|Re: HP-67 Protection|
Message #10 Posted by Tony Duell on 24 Apr 2005, 11:19 a.m.,
in response to message #9 by Jonathan
The HP adapter has 2 outputs. A constant voltage output to run the logic, and a constant current output -- about 15V unloaded -- to charge the batteries.
When you plug the charger into the calculator, the battery is disconnected from the logic -- that's what the spring contact on the charger connector is for -- and each section is connected to the appropriate section of the adapater.
BUT in the 65 and 67, the sense amplifier chip is connected directly to the battery terminals. Not even the on/off switch is between them. If you connect an adapter to one of these machines, it relies on the battery pack acting as a shunt regulator to pull the voltage down. If the battery is missing or open-circuit, it won't do this, and the sense amplifer chip will get 15V across it. Even if the machine is switched off.
The original HP65 sense amplifier can be damaged by this. The later one, used in the 67, is supposed to stand it, but I don't want to risk it.
As to whether your home-made charger will damage the chip, it depends on the open-circuit voltage.
The schematic of the HP adapter is available on this site as ASCII-art, and is also on the HPCC CD-ROM.