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light in parallel with the capacitor will turn on (labeled “flash-ready
lamp” on the camera) to let you know that the camera is ready to use.
The entire network from the 1.5-V dc level to the final 300-V level is
called a dc-dc converter. The terminology chopper network comes from
the fact that the applied dc voltage of 1.5 V was chopped up into one
that changes level at a very high frequency so that the transformer can
perform its function.
Even though the camera may use a 60-V neon light, the neon light
and series resistor R n must have a full 300 V across the branch before
the neon light will turn on. Neon lights are simply bulbs with a neon
gas that will support conduction when the voltage across the terminals
reaches a sufficiently high level. There is no filament, or hot wire as in
a light bulb, but simply conduction through the gaseous medium. For
new cameras the first charging sequence may take 12 s to 15 s. Succeeding
charging cycles may only take some 7 s or 8 s because the
capacitor will still have some residual charge on its plates. If the flash
unit is not used, the neon light will begin to drain the 300-V dc supply
with a drain current in microamperes. As the terminal voltage drops,
there will come a point where the neon light will turn off. For the unit
of Fig. 10.72, it takes about 15 min before the light turns off. Once off,
the neon light will no longer drain the capacitor, and the terminal voltage
of the capacitor will remain fairly constant. Eventually, however,
the capacitor will discharge due to its own leakage current, and the terminal
voltage will drop to very low levels. The discharge process is
very rapid when the flash unit is used, causing the terminal voltage to
drop very quickly (V � Q/C) and, through the feedback-sense connection
signal, causing the oscillator to start up again and recharge the
capacitor. You may have noticed when using a camera of this type that
once the camera has its initial charge, there is no need to press the
charge button between pictures—it is done automatically. However, if
the camera sits for a long period of time, the charge button will have to
be depressed again; but you will find that the charge time is only 3 s or
4 s due to the residual charge on the plates of the capacitor.
The 300 V across the capacitor are insufficient to fire the flash lamp.
Additional circuitry, called the trigger network, must be incorporated to
generate the few thousand volts necessary to fire the flash lamp. The
resulting high voltage is one reason that there is a CAUTION note on
each camera regarding the high internal voltages generated and the possibility
of electrical shock if the camera is opened.
The thousands of volts required to fire the flash lamp require a discussion
that introduces elements and concepts beyond the current level
of the text. However, this description is sensitive to this fact and should
be looked upon as simply a first exposure to some of the interesting
possibilities available from the right mix of elements. When the flash
switch at the bottom left of Fig. 10.72(a) is closed, it will establish a
connection between the resistors R 1 and R 2. Through a voltage divider
action, a dc voltage will appear at the gate (G) terminal of the SCR
(silicon-controlled rectifier—a device whose state is controlled by the
voltage at the gate terminal). This dc voltage will turn the SCR “on”
and will establish a very low resistance path (like a short circuit)
between its anode (A) and cathode (K) terminals. At this point the trigger
capacitor, which is connected directly to the 300 V sitting across the
capacitor, will rapidly charge to 300 V because it now has a direct, lowresistance
path to ground through the SCR. Once it reaches 300 V, the
charging current in this part of the network will drop to 0 A, and the
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