Testing trigger voltage

Discussion in 'Digital Photography' started by Guest, Mar 25, 2006.

  1. Guest

    Guest Guest

    How can the trigger voltage on a flash be tested?
    Old Bob
    Guest, Mar 25, 2006
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  2. Guest

    Jem Raid Guest

    I made a flash gun once bought the tube and some huge capacitors, first time
    i fired it the copper track on the circuit board disintegrated, repaired it
    with some thick wire and it was OK.

    As I remember the trigger voltage was taken off using a resisitor network to
    bring it down to 12 to 18 volts, from about 500 I think.

    The only way I can think of to measure the voltage of such a short pulse
    would be to use an oscilloscope.

    Jem Raid, Mar 25, 2006
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  3. Guest

    Bluto Guest

    Bluto, Mar 25, 2006
  4. Guest

    Tom2000 Guest

    Disconnect the flash from the camera. Turn it on, and wait for the
    Ready light to illuminate.

    Then, measure the voltage on the sync terminals (between the hot
    shoe's center contact and the part that makes contact with the
    camera's hot shoe rails) using a digital voltmeter.

    For modern cameras, the sync voltage should be less than about 12
    volts DC, with the positive side connected to the center terminal.

    If your sync voltage is higher than 12 volts or so, you can obtain a
    gizmo called a Safe Sync from B&H (or any good camera outfit) that
    limits sync voltage to about 6 volts. If the polarity is reversed,
    you need to get inside the flash and re-wire the sync wires to the
    hot shoe.

    Good luck!

    Tom2000, Mar 25, 2006
  5. Guest

    Allan Guest

    I was just doing this before I looked at these posts.

    One of the other posters and the site mentioned both suggest a DVM to
    measure the voltage and that was my first choice.

    However, I have discovered a problem. The problem is the readings on the
    DVM's (two of them) do not agree with my AVO. For those who know, AVO's do
    not lie. Both DVM's measure 94 volts on my Vivitar 283, while the AVO says
    212 volts.

    Update, using a third DVM, I now measure 257 volts. Obviously, the first two
    DVM's have a low input impedance - less than the 20K/V of the AVO - while
    the third DVM has a high impedance.

    The first two are DVM's bought from an auto store and it looks like they are
    different from the "normal" DVM.

    You should be using a high impedance meter for measuring flash trigger

    Allan, Mar 26, 2006
  6. Guest

    Roy Smith Guest

    I'm assuming DVM is Digital Volt Meter, but what's an AVO?
    Roy Smith, Mar 26, 2006
  7. Guest

    Roy G Guest


    I tested the Trigger voltage on my 2 Vivitar 283s, and both of them were 280

    I have heard that some of the even older ones can be over 300Volts.

    Roy G
    Roy G, Mar 26, 2006
  8. Guest

    Allan Guest

    Yes, DVM is Digital Volt Meter. AVO is the trade name of a British company
    (AVO Megger) that makes high quality multimeters and other lab grade test
    equipment - considered by many as the multimeter of choice.

    Allan, Mar 26, 2006
  9. Roy-

    It must be Analog Volt Ohmmeter, which explaines the reference to 20K Ohms
    per volt.

    The AVO (or is that VOA?) may not lie, but it can load down a
    high-impedance circuit. Assuming a 300 volt range, the impedance would be
    6 Megohms. This compares to some VTVMs and DVMs, but explains the
    slightly higher reading of his third DVM.

    Fred McKenzie, Mar 26, 2006
  10. Huh? You think 20K Ohm/Volt could not apply to a DVM?

    It could just as easily...
    So can a DVM (or an oscilliscope or a VTVM).
    Floyd L. Davidson, Mar 26, 2006
  11. Guest

    Davy Guest

    AVO, they'er Test meters (Amps Voltage Ohms) but we knew them as th
    'bees knees' of testmeters - built like a tank and normally used
    moving coil meter as opposed to a digital read out

    Digital meters usually have an input around 10 M ohms per volt
    meaning that if you are measureing 2 volts the circuit will see a 2
    M ohm resistor across it's terminals

    A moving coil meter could be anywhere from 1k ohm per volt to aroun
    100 k ohms per volt, meaning when we measure say two volts an 100
    ohm testmeter will represent a 200K resistor across the circuit you

    The lower the current drawn by a testmeter the more accurate th
    readiing and modern circuits use very low currents

    It's not easy to measure the flash voltage because of the rise time
    the moment of applying voltage to trigger the flash and the time i
    takes for the meter to register the voltage, moving coil or analogu
    meters as they are known are totally useless, digital meters fare n
    different although better, so slow not to give any readings that i
    anywhere near accurate, by the flash has gone the poor ol' meter wil
    still be responding

    The correct way is to use an oscilloscope, not only could you measur
    the voltage you could also measure the 'rise time'

    I am curious to know the reason why you need to measure a voltage,
    it's something that normally isn't done even in a microwave oven it'
    never necessary to know the magnetron's supply voltage...

    Flash tubes normally use a 'trigger transformer', charging a capacito
    to around 2,000 V (2kV), the firing circuit which could be transistor
    or thyristors

    A little off topic ...

