|Re: HP-29C problem|
Message #6 Posted by Tony Duell on 22 June 2001, 1:43 p.m.,
in response to message #5 by Mark Sims
Problem is, in the -C models (25C, 29C, 33C, 34C, 38C) of the Woodstock and Spice
series, not everything runs off the DC-DC converter. In particular, the RAM circuitry runs off the
DC-DC converter when the machine is turned on, but when the machine is turned off, the
RAM contents are maintained by a connection from the battery +ve terminal via a resistor (about 10k IIRC).
The charger output is about 14V off-load, but the charger has a high internal resistance (at least 10 Ohms). That, combined with the charger circuit resistor in the calculator means that when the machine is on, the voltages look 'normal'. But when the machine is off, the RAM alone doesn't draw much current
and the voltage rises. Often to a high enough voltage to cook a ROM/RAM chip.
That's why you should _never_ connect a charger to a 25C, 29C, etc unless there's a known-good battery pack in place.
Incidentally, the DC-DC converter in most HP calculators is designed so that the main +ve output about 6.2V) can never be lower than the input voltage. Normally this is OK (the battery is about 2.7V). But if you connect an overvoltage PSU to the battery contacts you can, again, damage chips.