HP97 The journey begins

[teenix]

Hi all,

For those that might be interested, here is a oscilloscope trace of the HP-97 printer circuitry printing 0.00

The top trace is the seven print head drivers working.

The middle trace is expanded to show the pulse width modulation for the print head pixels which controls the print intensity. The pulse width is determined by battery voltage (VB) and the formula:

Pulse width % (uS) = (Vref / VB)sqr * 100

This value is calculated continuously by some op amps and most of the smaller circuit board that is encapsulated in the red/brown coating as part of the print driver PCB. The ratio calculated by the formula alters if you change the print intensity resistor value. As per the printed info available for the 97 you can see the staggering of the print head pixels to reduce instantaneous power consumption. You might notice that the angle of the printed text is opposite to that of the actual printer. That is because of the right to left printing.

The lower trace is the PWM signal that is driving the motor forward. The duty cycle is again dependant on battery voltage and is continuously determined by the speed of the motor on that part of the PWM cycle when power is removed. ie The dc motor briefly becomes a generator. It also forms part of the encapsulated circuit board and a single op amp. The final speed can be changed by altering the resistor value that is part of the op amp circuit.

cheers

Tony