1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

ABout capacitor in photographic flash

  1. Feb 3, 2010 #1
    Hi All,

    What is the working mechanism in photographic machines that allows for obtaining High voltages from two ordinary 1,5 V batteries, using capacitor(s).
    In principle, charging a capacitor with two 1,5 V will preserve the 3,0 V between the terminals of this (these) capacitors.

    Thank you

    Sincerely

    DaTario
     
  2. jcsd
  3. Feb 3, 2010 #2

    berkeman

    User Avatar

    Staff: Mentor

    It uses a DC-DC converter, which is a type of power supply circuit. They will use either a "boost" or a "flyback" circuit.

    http://en.wikipedia.org/wiki/Buck–boost_converter

    .
     
  4. Feb 3, 2010 #3

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    "DC to DC converter"
    Look upon it as a low voltage DC to low voltage AC (high audio frequency- you can hear the whistle) converter, followed by a step-up transformer (3V'ish to a couple of hundred volts or so) and then a rectifier to get back to high voltage DC.
     
  5. Feb 3, 2010 #4
    Thank you and sorry,

    Could you express the answer in basic physical EM principles. I am not so acquainted with electronics.

    Thank you in advance,

    DaTario
     
  6. Feb 3, 2010 #5

    berkeman

    User Avatar

    Staff: Mentor

    A diode is a component that only lets current flow one way, and not the other. So when you put a voltage on the anode of the diode that is bigger than the voltage one the cathode, current flows in the anode and out the cathode. But when you reverse the voltage, no current flows from cathode to anode. Think of a diode as an arrow (the circuit symbol for a diode looks a lot like an arrow from anode to cathode).

    Now, you can do some things with magnetic components (inductors or transformers) that can temporarily give you high voltage pulses. You use the diode to "store" the peaks of those high voltage pulses on a capacitor, and then when you are ready for the flash, you close a switch to release the energy stored in that high-voltage capacitor.

    The squeal that you hear as a flash unit charges up are from those pulses being made to charge up the capacitor.
     
  7. Feb 3, 2010 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I think it would be better for you to read about how a transformer works than for me to try to describe it in words, using basic EM principles. Wikipedia does a fair job.
    The same goes for diodes an things. It may be unfamiliar but it's only School level Science and a lot easier than EM theory.
     
  8. Feb 3, 2010 #7
    In general if your convert (change or 'transform' as in a transformer) one form of power to another, you can boost voltages when convenient to do so. Power is voltage (E)times current(I) and if you assume an ideal EI (in) + EI (out)...you can manipulate the voltage (E) times current (products)....so maybe a modest EI in can be converted to a small current out but a large voltage...that's what a step UP (in voltage) transformer does by using more coil turns on the output than input...other means exist to transform dc voltages as mentioned above.

    A capacitor is a convenient means of storing such high voltages for brief periods.
     
  9. Feb 5, 2010 #8

    So I ask: Is it correct to conclude that inside an ordinary digital photo machine we have a transformer that converts 3 V into high voltage to feed the capacitor ?


    Best Regards,

    DaTario
     
  10. Feb 5, 2010 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Absolutely.
    However, because such a high frequency is used, the transformer can be made much smaller. Moreover, the average power handled by such a transformer is very low.
     
  11. Feb 5, 2010 #10

    berkeman

    User Avatar

    Staff: Mentor

    Either a transformer or an inductor. For the voltage levels in the flash, sophie is probably right that it's a transformer (flyback topoloty DC-DC).
     
  12. Feb 5, 2010 #11

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Yes - I was just making the point that, just because large voltages are involved, the transformer doesn't need to be as big as you might imagine a 'mains transformer' to be.
    Actually, with modern, more expensive flashguns, the discharge is truncated to give the right exposure and most of the energy gets 'put back' into the circuit. The cheap old ones just used to short out the tube in order to terminate the flash. Does that have any implications as to the probable of arrangement of inductances?
     
  13. Feb 13, 2010 #12
    Thank you all for the answers,

    Best Wishes

    DaTario
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook