HP HP-97 User Manual

HP-97 Theory of Operation
2-39. During a write operation, the CRC interrogates the write protect
switch when the head switch closes to determine if the card has a clipped
corner. If so, the CRC inhibits the write operation and informs the
microprocessor, which generates an "Error" display.
Figure 2-8. HP-97 Power Supply Circuit
2-40. POWER SUPPLY
2-U41. Quick-charge nickel-cadmium batteries are the primary power source
for the HP-97. The 4+5.0 nominal battery voltage is converted to +6.25 Vdc
and to -12.0 Vdc by the transistor inverter circuit shown in figure 2-8.
2-U42. Transistor Q2 and toroidal transformer T1 form the basic inverter
circuit. With feedback from winding A, Q2 oscillates at a frequency of
approximately 20.0 kHz. Winding B of T1 forms the transformer primary from
which VSS is derived; CR5 rectifies and C4 filters the voltage from wind-
ing B. The voltage from winding C is rectified, filtered, and doubled by
the combined actions of C1, C2, CR6, and CR7 to produce the output voltage
VGG. Voltage regulation of VSS is provided by controlling the frequency
of oscillation of Q2 through the combined action of zener diode CR9 and
transistor Q3.
2-9

HP HP-97 Specifications

General

Brand	HP
Model	HP-97
Category	Calculator
Language	English