How to properly measure flash trigger voltages

Discussion in 'Lighting Equipment' started by rodeo_joe|1, Mar 5, 2021.

  1. A subject that I felt needed a thread of its own.

    There's a fair bit of misinformation about the trigger voltage of older flash equipment. Mainly caused by people not having the equipment or skills to properly measure it.

    The issue is caused by the high series resistance of the trigger circuit used in most older flash designs.

    Here's a quick sketch of a typical trigger circuit used from the 1960s to the 1990s:
    Vx is the unknown trigger voltage we're trying to measure.

    Rx in the diagram is there to prevent the flash trigger connector from killing you! Or at least prevent it from giving you more than an uncomfortable tickle. The resistor needs to have a high resistance value to do this - usually several megohms.

    So just poking a 10 megohm input-resistance voltmeter onto the P-C connector or hotshoe won't give you an accurate voltage reading; not by a long shot. The reading you get will be low by a fair fraction of the true voltage.

    There are at least 3 ways around this, but only one of them is cheap to implement.
    1. Use a very high input impedance electrometer. These are rare and usually expensive bits of kit.
    2. Use a reasonably fast storage oscilloscope to 'grab' the peak voltage before it decays. Also expensive if you don't already own one.
    3. Use a cheap 10 megohm input resistance digital meter, and add a 10 megohm resistor (costing a few pence or cents), plus a bit of maths.

    These are the steps of option 3.
    Take a straight reading of the trigger voltage with the meter. Call this V1.
    Put your 10 megohm resistor in series with the meter and take a second reading through the resistor. Call this V2
    Done! Apart from a bit of long-winded maths.

    To find Vx, the unknown trigger voltage, you can use this formula:
    Vx = V1+(V1*(2*V2 -V1)/(V1-V2))
    I've used spreadsheet notation so you can easily copy it across.

    Case study.
    Here's a flash that has a particularly nasty high voltage.
    V1 on this beast measured 204 volts. You might think this would be safe on your 250 volt rated Nikon.... wrong! Because
    V2, with series resistor, measured 129.4 volts, and if we plug those numbers into the formula we get Vx = 353.9 volts. Ouch!
    Not safe on your Nikon at all.

    Just to verify that the method works; here's a storage 'scope trace of the open-circuit voltage quickly decaying as the 10 megohm 'scope probe is applied.
    You can see that the instantaneous peak hits over 350 volts (Y=50 volts/division, X=50 ms/division)
    The voltage discharges to just over 200 volts in 1/3rd of a second. Too fast to be caught by a cheap DVM.

    Once you know Vx, you can also work out the value of Rx if you care to. In the above case it's 7.35 megohms.

    Well, there you have it.
    That's how to accurately measure the trigger voltage of that old portable or studio flash.

    FWIW, the hotshoe adapter shown alongside the Sunpak AZ3600 does drop the trigger voltage to just over 12 volts. But the P-C connector voltage is still at 353 volts.
    Last edited: Mar 5, 2021
  2. Just looking at this for fun (its amazing what we call fun in the era of Covid), I think I understand this, but since I have very limited electrical knowledge, just a few questions:

    "Take a straight reading of the trigger voltage with the meter. Call this V1." - OK, assume I have a multimeter lying around that I barely understand, What basic setting do I use and where do I stick the black and red wires clips? (for hot shoe and PC cords). I assume you set off the flash to take a reading?

    "Put your 10 megohm resistor in series with the meter and take a second reading through the resistor. Call this V2" - OK, how do I put the resistor in series with the meter? (where do I stick the black and red wires, and I assume the multimeter setting stay the same as they were in step 1.

    The math I can do, but appreciate laying it out is spreadsheet format.
  3. Most digital multimeters, even cheap ones, will autorange on the DC voltage setting. So you just turn the switch to DC volts and read the LCD numbers.

    The clips need to go negative to the outer of the P-C socket, and positive to the tiny inner pin. On the hotshoe, the negative goes to a hidden connecter in the slide groove, and positive to the centre stud on the underside of the shoe.
    This might need thinner probes than are supplied with the meter. Especially for reaching the centre pin of a P-C connector. A bit of 1mm copper household wire with a slight overhang of insulation does the job for me.

    You do not want to set off the flash. That completely defeats the object of measuring the fully-charged voltage on the trigger terminals.
    There's only one way to connect a resistor in series. At the end of one or other of the probes. It really doesn't matter which one in this case.
    Last edited: Mar 5, 2021
  4. Thank you! This question comes up all the time and it will be great to have a definitive answer.
  5. Special thanks.

    In a moment of exuberance. , I actually paid out for a Wein Safe-Sync Hot Shoe to Hot Shoe with PC. I thought that I was accumulating a lot of old "strobes" and was worried about using them on newer cameras. Of course, like so many things of this sort, I used it only a couple of times and went back to "available light"o_O

    It does seem to work, however. Maybe it's just a macguffin?
    Last edited: Mar 6, 2021
  6. I'm sure the Wein Safe Sync works as advertised.
    However, the cheapest UK price I could find for the genuine Wein product was £67 (about $85 US), which is ridiculously overpriced for such a simple device.
    Whereas around £12 (15 US bucks) gets you a basic RF trigger kit that offers complete isolation of the flash from the camera with (usually) a 400 volt capable triggering slave. Plus the added facility of using the flash several yards from the camera, and extending that to a multi-flash setup.

    So even throwing in the price of a camera bracket to hold the trigger and flash, you're still well in pocket over the Wein product.

    Also, I see there are similar products to the Safe Sync at much lower prices.

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