferable. The old style needle meters, unless they are virtually professional quality, and expensive, probably have a low enough resistance to drop the voltage they're trying to measure. I'd imagine that the old vacuum tube VOMs would do the job, but they're ancient and costly. Any hams out there with tech chops on this sort of thing?

I'm getting old to remember vacuum tube VOMs......
Not even all digital meters have high enough input impedance.
05-30-2013, 02:04 PM   #14
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QuoteOriginally posted by Lowell Goudge Quote
Not even all digital meters have high enough input impedance.
Could you mention some that do have a high enough impedance?
05-30-2013, 02:10 PM   #15
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QuoteOriginally posted by grhazelton Quote
Could you mention some that do have a high enough impedance?
It may be simply be semantics, by saying not all have high enough impedance, it automatically implies some do.

What you are looking fore specifically is meters with a minimum input impedance of 10-20 meg ohms
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