USB powered HP41

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USB powered HP41

Postby Neve1 » Thu Sep 07, 2017 11:20 pm

Hi all,

I'm kinda new here. So excuse me if I ask questions that where asked of discussed many times around.
After some research, I camo across several posts On this subject. Like this one for example:

http://www.hpmuseum.org/cgi-sys/cgiwrap ... 722#183722

I thought of using an iPhone wall USB plug as the power supply. It delivers 5V/1A

Several issues come to mind, though:
1- Would the 5V provided by the USB suffice to properly power the card reader?
2- Shouldn't the battery pack be removed before plugging in the USB power supply? If not, it will probably add another 5V or 6V depending on the current battery charge! (I haven't seen the schematics yet). Could the circuit handle it?
3- Does anyone know where the + and Grnd are on a Fullnut circuit board if I decided not to use a module/slot? That would save me some time if I decide to go that route.

I have an idea that I will gladly share if I can make this work.

Thank you all.
Last edited by Neve1 on Fri Sep 08, 2017 2:15 am, edited 2 times in total.
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Re: USB powered HP41

Postby Garth » Fri Sep 08, 2017 12:13 am

You seem to have copied the URL from another reference to it which had the "..." in it. You need to right-click on wherever you found it, then click on "Copy link location" (or something like that), then paste. I have the module pinout at home, but I'm travelling and won't be home 'til Tuesday night. The battery voltage won't be added to the USB voltage, but you definitely don't want the USB to charge alkaline batteries. Whether or not it could discharge them down to 5V would depend on what's in the charger, I suppose.
http://WilsonMinesCo.com (60 HP-41 links on the links page)
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Re: USB powered HP41

Postby Neve1 » Fri Sep 08, 2017 2:20 am

Garth wrote:You seem to have copied the URL from another reference to it which had the "..." in it. You need to right-click on wherever you found it, then click on "Copy link location" (or something like that), then paste. I have the module pinout at home, but I'm travelling and won't be home 'til Tuesday night. The battery voltage won't be added to the USB voltage, but you definitely don't want the USB to charge alkaline batteries. Whether or not it could discharge them down to 5V would depend on what's in the charger, I suppose.


The charge issue is indeed a very good point! So I guess batteries will have to be removed.

I copied the complete URL. When I click on it, it works fine for me... It should be ok for you too I guess.

BTW, I have the module pinout, but the whole point here is NOT to use a module. Hence my question about the location of the + and Grnd points on the board.

Are you sure the voltages won't add up?

Thank you very much.
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Re: USB powered HP41

Postby Garth » Fri Sep 08, 2017 4:10 am

Neve1 wrote:I copied the complete URL. When I click on it, it works fine for me... It should be ok for you too I guess.

Ah yes, it works now, and I see the battery and ground connections labeled. So if you're going to open up the calc to solder to the board, any DMM or VOM or continuity meter should help you find suitable points. However, I do not recommend opening a working 41, since the problems with the broken posts usually come as a result of people opening them and then re-assembling. I don't have any experience with it myself, only read about it many times. My 41cx is in good shape after 31 years, so I'll leave it unopened.

Are you sure the voltages won't add up?

Yes. Current adds in parallel, not series. Voltages add in series; but putting the charger across the batteries is parallel, not series.
http://WilsonMinesCo.com (60 HP-41 links on the links page)
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Re: USB powered HP41

Postby Neve1 » Fri Sep 08, 2017 8:02 am

Garth wrote:
Are you sure the voltages won't add up?

Yes. Current adds in parallel, not series. Voltages add in series; but putting the charger across the batteries is parallel, not series.


This actually means that if I put rechargeable N Type batteries in it, it will charge them and power the calculator at the same time. Sounds neat...
I hope to make this work the way I would like too. I'll post pics when done.

I've opened mine several times. You just have to be gentle and careful.
My hp41C is an earlier tall-key version (1980) with hex screws to hold the board in place.
My HP41CX I have never opened since I got it in 1982. (Maybe once a long time ago...)

Thank you!!
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