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09-10-2021, 09:00 AM   #1
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Old Flash Units - Can Trigger Voltages Harm User?

Knowing that many old flash units can have high trigger voltages on the centre contact (I've measured 350v), and while we know that this can damage modern electronic cameras, is this dangerous to users?

I once read a claim that it is not, because that voltage is on a tiny capacitor (some nanofarads) so the energy is very small. And surely the manufacturers would not have an exposed contact if it were dangerous? However I would not want to put that theory to the test. Googling for electrocution, the data all seems to refer to dangerous current levels (typically over 100 mA) but nothing about the current duration or energy levels. Those trigger voltages could create that current, but only for a very short time (microseconds) before the tiny capacitor was discharged.

For a shock you would need to touch both the trigger contact and the unit's return contact, and the latter is usually recessed in the shoe's mounting grooves. However, Wikipedia mentions the fact that in many (older?) cameras the X socket is connected directly to the hot shoe receptacle, so plugging a flash unit via a flying lead into the X socket will make the exposed hot shoe live at the same voltage, with both poles now well exposed. Wikipedia says that there have been cases of electrocutions this way via the user's eyebrow, and some cameras separate the circuits for this reason.

Meanwhile I will avoid touching the trigger contacts on old flash units - in fact I don't give any of them house room unless I first measure them as safe. Does anyone have more knowledge (or possibly bad experiences) with this?


Last edited by Lord Lucan; 09-10-2021 at 09:02 AM. Reason: Format
09-10-2021, 09:38 AM   #2
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QuoteOriginally posted by Lord Lucan Quote
I once read a claim that it is not, because that voltage is on a tiny capacitor (some nanofarads) so the energy is very small. And surely the manufacturers would not have an exposed contact if it were dangerous?
In general this is true, an application are the electric farmers fences for animals. High voltage, short time, low charges: annoying but not killing the animals (or accidentally humans).
Question is now how strong these Flashes are. Never heard of massively dying photographers....
09-10-2021, 09:41 AM - 2 Likes   #3
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Let's just say that I would not put the foot in my mouth when the thing is charged.


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09-10-2021, 10:25 AM   #4
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Your skinís resistance makes the current drop to very small numbers. Typically with not tally dry skin you have about 1 million ohms resistance between two hands. It takes a LOT of voltage to supply enough current to kill in that scenario. But well below that level of resistance are wet salty sweaty hands or bleeding wounds which can conduct electricity much more readily.

Sign up here and read this if you want details:

https://www.allaboutcircuits.com/textbook/direct-current/chpt-3/ohms-law-again/

My assessment is you could get a nasty jolt but it is highly unlikely to harm you. And more likely if you are working inside the unit than when using as designed.

09-10-2021, 11:16 AM   #5
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I guess, but dont know, that pace maker users should be awarw
09-10-2021, 11:18 AM   #6
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My guess is that they're not dangerous. Give you a good tingle, perhaps a small burn, though. See the way they work is by cranking up the voltage big time, which decreases the amperage. Voltage is a measure of electrical pressure, amperage is a measure of quantity, and ohms is a measure of resistance. Your skin's got the resistance, and in order to overcome that resistance, it would take a pretty good amoung of pressure; the thing is that these three are similar to the effects of shutter speed, ISO rating, and aperture with respect to light. Take the shutter speed down, you've got to elevate the ISO or open the aperture more to get the same picture. The quantum of light can remain the same, albeit somewhat refracted this way or that. Same thing with the electrical stuff - if you decrease the voltage, you increase the amperage and vice-versa. So the xenon flash tube has a certain amount of resistance, and the voltage has to be high enough to overcome that resistance, but cranking up the pressure decreases the quantity that's getting through - like using a high-pressure nozzle on a water hose-pipe. (Amperage is actually a rate, volume transmitted over time, joules per second or some such.)

So, while the pressure is great enough to overcome the resistance of your skin, it's not likely to be sufficient quantity to cause any real harm.

That's all according to theory, of course, and coming from a guy whose career in electrical engineering was cut short at age fourteen when he built a short-wave radio but got all the color-coded resisters wrong - and being color-blind, of course he resorted to photography (after having been a software engineer for twelve years and an attorney for thirty). In other words, I hope someone will correct me if I'm wrong, and I don't suggest that anyone rely on what I've said until someone who actually knows something chimes in.
09-10-2021, 12:30 PM - 5 Likes   #7
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Nope, they are not dangerous. When i was working in an electronic workshop, just for the fun of it, we sometimes charged that kind of capacitors and trow them at our colleges. As a reflex they would catch them and in the process discharge them (i often had to run )

09-10-2021, 02:41 PM   #8
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I know from personal experience that they hurt like hell when accidentally discharged. I'm still very much alive though.

