|Re: HP-67 - no battery - No Danger ! -- Oh yes there is!|
Message #17 Posted by Tony Duell on 5 Sept 2005, 9:47 a.m.,
in response to message #11 by Bernhard
I disagree with you! There is a danger.
The classic-series (3 pin) adapter/charger has 2 outputs. One is a constant voltage of about 4.2V. The other is a constant current (about 50mA) to charge the NiCd cells. The voltage of that second output is unregulated and will rise to 16V or so if there's no load.
The 3 pins on the classic calculator connector are ground (in the middle), the +ve side of the NiCd battery, and the +ve supply to the on/off switch and thence to the logic. The last 2 pins are connected by a spring contact in the plug on the calculator, allowing the battery to supply power to the calculator. When you plug the charger in, the spring contact is forced aside, the battery pack is charged from the constant-current supply, the constant voltage supply feeds the logic.
_BUT_ on the card reader models -- the 65 and 67, the card reader sense amplifier chip, the one on the card reader PCB itself, is powered from the battery pack directly. When you plug the charger in, it's effectively fed from the constant current supply. If the battery pack is in place, the battery effectively clamps that voltage to about 4.2V. If the battery is missing, dead, open circuit, etc, then the card reader sense amplifier gets 16V across it.
The sense amplifier was redesigned between the 65 and 67 (the latter chip is also used in the 97 and 82104 (HP41 card reader). The 2 chips have the same pinouts, and seem to be interchangeable. I conjecture that one of the changes was to make the chip less likely to be damaged if the battery back is missing (the 65 manual has a large warning about this, the 67 manual doesn't). However, I don't like risking it. This is a custom chip, spares are very hard to find.
There is no danger in running the 35, 45, 55, 70, 80 from the charger with no battery in place.