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I think this might be a bit off topic, but communicating through RS232 with a HP 30b while it has batteries annoys me because I don't know how much power it's drawing while the cable is attached, and, because you can't see whether it's on or off if its firmware is empty.

Would connecting the output of a 3V power supply to one of the battery holders work?

What kind of power supply could do that? If I search for linear dc power supplies on the internet I mostly find expensive ones.


Edited: 30 Aug 2011, 6:56 p.m.

The 20b/30b do use a lot of power, 30mA or more, when communicating via SAMBA and will run down new cells in very short order, maybe under 1 hour. You can use an external power supply but you'll need to remove both cells -- not just one -- since you'll be feeding current into the remaining cell (there's no blocking diode in there).

Instead of a linear power supply, I'd recommend 2 AA cells in a plastic battery holder with some short wires to aligator clips. That will be able to supply 30mA for 100 hours if you use alkaline cells and be very inexpensive to put together.

The cable alone draws power while connected, even with the caculator turned off. The level shifters are driven by the 3V contact in the socket.

You may feed three volts through the serial connector if you remove the batteries before you connect. On my development machine, I've connected a short cable to the battery terminals and run it off a cheap wall wart (switching PS).

On my carry around device, I've connected a stereo socket for the serial communication and connect it directly to a 3V FTDI USB cable.

WP 34S and USB.