Around 2012, I had taken apart an old Vivitar 285HV to find out what was wrong with the thing and dropped the thumb sized capacitor in the process. Out of reflex, I grabbed the capacitor midair and got a burn on my palm between the contacts of the capacitor and my head snapped back for my foolishness. If I recall correctly, it was ~300V.

The experience can be likened to stubbing your pinky toe on a coffee table while having someone sneak goalie salts under your nose at the same time.

2/10. Would not recommend trying it for yourself.
09-10-2021, 04:10 PM - 1 Like   #9
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QuoteOriginally posted by Gerbermiester Quote
dropped the thumb sized capacitor in the process
That would be the main capacitor that provides the energy of the light discharge itself. My question involved the much smaller capacitor used in the trigger circuit.

This is the circuit diagram of a typical older flash unit. S2 (Switch 2) near the bottom right is where the trigger contacts are shorted by the camera to fire the unit, and when this happens the small capacitor C3 is discharged (via the coil marked 12T). The capacitor you picked up would be like the much larger C2 (right of centre). Their respective values are 0.022 microfarads and 160 microfarads, so although both are charged to around 300V, C2 contains over 7000 times as much energy as C3. You seriously don't want a shock from C2, but C3 might just be a tingle - the subject of my original post.

If you touch the positive contact of S2 (which will be the centre contact on the flash foot) you would be touching C3's voltage more-or-less directly but you will be protected from C2 by the 1,000,000 ohm resistor R2 which would only allow a very small current to pass into you from C2.

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09-10-2021, 07:09 PM   #10
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From my understanding that to be fatal the current has to pass through the heart or the cardiac nerves. Electricity follows the path of least resistance so if you are touching the contacts on the foot of the flash the electricity is going to follow the path of skin between the contacts.

If you were to touch one contact per left and right hand then maybe the current might go through the heart.
09-10-2021, 07:10 PM   #11
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One thing which greatly reduces any hazard (if there is one to begin with) is that touching exposed contacts, discharges the current through one's finger or hand and the current isn't routed through the heart. Electrocution often occurs because the path is hand to hand, hand to foot, or foot to foot (often the case for lightning). Inter-pinkie isn't a heart stopper under normal circumstances, and with 300 Volts on a 0.02 microfarad capacitor less than 1 milli-Joule of energy), not much to worry about. I've had a finger discharge (450 Volt microwave power supply) which left a small entry hole charred in the finger, yet no electrocution because it exited my watchband (same hand, and yes the hole did heal up over time). Still, does one like being stung by a hornet????

Last edited by Bob 256; 09-10-2021 at 07:19 PM.
09-10-2021, 07:16 PM   #12
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Lord Lucan - thanks for the circuit - I've never seen that before - elegant yet simple. I see that the oscillator/charging circuit on the left basically charges up C2. T1 gives you a ~300:1 voltage step up! And, a good old prosaic 2N3904 in there. I've got oodles of them in my junk box.

D2 turns things off when the voltage across the NE2 gets high enough?? (For you non-circuit folks: NE2 is a little neon bulb - needs ~60 volts to turn on)

As you note, C3 may give you a little tingle, but it's C2 that will hurt.
09-11-2021, 02:35 AM   #13
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QuoteOriginally posted by AstroDave Quote
D2 turns things off when the voltage across the NE2 gets high enough??
Yes, and your reply made me notice for the first time that this flash unit has a negative polarity at the unit foot! That's if it has a foot, and the bottom rail of the circuit is earth for the camera, because it might be a built-in flash for a Kodak P&S, but that makes no difference to the principle.

It looks like when the neon turns on, ie when its upper end connected to the xenon cathode has dropped to ~ -60v (that's a minus), it pulls down the voltage at the top of R3. C2 will continue charging until the the top of R3 reaches ~ -110v which will turn on the zener diode D2. D2 then pulls a small current from the base of the switching transistor Q3 which turns that on, effectively shorting to earth the Q2/Q1 transistor pair and the main switch S1 (via R1). That stops the oscillator and the charging.

