Modding a Vivitar flash unit

The original automatic flash was invented by Harold Edgerton back in the

1940s (one of his very large number of contributions to the high speed photographic art, including the original flashtube).

Edgerton's worked by putting a lower-voltage, blacked-out flashtube in parallel with the main one. Triggering the low voltage tube turned off the high voltage one, reducing the total light output accordingly. Nice idea.

The bad news was that it dumped the whole charge on the capacitor regardless of the flash duration. By about 1975, manufacturers were using high voltage transistors to turn the tubes off, which saved a tube and, more importantly, saved battery power and improved the recycle time.

If you aren't comfortable tracing out the schematic of the flash and figuring out how it works, the simplest thing would be to put a piece of white card in the field of view of the flash but outside that of the camera. Moving it in and out will cause the sensor to turn the flash off at different times.

Cheers,

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
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email: hobbs (atsign) electrooptical (period) net
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I may have misspoken--a couple of neurons connected with the idea of SCR commutation--and SCRs were cheaper than HV transistors back in the '70s.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

My wife worked for "Doc" while I was a student at MIT.

What I did in the mid-60's...

Patented in a flurry of applications when I left Motorola in 1968 and went off to Philco-Ford with John Welty :-)

IIRC this also dumps all the charge remaining :-(

...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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      Remember: Once you go over the hill, you pick up speed

I've seen series IGBTs used to cut off the flashtube.

John

While I've not done the flash mod, I know of the technology they're using, put an IGT in the ground lead of the strobe tube and can turn off the flash at any point for the lower energy output. This is also used for pre-flash to avoid red-eye and perhaps focus.

So you need to add one of the gate transistors in series with the tube to turn it off, turn on is still triggered by that small scr switched pilot pulse to the strobe's reflector (in a P&S cam I recently dismantled).

Grant.

That's current technology, the series switch is tiny too, I got two examples from small cams I opened up now. I mistakenly called it an IBT or something in my reply, couldn't remember the name for the letters.

Grant.

Do you want to control the power, or the duration of the flash? Or, do you want to make multiple repetitive flashes at part-power?

To control the power, a few neutral density filters (or a pair of polarizers you can cross) is easy.

To control the flash duration, the builtin sensor (in 'auto' mode) just looks at the light it gets in; you can put a periscope from the sensor to the flashtube, and play with neutral density filters... or relocate the sensor so it sees the flashtube through a pinhole.

To make multiple flashes, consider charging a coil (auto spark style) instead of using a capacitor. Ripple current can kill cheap capacitors easier than cheap coils. Roughly speaking, it's just a matter of driving the coil primary with fixed frequency/variable duty cycle, exactly what all PWM schemees do well at.

Grant wrote in news: snipped-for-privacy@4ax.com:

yes,it uses a photocell to measure flash output reflected off the subject and surroundings,then cuts off the thyristor.

he already said it was a -thyristor- flash unit,cheaper flashes used only a trigger coil and always dumped the full charge into the flashtube,they had no control once the flashtube was triggered. So,all he needs is a circuit to control how long the thyristor stays on,tied into the existing photocell unit. no need to add another thyristor.

cutting off the flash early also leaves some charge in the capacitor,so it takes less time to recharge to full.

--
Jim Yanik
jyanik
at
localnet
dot com

If I might interrupt my own thread to address a more fundamental problem.

I determined that the voltage on the flash's trigger pin was ok for my Canon XT, so this morning I mounted the flash on the camera. And it doesn't fire. I bought the camera used, and have never mounted anything in the hot shoe before, so it may have been dead from the beginning.

So, in troubleshooting, I've found that:

The built-in flash pops up and flashes properly.

When the flash is mounted in the hot shoe, the built-in flash will not pop up. That means the little switch in the hot shoe is working, and the camera is responding properly.

When I short the flash center pin to its shell, the flash fires. The test button on the flash also works.

I inserted a popsicle stick on one side of the hot shoe to depress the "external flash is present", then tested resistance between the center pin and ground. I get infinite resistance measuring one way, and a diode drop the other way - I assume this is a protection diode of some sort for when the current shuts off.

I then measured resistance when the shutter is triggered, and see nothing on my analog meter. Not a twitch. But, I don't know how fast it would be, so I don't know whether it should twitch or not.

Well, unless there's a camera setting I haven't found to enable external flash, it seems my hot shoe isn't operating.

Anybody have any words of wisdom on this?

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

"let's do something useful for a change" (place a comma wherever you like...)

here is link to an interesting pdf document about Doc:

Thanks, Jure Z.