On 2009-04-15, James Waldby wrote:
On Sun, 12 Apr 2009 03:00:06 +0000, DoN. Nichols wrote:
On 2009-04-09, James Waldby ... wrote:
On Thu, 09 Apr 2009 22:31:31 +0000, DoN. Nichols wrote: ...
Just for the fun of it -- let's see what I can remember picking
up at hamfests which might be of interest he

1) B&S 0-6" digital calipers (now not usable because they need
PX-13 mercury cells in a holder which would not be happy with any
modern cell, even if the slightly higher voltage does not harm
it.)
I'll try that someday.
...

A WeinCell MRB625 battery is supposed to be a proper $4 substitute for
a PX-13 mercury cell,

But since the B&S caliper needs *four* of them, that is a $16.00
replacement. (It is a "DIGIT-CAL II", FWIW.) (At least it is better
than some adaptors which were supposed to convert something like a SR357
to fit the PX-13 profile, which were selling for something like $25.00
each. Five Chinese calipers are cheaper enough of those to run the
caliper. :-)

I can't quite parse that last sentence, but get the drift that
the batteries get pricy (even if you buy 12/$40 from earlier link,

The sentence *should* have read:

Five Chinese calipers are cheaper than enough of those (the $25.00
adaptors) to run the caliper.

http://www.micro-tools.com/store/item_detail.aspx?ItemCode=MRB625-12),
possible costing more than a new cheap caliper.

Yes.

[snip re shape]

[ ... ]

mercury batteries." ... "the WeinCell is not activated until the
pull-tab is removed (removing batteries from equipment and replacing
tabs prolongs battery life)." ...

Snapping off the battery holder, removing two small Phillips
screws, removing the four cells, and relacing the tabs -- certainly
sounds convenient. :-) Well ... three of the four cells are held + side
up, visible through apertures in the holder, but (assuming that is the
surface with the holes to be covered) that still leaves one which
requires the same disassembly sequence.
...

True, putting the tabs back on doesn't sound like an easy fix for
longer battery-life in that caliper. I included that bit in my
earlier post not because it's useful, but because it's amusing
that the seller suggests it. However, it might be practical to
store the caliper in a box purged with nitrogen, argon, or CO2.

Hmm ... if I had a TIG welding setup, the argon would be a good
choice (along with machining up a metal case with an O-ring seal and a
fill and vent valve). I wonder what would happen should I pump it down
to a fairly good vacuum (other than it leaking air back in over time).
I wonder whether the cells are allergic to vacuum.

Since this does not require the stable voltage that an exposure
meter (or one built into a camera), I'm more likely to machine a
replacement holder using two CR3032 3V cells, with a silicon diode and
two germanium diodes in series with them (for a total of about 1 V drop,
since four mercury cells add up to 5 V, and the CR3032s are 3V each.)
I'll first check how forgiving it is using a bench power supply to apply
a full 6 V (working my way up from 5 V) and see whether it has problems
with that.
[snip rest]

Depending on what the current draw is and how stable it is, you
might be able to use something like the dual-diode BAV199 in a
0.05"x0.12" SOT-23 package.

Hmm ... I've not measured the current draw yet -- but I suspect
that there will be a significant variation from displaying '1's to
displaying '8's, since it is a LED readout, not LCD like most more
recent ones.

Under a milliamp, the drop per diode
would be about right. Or perhaps use a couple of Schottky diodes
at about .4V each, a little low. Of course if you've got those
germanium diodes already, wouldn't 3 in series be about right?

About, yes.

(Per, eg, (http://baec.tripod.com/articles/crystal.htm) If
the current doesn't vary, just use a resistor.

Hmm ... he is using unfamiliar diode part numbers, except for
the mention of the US 1N34A (which I have used in the past).

The mention of the gold bonded diodes reminded me of my first
employer -- Transitron -- who got their start in the semiconductor
industry by making good quality gold bonded diodes. I *think* that they
actually invented the process. At the time, they were used heavily in
logic circuits by the then growing mainframe computer industry. By the
time I was working for them, they were making lots of silicon diodes and
transistors -- in competition with Texas Instruments. I also remember
interesting variants on the diodes called "stabistors" -- made
specifically for the stability of their forward voltage drop -- and the
temperature coefficient of that forward voltage. They were combined
with a zener diode with the opposite temperature coefficient, and a
selected transistor to make what was sold as a "ref-amp" (reference
amplifier) which had a precise and stable forward voltage drop from the
base out the emitter, and through the zener and the stabistors to
provide a very stable reference, and to amplify any variation applied to
the base to control a voltage regulator. The transistors, zeners, and
stabistors were all tested in large racks under silicone oil at -50C,
50C, and 150C, and it was fed into a very expensive (at the time)
mainframe computer to spit out selections of which ones would give the
most stable results. (Sometimes, some would give poor results at
intermediate temperatures, as I remember having to test some returned
units as they were cycled through the full range from -50 C to +150 C,
and which meant that I had to use a mix of silicone oils, as that used
at -50 C would boil at +150 C, and that used at +150 C would become
rather hard silicone grease at -50 C. :-)

I think that the company name is still around, but that they
don't make semiconductors now.

They also had another device -- a 4-lead device in a TO-5 can
which they called the "binistor". It was a mesa style silicon
transistor with an extra lead bonded wrong -- overlapping two zones, and
it behaved as a bistable device. So -- you could make a flip-flop with
a single device -- before integrated circuits allowed it in a single
chip. :-)

Enjoy,
DoN.

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