20 Jul
2018
20 Jul
'18
8:57 p.m.
On Fri, Jul 20, 2018 at 7:40 PM, Brent Hilpert via cctalk
<cctalk at classiccmp.org> wrote:
If you want to power it through the charger jack, then
you need to assess
whether there is any internal charging circuitry (rectification, aforementioned
current-limiting R, etc.) sitting between the jack and the cells.
It is not uncommon for the NiCd cells to act as a shunt regulator in such
calculators. The charger is of relatively high voltage (say around 9V), it is
applied to the cells through a current limiter (often just a simple resistor as
you say), and the fact that the on-charge voltage of the cells is perhaps 2.5V
(for a pair of NiCds) limits the voltage applied to the rest of the calculator.
What this means is that if you connect the charger with no cells in place
then either
1) Nothing will work as the charger circuit can't supply enough current to
power the calculator
2) Nothing will work ever again as the voltage across the battery terminals
with no cells to clamp the voltage is sufficiently high to damage the IC.
The latter is a particular problem in some calculators with continuous
memory (meaning the memory is powered from the battery even when
the machine is turned off). Connecting the charger to one of those with
no battery in place will damage ICs even if the machine is not turned
on.
-tony