HP 15C CE ships with mediocre batteries

[rprosperi]

(Yesterday 08:01 PM)mfleming Wrote:  Here's something to ponder. The HP15C CE I received months ago worked fine, no key bounce problems at all. Then more recently I began to see certain keys repeating when pressed, often enough that I was going to pull out my cable and update the calculator software.

Remembering this thread, I thought, why not change the batteries? Replaced the 2032's that came with the calculator with some Amazon Basics in their adult-proof packaging. Yep, no more bounce. Puzzling...

~Mark

Thanks for posting this Mark!

I've been wondering if there was any meaningful correlation between battery state and presence of bounce. I got one of the very first units and have never experienced any bounce issues at all.

And for several months there were almost no such reports (perhaps 1 or 2?) of bounce issues. Then as time went by, there suddenly were a notable number of reports, and then it seemed to quiet back down again.

No reports, then some, then none again might correlate with batches of batteries used in manufacturing, if there is some cause and effect, but I've never heard of such a case, and frankly don't have enough EE skills to even consider if this is feasible.

Any experts out there?

[RPNerd]

(Today 03:44 AM)rprosperi Wrote:  No reports, then some, then none again might correlate with batches of batteries used in manufacturing, if there is some cause and effect, but I've never heard of such a case, and frankly don't have enough EE skills to even consider if this is feasible.

Any experts out there?

It is plausible.

Whenever a key is pressed, a contact is made, which means that a small current is flowing somewhere. That current, however small, inevitably has to come from the batteries. This means that the voltage from the batteries is going to drop a small amount, but it could be just enough for a gate somewhere in the calculator's guts to no longer register the key press, leading to another condition that draws less power from the batteries, in turn causing the output voltage to go back up and the key press to be registered once again.

If this is the case it could probably be avoided with better decoupling of the +3V rail.

[mfleming]

(Today 11:55 AM)RPNerd Wrote:  If this is the case it could probably be avoided with better decoupling of the +3V rail.

Reasonable conjecture. Lack of a capacitive reservoir might also account for memory loss when replacing depleted batteries one at a time. Anyone willing to slap a big cap across the input rails?