HP-41CL and 82120D Power Bank.

[Diego Diaz]

Hi there,

I've taken a while to check the technical info from the CL board and schematics, and the short answer is that it is perfectly safe to run it with a battery voltage of 6.3v (even 6.4v for that matter).

The long answer for those technically biased (quite like myself :-) is as follows:

The VIN line in the CL board connects the 41 battery + terminal with two diodes (D1 & D2) MBR0530.

According to the datasheet, these diodes show a forward voltage drop of about 0.375v, so the voltage input to the board will be quite close to 6.0v.

D1 feeds the VBAT line for the I/O ports and it's coupled with C7, a multilayer ceramic capacitor with an operational voltage of 16v (at least).

D2 feeds the VCC converter and is coupled with C3 (68uF 16V, so no problem here either); and C1//C2 (aluminium 330uF 6.3v!) But, the marking in the case of C1 & C2 shows 330 6V.

And here is where I think the alarm rings. The 6V marking can lead to believe that these capacitors are rated to 6.0v. This is not the case, some manufacturers mark just "6V" or "6" to refer to the 6.3v standard rating.

Hope this clarifies the point.

@brouhaha, There's a small, but I think relevant, benefit in using the voltage equivalent to the one provided by a new set of alkalines in all the -non CL- 41s. Since these venerable bipolar circuits can run a bit more unstressed as the input voltage is closer to the output they supply.

In most cases the Vcc line in a standard 41 when ON is about 6.4v, so not a big deal, but every bit helps. ;-)

Best wishes.

Diego.