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I have resumed an old gloriful redled calc
that I owned many years ago.
Since the akku-pack is no longer working,
but it is needed in order to use the calc with
its charger/adapter, I'm thinking to try connecting
a variable resistor to the battery terminals,
in order to emulate the load of a working akku-pack.
I hope this doesn't sound too ridiculous...
Has someone ever tried something like that ?


--Rupert

Most of the HP calculators use the battery pack as a (rather poor) filter capacitor for the raw output of the AC adapter. Without the pack you have a large amount of rather high voltage ripple on the internals of the calculator.

A resistor alone will not help. I have used an elecrolytic filiter capacitor paralleled with a resistor as a substitute for the battery pack. Since the capacitor is a much better filter than the battery pack, the voltage can rise more than I feel comfortable with if you don't parallel the cap with a resistor. If memory serves I used a 500-1000uF 20 volt capacitor and paralleled it with a 220-470 ohm resistor. They were chosen because they happened to be within arms reach at the time. Experiment at your own risk though...

DON'T DO IT !!!!

The batteries act not only as a filter (a capacitor will do as a mere filter). but as a VOLTAGE REGULATOR. A capacitor will not average voltage, it will keep the voltage at the maximum value (more or less). A battery will LIMIT the maximum voltage applied.

Suggestions:

a) Rebuild the battery pack and verify clean contacts (much recommended).

b) If you don't want to rebuild the battery pack, a good option AT YOUR OWN RISK is to use standard alkaline AA batteries for a while. Voltage is greater than NiCads, but in most cases seems to be within tolerance. I CANNOT ASSURE THIS FOR ALL MODELS.

NEVER USE THE AC ADAPTER WITHOUT A RECHARGEABLE, WORKING BATTERY PACK.

NEVER USE THE AC ADAPTER WITH ALKALINE BATTERIES.

i agree with andres. if you just take an electrolytic capacitor *alone* to do the job, this can be hazardous to the electronics. however, if you put a proper zener diode in parallel with the cap, there should be nothing wrong with taking it as a accumulator replacment.
what is a proper zener diode?
1) the voltage rating of the diode should match the accu voltage: if the accu was rated 4.8 volts, then take a zener diode of 5.1 volts. (this 'odd' value is most likely in stock of any electronics vendor)
2) the power rating of the diode must be high enough, so that the diode is able to absorb the incoming charger energy without damage. example: if your charger can deliver 8 volts with a power of 3VA, then it can deliver (roughly) a current of 3VA/8V = 0.375A. the power rating of the diode should then be at least 5.1V * 0.375A = 2Watts. (some of you may state, that the current of the transformer is rms, and that the current through the diode has a form factor, and that there must be a current limiting resistor etc, etc...: let's keep it simple!)
3) when parallelling diode and capacitor, check proper polarity: the *cathode* of the zener goes to the *plus*- pole of the capacitor.

happy soldering

For the 2 cell devices, a 2.5V to 3V zener is appropriate.

But remember the cautions -- overvoltage WILL kill calculators. And the charger is a nice convenient source of over voltage.

Well, thanks to all for the replies.

If you are going to use a zener diode, you really do want a current limiting resistor in there. I would start with a 22 - 47 ohm resistor in series with the output of the wall wart connected to the cathode of the zener.

The internal current limiting resistor used for charging the nicads should be sufficient I believe.

> If you are going to use a zener diode, you really do >want a current limiting resistor in there.

At present, I've just got rid of the old rechargeable
batteries and put three alkaline batteries in their place.
It is just a temporary solution, but allowed me to
power-on the calc and to play a bit with it.
Maybe I should confess :) that I'm talking
about a........... Texas Instruments TI-59.
I asked here because I'm mostly an HP addict.
In any case I think the 59 should be interesting for
any classic calculators nut. :)

--Rupert.

Wear rubber gloves when you touch that '59, lest you get TI cooties... I know one of the guys who designed it.

if the gaping rends and gaps in my grey mater haven't done a complete memory lost on me the 59 was the one with the optional printer and a port for one rom, wasn't it??? yes; it was interesting. kind of a second string 41 for the algebreic entry crowd and as i remember the surveying chip would actually save about 20 (!) or so coordinate pairs. knew a man in reno who swore by his. or at it. i can't remember which. glad you are able to keep yours going.

I can't imagine how a TI anything could be interesting to an HP user.

After all, I hear they intermittently let the door close.

The real question now may be which one will drop the calculator market first, TI or HP and when in the next 5-10 years? Has anyone seen any financials for these segments?

the point is that even though the ti calculator wouldn't make an effective doorstop; the programing in the ti survey rom was .... well .... more real-world-useful in at least the most important routine.
btw - i held that tds data colector/field computer in my hands and while it is not a calculator, i can swear that someone somewhere still knows how to build a keyboard. it felt sweet like a new 41.