Topic: HP-35/45/55 etc. Charger Circuit
Hi Jacques,
I have been experimenting with the charger circuit. I found that when the charger is coupled to a calculator, the output voltage drops considerably, to around 3.5V. When the charger logic supply is connected to a multimeter, I get more like 4.5V. These numbers come from a couple of chargers I repaired by replacing the Zener diode with a 4.7V 1W Zener.
I had been assuming that the output voltage would be fixed at the Vz less the .6V drop across the NPN darlington. However it appears the output voltage is not fixed at all.
To confirm I created a PSpice model. I found a model for the MJE800 Darlington at the On Semi site. Added a 470 Ohm resistor, the Zener, and a constant voltage source at 15V. When I simulate a load across the output voltage, the voltage varies considerably depending on the load. It routinely goes down to 3.5V with a load resulting in a current of about 150mA, which is about what we would see from the calculator.
Has anyone else done this? If this is correct, the 4.7V Zener should probably be more like 5.1V, since a true output voltage of 3.5V is just barely above the low voltage indicator of the calculator, which is at 3.4V, and the calculator stops working about 3.3V. Most chargers I see have output voltages close to 5.0 volts (when connected to a multimeter which has a very high impedance). I had always thought this was too high. Perhaps not. Comments?