HP-41CL and 82120D Power Bank.

[Diego Diaz]

Hi all,

A user has referred me to this old (2011) thread.

Original poster is Geir Isene, and he proposed:
"...building a new battery pack using cell phone batteries, like stacking two of these or some such."

In msg. #16 Monte Dalrymple warned:
"The board is designed to accept 6V. Applying a higher voltage will reduce the lifetime."

Previously, in msg. #7 Garth Wilson had already noted that:
"It should be ok to exceed it a little. A brand-new alkaline is really more like 1.6V per cell, meaning 6.4V for four."

At first glace it seems to me that Monte's warning was referred to the fist post: stacking two Li-Ion batteries will rise 8.4v at full charge, which is by all means excessive.

But, since I designed the Power Bank to supply 6.3v, I'd like to confirm that this voltage (chosen on purpose to mimic a brand new set of alkalines) is safe enough to run a 41CL without compromising its life expectancy.

Any comments/insights are welcome.

All the best.

Diego.

[brouhaha]

(11-22-2023 01:57 AM)Diego Diaz Wrote:  But, since I designed the Power Bank to supply 6.3v, I'd like to confirm that this voltage (chosen on purpose to mimic a brand new set of alkalines) is safe enough to run a 41CL without compromising its life expectancy.

I don't know how high a battery input is safe, but since the nominal voltage of alkalines is 1.5V though much of their life, and when the calculator is "on" they are feeding a boost regulator (bipolar chip on the 41 logic board), I'd recommend setting the feedback for your regulator to produce 6.0V rather that 6.3V. I don't think there's any drawback to using 6.0V, nor any benefit to 6.3V.

I can't speak to the 41CL design, but if it's not designed for about 6.0V from the batteries, that's quite unfortunate.

[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.

[Monte Dalrymple]

6.3V from the battery is fine. As Diego noted, the D2 polarity protection diode drops the battery voltage by about 0.3V immediately, and everything from that point is rated at least 6.3V. I wanted to use higher voltage-rated capacitors for C1 and C2, but a higher voltage rating means physically larger, which won't fit. And I didn't want to go the "cut off the corner of the PC board and lay the component sideways" approach used with most of the original HP designs.

[Diego Diaz]

Hi Monte,

Thanks for dropping by and confirming that point. ;-)

All the best!

Diego.

[Gene]

Diego, any ETA on when these 82120D packs will be available to order?

Gene

[Diego Diaz]

Hi Gene,

And thanks for your interest.

I've already built and sent several units to users around the world.

Next batch, which was supposed* to be ready by now (mid January) is already sold out.

Please PM me or email to "clonix41" -at- "gmail" -dot- "com", for ordering details when a new batch becomes available.

*Shipping the batteries to the Canary Islands seems to be taking longer than expected.
Still in transit since Dec. 18.

Best wishes.

Diego