Ever since the 41C has been built discussions came up because of the power
supply. Although the standard machine is very "green" and has a minimum power
consumption users get nervous when devices are connected that do not fit in that
line. The first to mention is the magnetic card reader which eats lots of energy
and renders a BAT signal pretty fast after not too abundant usage.
With the rise of the 41CL the energy question has become even more pressing,
first and foremost if you want to perform writing operations on flash.
To make long things short, I pursued the idea to feed the 41C "backwards". The
IO ports have connectors over which an external power source may supply the
calculator, thus being independent of battery power (Figure 1).
Figure 1
I (ab)used a simple memory module and withdrew the memory chip for the purpose
to solder wires on the respective port outlets and connect them to a 6V DC
supply (with VBat being positive and GND negative polarity).
Any other module would work just as fine but I think that a memory
module is the most easily obtainable and cheapest. The power supply of a cell
phone was a good solution for me. The voltage is stable and the output current
of 800 mA is enough to drive a magnetic card reader and is sufficient to warrant
accurate flash writings in the 41CL (Figure 2).
Figure 2
The battery compartment can be left operational while the external power source
is used. I measured a "loading current" of about 2.5 µA when the power supply
was used in parallel. A such a low current does not produce any heat even if the
calculator is active over night and does not harm the batteries as far as I made
the experience. Of course, the battery compartment may be pulled out while the
external source is used. Caution has to be exercised if the external power
supply is taken from the socket. I made the experience that in that case the
batteries feed the voltage converter, in my situation with a current of about 8 mA.
Interestingly, this solution works well on all 41 fullnuts (HP41C/CV/CX) but not
so well on the halfnuts (41CX with rounded display corners). The main problem in
the halfnut models is that the external power supply appears not to render
enough power to the circuitry to shut off the low bat annunciator (BAT). Even a
rise of up to 8V (I didn't dare more) was not able to toggle off the BAT signal,
i.e. Flag 49 was still active. However, the halfnut performed well with
exception of the CLOCK (resp. SHIFT ON) operation - which of course is a default
behavior when the battery is low.
I do not know if this kind of solution has been suggested before and if I have
"carried coals to Newcastle" (the German proverb I am familiar with would be "to
carry owls to Athens"). I have used this form of external power supply for a
couple of months now and I am very satisfied with it. I think that this may be
generally a good solution for people who do not have a battery pack with
rechargeables as it is a "minimal invasive" way to supply the HP41 with external
power. The basic prerequisites are a free physical port, some basic tinkering
and the exertion of blasphemy on an original HP41 module.
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HP41C "Power Module"
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