HP HP-97 User Manual

Theory of Operation HP-97
2-U3. An over-voltage circuit consisting of QU4, Q5, and R8 through R10, as
shown in figure 2-9, prevents VB (battery voltage) from rising above VSS.
When VB approaches VSS, CR10 conducts, turning on transistors QU4 and Q5.
Current is drawn from the battery through R10 until VB falls below VSS.
Figure 2-9. Over-Voltage Circuit
2-44, BATTERY CHARGING
2-45. Figure 2-10 illustrates the battery charging circuitry. The ac
adapter/recharger is a transformer that drops the line voltage to 12.8
Vac at the input terminals of the calculator. Diodes CR1 through CR4
rectify the alternating current, and the resistor R4 limits the dc current
applied to the batteries. When the ON-OFF switch is turned ON, limiting
resistor R3 is shunted, and the dc voltage is applied directly to the
battery pack and the calculator power supply. Transistor Q1 turns on
during periods of high display current demands.
Note: The calculator will not be damaged by connecting the ac adapter/
recharger to the input terminals when the batteries are removed; however,
it will not operate correctly until the batteries have been reinstalled.
2-46. POWER-ON RESET
2-U47. To ensure that the logic contained within the ACT comes up in the
correct logic state when power is applied to the HP-97, a power-on reset
circuit is included. (See figure 2-11.)

HP HP-97 Specifications

General

Brand	HP
Model	HP-97
Category	Calculator
Language	English