Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Camera Flash Circuit

  1. Jul 22, 2012 #1
    These two websites:




    have very similar circuits for a standard camera flash circuit...and overall do a decent job explaining them.

    There is one question I have though......one thing they do not explain in detail and I'm not quite sure what the answer is.

    The question is WHY the transistor ever shuts off? WHY does it go into the cutoff mode for a short period of time? What is causing this?

    So........what causes the transistor to go into cutoff? Does anyone have a good (detailed) explanation?

  2. jcsd
  3. Jul 23, 2012 #2
    Anyone have any thoughts on this?
  4. Jul 23, 2012 #3

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    No takers yet ?

    i think it has to do with inductance of the little transformer.
    Recall current flowing in an inductance causes magnetic flux
    but iron is non-linear it can only hold so much flux,
    so at some amount of current an iron core inductor quits behaving like an inductor , its flux quits increasing with current...

    from your first link (and credit to those folks at http://www.increa.com/reverse/dc/ )


    when transistor conduction starts,
    current begins to increase in the transformer primary increasing flux in its core
    and voltage in every turn of every winding is in proportion to rate of flux increase

    as he said, voltage in secondary during increasing flux interval turns transistor on more(note dots) and that's positive feedback

    now here's the key to it -
    when transformer core has all the flux it can handle
    it "saturates" and flux stops increasing
    so there's no more volts induced in any of the turns of either winding
    so secondary winding no longer promotes primary current
    and when primary current begins to drop,
    flux starts to decrease,
    reversing polarity of induced voltage in all windings
    so secondary winding now turns the transistor off and there it stays until flux settles back near zero.

    note his positive feedback operates in BOTH half cycles ! That's why it gives that snap-action full-on full-off cycling.

    we are taught to usually think of transformers as processing sinewaves
    but they are at heart just inductors.

    old jim
    Last edited: Jul 23, 2012
  5. Jul 24, 2012 #4

    Jim....thanks for the reply. I need to think about this more, but on a first look I think this might be what is happening.

    Thanks again!
  6. Jul 24, 2012 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    indeed that's the thing to do.
    You need to get to where you can work a machine in your head
    and check the formulas that naturally fall out from your understanding against textbook formulas
    when they converge you are probably right.

    Observe that so long as flux is increasing, the induced voltage helps push current through the 220 ohm resistor.
    When flux stops increasing due to saturation,
    induced voltage drops off
    reducing base drive
    decreasing base current hence collector and primary current
    which begins the 'decreasing flux' half cycle.

    That works for me.

    Now consider this drawing from another site

    i think he has his 15 turn winding connected backward , 220 ohm should go to the tap not the end (or else he needs another dot on bottom of winding and reverse direction of those turns)

    that had me stumped last night.
    Somebody please correct me if i'm wrong ?

    old jim
  7. Jul 24, 2012 #6
    In my short career..........I have found that to usually be the case.

    I think you are right......I also think this is why the OFF time of the transistor is so short compared to the ON time....the reverse EMF quickly dissipates and the process starts all over again......

    That looks wrong to me also.....
  8. Aug 16, 2012 #7
    Okay I have been thinking about this a bit more....

    And now I am thinking it is not due to the transformer saturating.....

    I think it is the transistor.

    Once the transistor saturates we have maximum current flowing through it....so the curent stops increasing and is constant....this causes the flux/induced voltage in the secondary to quickly drop to zero.....and this is what causes the transistor to go into cut-off.

    Of course the transistor goes into cut-off before "zero volts" is reached....

    So I think it has more to do with the transistor saturating than the actual transformer itself...

    Let me know what you think.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook