The HP calculator internal power supplies


This page describes the power supplies found in the LED series HP calculators.


The Classic series

Schematic of the internal power supply of the HP-35 and the HP-45.

HP35 power supply HP35 power supply
HP45 power supply HP45 power supply


The Woodstock series

HP25 Powersupply pcb HP25 power supply

Photo of the power supply of the HP-25. D1 and R1 are not on this picture. and capacitor C5 is located under the LED display.


The Topcat Series

The next schematic view and description of the HP-97 power supply are copied from the HP-97service manual.


HP97 Supply

Transistor Q2 and toroidal transformer T1 form the basic inverter circuit. With feedback from winding A, Q2 oscillates at a frequency of approximate 20.0 Khz. Winding B of T1 forms the transformer primary from which VSS is derived. Diode CR5 rectifies and capacitor C4 filter the voltage from winding B. The voltage from winding C is rectified, filtered and doubled by the combined actions of cpapcitors C1, C2 and diodes CR6 and CR7. This gives the output voltage VGG. Voltage regulation of VSS is provided by controlling the current through transistor Q2.

HP97 Powersupply PCB HP97 Powersupply PCB


The Sting Series

Also the Sting series uses a blocking oscillator to generate the internal supply voltages from the battery.
Below the power supply print and schematic is given from a HP 10 calculator. This calculator is supplied by 4 cells.

HP10 power supply HP10 Power supply

The pins numbers relates to the power connector on the left side of the CPU. Pin 1 is the upper pin and 8 the lower pin.
The voltage levels on the right are measured on a functional HP-10 calculator.


The Spice Series

The power supply of the Spice models have their own PCB. See also repair of the HP-32E. HP32E supply

The schematic given for the HP 32E model is also applicable for the other E models.

HP32E supply

The C models, with the continuous memory have some additional components to supply the Vmem from the battery when the calculator is switched off. The next picture shows the supply of the HP-38C model. The memory is supplied by resistor R2 directly from the battery. Diode D6 prevent supplying of the logic in off mode.

HP32E supply



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