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Safe Measurement

Discussion in 'Electronic Basics' started by Captain Blammo, Jun 21, 2004.

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  1. I have a couple of flash units ripped out of disposable cameras. They both
    need to have two contacts physically pressed together to trigger, and these
    contacts are relatively large, so I assume they could be conducting large
    amounts of power. The caps only have "9(2)" and "4 a" written on them, so I
    have no idea what they hold.

    Does anyone know from experience if these are likely to fry a multimeter if
    I complete the circuit with the probes? Mine has a 1000V and 10A ranges, but
    I don't know if such a brief burst would register. If it did survive, would
    it even detect anything?

    If this isn't recommended, what would be a good way to measure if large
    amounts of power are passing between the contacts? I'm just trying to
    determine if I need tough relays to remote trigger the units.

    If I do need tough relays, then for a 300V, 300uF capacitor, would I be
    right in saying that I'd need something rated at 300V, 45mA? My math and
    physics are a bit shaky, so I'd appreciate a check :)

    P=0.5 x C x (V^2)
    P=0.5 x (3x10^-4) x (300^2)=13.5W


    Thanks hugely for any help!

  2. JeffM

    JeffM Guest

    ...flash units ripped out of disposable cameras.
    They may be large to make them more physically rugged.
    Do you see an SCR?
    Not likely.
    Now you're getting close to the real problem.
    If they are conducting large currents, it's going to happen fast.
    To see that would require measuring across a resistor with a scope.

    My gut feeling is that
    the button puts a tiny DC current into the gate of an SCR.
  3. John Fields

    John Fields Guest

    More than likely, if it's like the one I'm looking at right now, all
    those contacts do is discharge a small cap (0.022µF) through the
    primary of a high voltage pulse transformer and use the output of the
    secondary to trigger the flashtube into conduction, which will dump
    the charge in the large cap into the flashtube, making the flash.

    The only actice device in the whole thing is a little TO-92
    something-or-other used in the switcher to pump up the large cap.
  4. More than likely, if it's like the one I'm looking at right now, all
    The ones I'm looking at only have the one large cap, as well as two
    transformers (one large, one small), *something* in a TO-92 package, and a
    flat translucently packaged thing labelled "233K" that I don't recognise.

    Also, when I discharged through my multimeter, it registered 412V for a
    moment, even though the only caps I have seen labelled on these are <300V.
    Is it possible that the meter misread due to the speed?

    Would I be correct in getting relays rated for 45mA, as per my earlier
    calculations? I'm thinking that perhaps the power is getting further stepped
    up on discharge, so perhaps I should get something rated for a higher
    voltage, to be on the safe side.

    Thanks for any further hints!

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