battery education sought
ard at p850ug1.demon.co.uk
Sun Oct 16 17:36:17 CDT 2005
> >I noticed my HP 262X terminals have a "Duracell TR133" battery in the back
> >to maintain settings when powered off. The battery indicates "Mercury" on
> >the side.
> Extinct due to envromental concerns. Common use these days are the
I am not at all convinced that the environmental harm done by mercury
batteries (particularly if correctly recycled is worse than the damage
done by all the equipment that was replaced becasue you can no longer get
the right battery. But anyway...
> Mercury batteries were used for many reasons, high energy density,
> extremely long shelf life at near zero power consumption and very
> stable voltage over life. Lithium cells have replaced them in
To go OT for a momnet, a lot of 1950's/1960's cameras used a mercury cell
to power the exposure meter. The constant voltage meant the circuit could
be very simple (just the CdS photoresistor, the mercury cell, a
microammeter movement and a couple more resistors, generally). Now that
such cells are unobtainium, there are various kludges, some of which work
better than others, but it's still a pain.
Incidentally, I was told that the Eastern Bloc cells were anything but
constant voltage, which means that cameras from those countries, like
Exaktas and Prakticas tended to use a wheatstone bridge circuit for the
meter, with correct exposure being at the balance point of the bridge. Of
course that design doesn't care about the battery voltage, so such
cameras are still easy to keep going. Unlike that ridiculous circuit in
the Pentax Spotmatic. It's a weatston bridge alright, but the correct
exposure point is not when the bridge is balanced, it's when a certain,
small, current flows through the meter. It therefore is dependant on
battery voltage. The official justification is that a totally flat
battery can't give the correct exposure indication, to me it appears they
got the design wrong....
> most apps where battey drain is low and extremely long stable
> life is desired.
> >The TR133 battery also indicates "4.2V". It seems the Duracell replacement
> >for this battery is Alkaline, and 4.5V. Should I use it?
> I'd expect that would work just fine. If your really uncertain add a shotkey
> rectifer in series (Vf about .3V). My bet is the logic they keep alive
> is a CMOS RAM and in operation that device Vcc is 5V.
I've not pulling mine apart yet, and the 'service' manual on hpmuseum.net
doesn't include schematics, but I will bet you're right. I am pretty
sure the HP150 (original version ) used a similar battery arrangement,
and it maintains a 5101 CMOS RAM chip. I have schematics for that.
 On the 150-II, there are a couple of lithium (?) cells soldered to
the PSU/sweep PSB. With a bit more circuitry, they supply a backup supply
to a pin on the Elmer custom chip, which contains the configuration RAM.
Or at least that's what happens on most machines. On some early ones,
including mine, the Elmer chip wasn't fitted, becasue apparently it
wasn't ready in time. Instead, there's a header plug soldered to the main
PCB in placeof the Elmer chip, and a little daughterboard containing a
handful of chips is plugged into it. One of thsoe -- and the one that the
battery maintains -- is a 5101 CMOS RAM.
FWIW, I do have HP150 schematics (both versions).
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