QuoteOriginally posted by Bob 256 Quote
One thing which greatly reduces any hazard (if there is one to begin with) is that touching exposed contacts, discharges the current through one's finger or hand and the current isn't routed through the heart.
I have a procedure when I know that I will be working on a circuit that could, if I have overlooked something, have live voltages in it. In something like a flash unit I short any capacitors with a 1k resistor connected to prods. I then check my multimeter works and then use it to check for voltages at salient points in the circuit. I then re-check that the voltmeter works (in case it happened to fail after the first check). Knowing that I will need to touch things sooner or later I then flick my finger over a few salient points with my other hand not touching anything - the best way to take any shock if there is one.

I also follow a rule that my father (a BBC technician) had, which was when working on mains equipment I always have its disconnected plug lying visible in front of me at the back of the workbench. I have been a ship's engineer, a railway engineer (UK sense, on a railway with ground-level live rails) and a power station engineer, so I could tell a few horror stories.

To anyone reading this generally, none of this stuff should be tackled unless you have a good understanding of electricity.
09-11-2021, 07:39 AM   #14
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I am always cautious around high voltages because my work (now mostly retired) involves AM and FM radio transmitters with voltages up to 15 kV.

I own three of the original Vivitar 283 flashes built with 280-290 volts on the shoe, but I have modified them for 5 volts on the shoe for use with my K-70 and Neewer wireless triggers.
09-11-2021, 08:23 AM - 2 Likes   #15
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QuoteOriginally posted by AstroDave Quote
Lord Lucan - thanks for the circuit - I've never seen that before - elegant yet simple. I see that the oscillator/charging circuit on the left basically charges up C2. T1 gives you a ~300:1 voltage step up! And, a good old prosaic 2N3904 in there. I've got oodles of them in my junk box.

D2 turns things off when the voltage across the NE2 gets high enough?? (For you non-circuit folks: NE2 is a little neon bulb - needs ~60 volts to turn on)

As you note, C3 may give you a little tingle, but it's C2 that will hurt.
This is a classic "old style" flash circuit. The left portion of the circuit is an oscillator which produces a high voltage on the right side of the main flash transformer (more of a pulse type waveform rather than a sinewave). Resistance in the transformer and current limiting by the oscillator circuit produces only a certain amount of current out of the transformer (capacitor C1 also has a role in limiting current). This is rectified by the left facing diode and the main capacitor (C2) charges over time. This circuit is totally dependent on the firing voltage of the (neon) lamp for its final flash lamp voltage. When that (neon) lamp fires, a negative voltage (limited by the Zener) is fed back to transistor, turning it on and killing the oscillator which stops the charging process. That (neon) draws very little current and will stay lighted until the main capacitor voltage falls significantly, at which time it will go out and the oscillator will again begin charging the main capacitor until the charging lamp again lights. In the meantime, the trigger capacitor (C3) charges through the 1 MegOhm R2 resistor to the same voltage as is on the flash lamp. When the flash contacts are closed, C3 discharges through the trigger transformer primary (R2 is essentially and open circuit during this time), producing a much higher voltage within the flash lamp which creates an ionizing path for the main discharge. Of course, that turns off the (neon) lamp and the charging process starts all over again until the (neon) lamp fires. I've seen a lot of these which run the flash lamp at 300 Volts or thereabouts, but more powerful flashes can run the flash lamp at considerably higher voltages. High speed flashes use a smaller flash capacitor to get the high speed so they must run the flash lamp at 1000 volts or more to get adequate output (and the flash lamp must be designed to take the extreme mechanical shock of the discharge). Newer circuits have greatly improved on this old classic, but the charging process is similar. New designs can stop the flash lamp discharge before it completely discharges the charge capacitor, meaning a saving of energy, and controlled length (output) discharges. This also allows very short discharge duration but at lower outputs compared to flash systems designed for high speed use.

Note: I placed (neon) in parenthesis because this is a special lamp designed to fire at higher voltages than a typical neon lamp which can fire as low as 50-60 volts. The gas mixture and electrode spacing is modified to get a higher breakdown voltage for the lamp used. As the lamp ages, it's trigger voltage will typically increase causing the flash output to increase - not that reliable a control over flash output, but it worked in "those" days.

Last edited by Bob 256; 09-11-2021 at 08:42 AM.
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