Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Flash & RC circuits



All electronic flash units use a solid-state oscillator in the 1khz to
20khz range; you should be able to hear some audio (a high-pitched whine)
when the circuit is operating. The 1.5v AC produced by the oscillator is
then stepped up to hundreds of volts to charge a capacitor, which fires a
Xenon gas tube. No way to fire a gas tube without getting up past the
threshold voltage, right? No way to step up DC without pulsing it or
converting to AC, right? The output voltage is pretty high but the
current is small.

Removing the battery (energy cell, actually) while the oscillator is
running could subject that circuit to surges that could cause damage to
the chip in some units or under some circumstances. There's nothing going
on that could harm the cell, capacitor or gas tube. In some circuits you
can get a nasty HV jolt if you make contact with the connectors while
removing the cell; that backcurrent might be capable of frying the chip.
No more oscillator, no more flashes.

This kind of damage is likely to be what technicians call "an
intermittent"; i.e. you might successfully remove the cell 16 times but
the circuit trashes on try 17. Intermittency lulls people into thinking
that warnings are over-reaction, then it bites them in the butt.

Best wishes,

Larry

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Larry Cartwright
Physics, Physical Science, Internet Teacher
Charlotte High School, 378 State Street, Charlotte MI 48813
<physics@scnc.cps.k12.mi.us> or <science@scnc.cps.k12.mi.us>

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I am very good at learning from my mistakes.
Undoubtedly, I shall learn a great deal today.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On Tue, 30 Mar 1999, Mike Wilson wrote:

<From mwilson@colosys.net>

In class a question about the electronic flash attachments used in
photography was voiced.

Apparently the photography teacher warned students that charging
the flash unit, then removing the battery, and then using the flash
would cause extensive damage to the flash unit or battery.

I had just explained how flash units use an RC circuit for the flash,
so failure to let the capacitor reach full charge would damage the
battery.

It does not seem to me removing the battery from a charged circuit
could cause damage (which is how we do the tau experiment in
class).

Am I wrong?

Obviously I told the involved students it could be a rather expensive
lesson to ignore their photography teacher, and it is never smart to
ignor teachers. Of course, being teenagers, they have already tried
all of the warnings on their own.