spersed between the two electrodes. This trigger pulse ionizes the gas
and causes current to flow through the tube, thus generating an intense
flash of white light.
3.2.3 STROBOTRON CIRCUIT.
The high -voltage output from the trigger circuit is capacitively
coupled from T2 to the strobotron. The coupling capacitors are built into
the ceramic insulator in the swivel neck.
(2) The energy to flash the strobotron is stored in the discharge cap-
acitors C10, Cll, and C14. The correct capacitance for each RPM range
is connected across the strobotron by the range switch. After the stro -
botron flashes, the active capacitors are recharged to 800 volts dc. The
unused capacitors are kept charged to 800 volts to reduce arcing at the
switch contacts when the range-switch setting is changed.
The flashing rate of the instrument is controlled by an internal os -
cillator. The two sections of the oscillator tube (V1) constitute a bistable
circuit; in such a circuit, one section conducts while the other section
is shut off. Then, very rapidly, the two sections reverse states. Each
section is alternately turned on and off at a rate determined by the values
of resistors and capacitors in the circuit and the voltage setting of R3
(RPM control). Several of these components are adjustable by panel con-
trols. The RPM control is R3; R1 and R5 are the calibration screw -
driver adjustments. The range switch (S2) introduces the proper timing
capacitor into the circuit to step the flashing rate up or down by a factor
3.2.5 TRIGGER CIRCUIT.
The output of the oscillator (V1) is applied to the thyratron tube
(V2) through C8. The thyratron, together with C9 and the pulse trans -
former T2, is used to produce the high-voltage pulse necessary to trigger
3.2.6 POWER SUPPLY.
A voltage -doubler power supply furnishes +400 and -400 volts dc
to operate the strobotron. The +400 -volt supply is filtered by a two -stage
R-C filter to obtain the +250 volt supply. The power transformer is cap-
able of operating on line frequencies ranging from 50 to 400 c/s, and is
normally wired for either 115- or 230-volt operation (see Figure 4-7).
3.2.7 EXTERNAL SYNCHRONIZATION.
For operating with an external synchronizing signal, the oscillator
circuit is converted to a conventional, amplitude -sensitive, bistable cir-
cuit. The RPM control varies the bias on V1 so that optimum sensitivity
for sine wave or pulse input signals can be obtained.
(2)The greater the amplitude of the input signal, the greater will be
the range of bias values that will allow proper flashing of the stroboscope.