Charging 'classics' from a USB port


I'm not promoting this e**y item at all, 170502089219, just curious if anyone here has tried it. It seems like a good idea even if a bit expensive.



Dear Katie:

I am no electrical engineer, but the calibrated eyeball tells me that there may be excessive current pulled from the USB port resulting in damage to the former, and, if regulated by the adaptor, insufficient current to charge the calculator timely.

My $0.02.

John Stark



I see no reason why this cable shouldn't work well from an electrical point of view. USB ports are rated at minimum of 500ma, so unless there's something wrong with the calculator any USB port should be able to power it with a lot of current to spare. Most external USB hard drives require a lot more current than a classic calculator does when powering just the LEDs (a card reader will raise the current draw significantly).

I'm most curious about the calculator-side plug. It's looks pretty flaky to me on the closeup picture on the web page that the seller links to.



Dear Katie:

Thanks for the clarification. Did not know of max current rating of USB ports.


[Beware of programmers carrying screwdrivers]


Could one build a USB to Calc that works for HP-41 and HP-71 as well? I know the 41 needs 6v but could that gap be bridged savely? the main reason I am asking is that it would provide for a simple way to savely power a bunch of calcs that have volatile RAM-Boxes which need constant current but doing it with Batteries exposes the calc to leaking battery risk...

Thanks for any tips!




The HP-41 will run at 5V I have a CV that I am using with a homemade ROM box and it runs off of the 5V from the ROM box fed to the calculator on pin 1 of a I/O port.

The HP-71 would be a different story, I just went and tried it and if I connect +5V to the battery terminals (mind the polarity) a base 71 with HP-IL powers on and functions normally.

The adapter input on the 71 is designed for the 8V AC adapter that is also used by some peripherals, however since there is no transformer in the circuit it should work off of DC as well, but you will have the voltage drop of two diodes in the internal bridge rectifier, plus the transistor used to regulate the voltage from the adapter input. I thought that this would drop the drop the 5V input to he point where the 71 would not power on however much to my surprise it did turn on, but the low battery indicator was on and it did not function correctly, the orange f key would come up as 'CAT'.

Edited: 5 Oct 2010, 3:09 p.m.


Peter wrote:

I know the 41 needs 6v but could that gap be bridged savely?

The 41-series doesn't need 6 Volts. That's just the nominal (!) voltage of four alkaline batteries. In real life their voltage may be something like 6.5 V (new out of the box) or 5 V (after some time of use) and finally it's 4 V and below when they're essentially depleted.

HP's dedicated rechargeable battery pack 82120A uses four NiCds that provide 5 V on average and once again 4 V when they can be considered empty. So, if the 41 really required 6 V it would work neither with alkalines (except brand new ones) nor with HP's own NiCd-pack.

BTW, the 41 knows its limits: the battery announciator BAT comes on right at the point when the voltage drops below 4 Volts. That's why I'm also thinking about powering a 41 directly from an USB port. Its 5 V looks just fine here - same as HP's NiCd pack or an average set of alkalines. Since many cell phones use Mini-USB connectors for their chargers this might be a possible option for a new connector in the calculator case. :-)



good points, forgot about the 1.2V NiCads. And I love the mini-usb port idea, it would fit just nicely into the side port of the hp41... Maybe I give it a try.




Perhaps Mark Henderson can give an update, assuming this is the same seller and device.



I have bought one of these from David a couple of months ago.

My Hp-80 with Ni-Mh has been connected to it ever since.

When in my office, I use the Hp-81 and keep the 80 connected to the USB port through this adaptor.

Excellent slow charging ability. Ni-Mh don't heat up during the charging process. Robust prong design.

Bottom line: perfectly satisfied with this adaptor and will buy a couple of others.

I have NOT tried it with my HP-65s (I use Eneloops in these calculators).

Usual disclaimer applies: "I have no business relationship with this seller nor interest in promoting his products...etc...

...and David is a very kind person IMHO.



I bought one and have had no trouble with it and my 45.
The calculator end plug seems sturdy enough and hasn't shown any sign of wear or breakage over dozens of insertions and removals.

- Pauli


Thanks for all your responses.

Jeff, I somehow missed the thread on these cables back in March and didn't find them on my Google site search (apparently because 'charge/charging' was never mentioned in the thread).



It sounded familiar. I think I found it by searching for "classic."


I took a chance and bought one. Will let you know how it fares when I get back from my sailing vacation in Greece next week.


... when I get back from my sailing vacation in Greece ...

Trying to generate some envy, are we?


Oh no, it's arduous, it's painful, wouldn't want anyone to endure this ;-)


Do you have your HP-41 with Bobby Schenk Navigation module with you? It might make it a little easier to endure this painful vacation ;-)

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