    Davy, Mar 26, 2006
  12. The voltage you are measuring has *nothing* to do with it.

    If you use a 2 volt scale, the meter will have 20M Ohms across
    the terminals (whether you are measuring 2 volts or 20 volts).
    And it is also true that a DVM might be just about any value too.
    The OP wanted to measure the trigger voltage, not the flash voltage.

    And that is exactly why the OP wants to know what the voltage
    is! The electronics in the camera may, or may not, be able to
    handle any given voltage on the circuit it uses to trigger the
    flash. Modern flash units will have less than 15 volts on the
    trigger terminals, while 20-30 year old units might have
    hundreds of volts, and may damage the electronics in a modern
    Floyd L. Davidson, Mar 26, 2006
  13. Never heard of them in North America, so AVO probably means nothing to a
    lot of readers.

    It doesn't matter what specific brand of DVM you use, but it should be
    something with at least 10 megohm input impedance to measure the
    trigger voltage accurately. Any *good* lab or portable DVM should meet
    this, though I wouldn't assume it was true of a cheap meter bought from
    an auto store.

    Dave Martindale, Mar 26, 2006
  14. Actually, no. Most digital meters are either 10 Mohm or 11 Mohm on
    *all* DC ranges except perhaps for the lowest (200 mV) range. The
    measuring circuit itself has extremely high input impedance, thousands
    of megohms, and the impedance across the probes is the impedance of a
    voltage divider. For an 11 M input meter, the divider is the series of
    resistors 10 M, 1M, 100k, 10k, 1k, ... connected in series, with a
    selection of taps that allow input voltage, 1/10 input, 1/100 input, ...
    to be connected to the measuring circuit. The 10 M input meter is
    similar, but the voltage divider chain is 9 M, 900 k, 90 k, 9 k, 900, ...
    A moving coil meter is driven by current; one rated at 100 kohm/volt
    really means that the meter is 10 uA full scale. Resistors are
    connected in series with the meter to convert voltage to current.
    So if you select the 10 V *range*, a 1 Mohm resistor is used to provide
    10 uA at 10 V. But the resistance depends on the setting of the range
    switch, not the current measurment. Measuring 2 V on a 10 V range, the
    resistor is still 1 M, not 200 K.
    True, but he's trying to measure the trigger voltage, not the flashtube
    or flash capacitor voltage. The trigger voltage is DC.
    Triggering the flash is normally done by charging a small capacitor,
    then dumping that capacitor through a switch into the trigger
    transformer primary. The transformer generates a few kV to ionize the
    gas in the flashtube, which then conducts the main flash current.
    The voltage in this trigger capacitor can be a couple hundred volts,
    and sometimes this is present right on the flash trigger contacts.

    The mechanical switches in old film cameras weren't bothered by
    switching this, but some newer digital cameras fire the flash using a
    transistor, not a mechanical switch. The safe trigger voltage for these
    cameras can be as low as 6-12 V. So the flash needs to provide a low
    voltage at the sync terminals which is used to trigger an SCR which
    switches the trigger capacitor internally.

    If you connect an old high-voltage trigger flash to a new camera with a
    low-voltage flash interface, you'll damage the camera. Permanently.
    That's why he cares.

    Dave Martindale, Mar 26, 2006
  15. Guest

    Cinder Lane Guest

    The voltage drop across the trigger is NOT measured during the brief
    interval that the flash is being discharged. AMPERAGE would be measured
    then. The voltage is continuously present -- for MORE than enough time
    to be measured by even the SLOWEST response meter.

    High-voltage damage will occur to a camera's circuit as soon as it is
    applied, whether the flash is triggered or not.

    Voltmeters are all high impedance. Although a HIGHER high impedance will
    theoretically draw less power, this application does NOT require it. It
    makes no difference whether the meter shows 185 or 300 volts -- if it is
    over 6 volts, it is too much.
    Cinder Lane, Mar 26, 2006
  16. Guest

    Davy Guest

    Yes of course Dave... You are right about the 10 mohm input business
    of Digivom's, thanks for the correction silly me the movement is
    static and not a mc.

    Davy, Mar 26, 2006
  17. Guest

    Tom2000 Guest

    Well said, Dave. Best response in the thread.

    Tom2000, Mar 27, 2006
  18. Correct, for both types of meters.
    Not for the DVM/DMM. Its input impedance is generally 10 or 11 Mohm on
    *all* scales, because the same voltage divider is always bridged across
    the input leads. The resistance is a shunt resistance, not a series
    resistance, and it doesn't change with the scale selected. So it's just
    "10 Mohm", not "10 Mohm/volt".

    (The exception is some meters that remove the divider from the circuit
    on the lowest range, often 200 mV, giving an impedance of hundreds or
    thousands of Mohm).

    Dave Martindale, Mar 27, 2006
  19. Guest

    Prometheus Guest

    Amp volt Ohm.
    Prometheus, Mar 27, 2006
  20. Guest

    Prometheus Guest

    Nah, it predates digital meters by so much that analogue was the only
    sort and there was no distinction to make.
    Prometheus, Mar 27, 2006
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