CALIBRATION CIRCUIT.

The input circuit can also be driven by an external contactor. In

such operation, half of the dc voltage divider, comprising R32 and R33,

is short -circuited by the closing of the external contactor. The positive

pulse required to operate the thyratron is generated by the input circuit,

upon the opening of the external contactor switch. Thus the flashing is

synchronized with the opening of the switch, not with the closing.

The input impedance at the panel input jack is about 500,000 ohms.

Because of the dc voltage divider, about 50 volts dc is present at the input

terminals.

Three frequency ranges for external input are provided: LOW

INTENSITY, MED INTENSITY and HIGH INTENSITY. Operation above

the maximum frequencies indicated will not damage the strobotron, but

operation may become erratic. On the LOW INTENSITY range, the in-

strument can often be synchronized at frequencies up to 600 c/s (36, 000

rpm) or higher, before the strobotron begins to misfire.

In LINE/EXT INPUT position, the Type 1531 Strobotac is synchroni -

zed with the power-line frequency, by means of an ac voltage from the

power transformer applied to the input circuit. This voltage is removed

from the circuit when a plug is inserted in the INPUT jack. Enough voltage

is available to permit synchronization at all bias (RPM control) settings.

3.2.8 CALIBRATION CIRCUIT.

To calibrate the frequency dial against power-line frequency, volt-

ages at both the power -line and the flashing-rate frequencies are super -

imposed across a neon bulb (V5).

When the flashing rate of the strobotron equals the power-line fre -

quency, or a multiple or submultiple of it, the voltage across V5 will

remain constant and the bulb will not vary in intensity. Depending upon

the phase relation between the strobotron oscillator and the power-line

voItage, the steady-state condition of the neon bulb may be maximum

intensity, partial intensity, or zero intensity.

If the flashing rate of the strobotron differs from the power-line

frequency, the voltage across the neon bulb will vary, and the intensity

will change, at the difference frequency.

The above calibration can easily be made at many flashing rates

between 600 and 7200 rpm. Below 600 rpm, the flashing rate will pro-

duce the appearance of flicker. The flicker frequency will be super-

imposed on the difference, or beat , frequency described above; when

the flicker is pronounced, it may be difficult to distinguish between the

flicker and the beat frequency. Above 7200 rpm, the variation in inten -

sity may be so slight that calibration is difficult.