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Design of an AC adpater for hot-shoe flash - where am I wrong?

  1. Feb 22, 2012 #1
    Hi everyone!

    First post on this really awesome forum!

    I'm thinking of an AC-adapter for hot-shoe flashes, which are usually powered with batteries.

    Here a guy describes his unsuccessful attempt: he built a wooden battery pack with contacts at the right place and connected to a universal power supplier giving 300 mA @ 6V.

    The flash does not even switch on. The reason is simple: the current is not enough. My first thought was: high current is only needed to charge the capacitor of the flash. How about I add an external capacitor that stabilizes the 6V at higher currents? But how big does it need to be?

    Easy to calculate. A flash takes about 5 seconds to reload after a full discharge. A flash can flash about 120 times with 4 AA batteries of 2500 capacity connected in series. Then, the discharge current during reload must be about 2.5 Ah / (120 x 5s / (3600 s/h) ) which makes 15 A (massive!).

    So, even if I allow a drop of 2 V, I still need a 7.5 F capacitor, which is pretty expensive (although easily available on car hifi stores).

    Am I doing something wrong? Is what I'm trying to do impossible? Is it necessary to SKIP the low-voltage stadium and connect the output of the AC/DC converter to the high-voltage section of the flash, so that the required currents are lower? (there is a product on the market, which I can't mention because it would be spam, which does exactly that, but it only works with flashes that have a plug for an optional external battery pack)

    What sucks, though, is that flashes that can use power from an external supply are twice as expensive as the ones that don't! Is there a way to "hack" into the flash and connect an external power supplier even if the flash does not natively accept it?

    Thanks a lot!
  2. jcsd
  3. Feb 23, 2012 #2
    Years ago, when a hot-shoe was nothing more than a switch, I installed an external power jack on my strobe. I simply wired it to the original battery terminals. I made a belt hook for a 6 volt lantern battery and plugged it into the jack. Worked flawlessly.

    ETA: I realize that I haven't answered any of your questions -- I'm very tired. I'll try to come back to this later.
    Last edited: Feb 23, 2012
  4. Feb 24, 2012 #3
    Hi Pantaz, thanks a lot for your reply!

    It does answer some of my questions! You proved that it is possible to use a self-made device and external battery to power a flash, which is not obvious given the currents involved.

    It would be even more convenient to connect the flash straight to the AC, but maybe I can use an external battery as if it was a capacitor, just in parallel to the flash.

    Thanks again & bye bye!
  5. Feb 24, 2012 #4


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    If your problem is that the PSU won't supply a high enough peak current, then why not float a rechargeable battery across the system (or even a large capacitor) to help you with peak current? The battery solution would be cheaper than a large capacitor.
  6. Feb 24, 2012 #5

    jim hardy

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    In the thrift stores there is usually a basketfull of these "Wall Warts".

    A transformer type capable of an amp will be pretty large and heavy.

    A "switcher" will be much lighter.

    Look carefully through them and find one marked 1000ma or higher that seems too light to be true.

    either of these would probably work okay



    though at light load it'll put out slightly more than 6 volts.
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