On Nov 24, 5:20*pm, Jeff Liebermann wrote:
On Wed, 23 Nov 2011 06:45:46 -0800 (PST),
wrote:

The max. solar panel voltage doesn't matter--the LED regulator clamps
the solar panel voltage, taking care of that.

Yeah, but shunt regulators and leaky super-caps are not really
appropriate for micropower devices. *They waste power.

Small, cheap and simple are the main factors here. The r.c.m. guys
aren't going to be building switching regulators, and switching
regulators generally aren't more efficient at these power levels
anyhow--their quiescent current draw's too high.

(I've made a study of designing microwatt switchers, from scratch.
It's possible, but wholly inappropriate here.)

In a previous message, James Arthur measured:
* Drain: 13.5uA (off), 14.5uA (on)
* Battery low threshold (blinking display): 1.37V
* Lowest operating voltage: 1.01V

Nominal voltage on a silver oxide battery is 1.5V. *Therefore, the
operating power is:
* *1.5VDC * 15uA = 22.5 microwatts.
From the standpoint of a resistive load, that's about:
* *1.5VDC / 15 uA = 100K ohms

The first question is whether a small solar cell will product 22.5
microwatts. *Testing a somewhat oversized polycrystaline cell that I
found in my junk box (quality unknown), it produces 3.0VDC at 6ma with
a short circuit load (my milliamps guesser). *My guess(tm) is that
this cell is about three times as big as will conveniently fit on the
calipers, so I'll just cut the current to 2ma . *Delivered power with
my desk lamp is 6 milliwatts. *Yeah, it will a 22.5 microwatt load.

Not so fast... The advantage of the thin-film PV panels is that
(appropriate) panels excel at producing power even in dim light.
Polycrystalline silicon panels don't.

The array I suggested for experimentation is thin-film for that
reason--so it can work in indoor light levels.

The next question is for how long will it run? *Assuming the calipers
can handle 3.0VDC without damage, how long will a junk 100UF
electrolytic cap run the calipers?

a) How long will it run? Not nearly long enough, and b) 3.0VDC is
waayyy too risky for my blood. 20uA will discharge 100uF from 2.0V to
1.35V in 3.25 seconds.

Of the setup I suggested, the most marginal part is the itty bitty PV
panel (its output is on the low side). Dark leakage on my much-larger
10x55mm calculator panel is about 8uA @ 1.7V bias.

The supercap works wonderfully well. Charge 0.6F to 1.8V, and you've
got 4 hours' runtime until you reach the 1.35V battery-low display-
starts-blinking level. (Assuming 20uA total draw, to allow for some
leakage.)

http://www.kpsec.freeuk.com/capacit.htm
From 1.37V is roughly 50% of full 3.0VDC charge. *That's about 80% of
1RC time constant. *1RC is:
* *0.8 * 100K * 1000uF = 80 seconds
That's probably enough to make a few measurements. *Any longer and a
super-cap will probably be needed. *Picking 50% of full charge out of
the hat is rather convenient, as it makes the time to charge from zero
to the dropout point the same 80 seconds (yes, I'm lazy). *Whether the
user really wants to wait 1.5 minutes under a desk lamp for the
calipers to be usable is dubious. *Of course, a longer run time, means
a longer charge time. *For example, a 1F 5V 1ua leakage super-cap,
will run the calipers for 80,000 seconds, but will also take 80,000
seconds to charge.

Not 80,000s. Expose the PV to sunlight (or directly to a lamp), and
it'll charge (initially) 50x faster. You'd only have to do that
once. Indoors, the PV would keep it topped off, that's the idea.

Alternatively, an electrolytic works, but gives a caliper that quickly
quits if you accidentally shadow it.

There are much smaller supercaps--0.02F--used in cellphones. That's
another option / compromise. Leakage should be better too.

There are low voltage DC-DC boost/buck switching regulator chips
available that can tolerate a wide range of input voltages, and
deliver a constant 1.5VDC.

In my never humble opinion, what makes more sense is to do it exactly
like the typical solar powered calculator. *They all have one or two
LR44 batteries inside. *However, the solar cell does NOT charge the
battery. *When you turn the calculator on, and there's enough light to
run from the solar cell, the battery is essentially disconnected. When
there's not enough light to run the calculator, it runs off the
battery. *No waiting to charge a capacitor from the solar cell.

That uses the PV as, basically, a battery-extender. That's fine, but
complex--you need a micro-power switch to disconnect the battery, etc.
(A diode drops waayyy too much voltage.) That puts it out of the
realm of a simple project that can fit into the existing caliper.

If you're into high tech, there are various energy scavenging devices
that can also power the calipers.
http://en.wikipedia.org/wiki/Energy_harvesting
With only 22.5 microwatts required, it might be possible to power the
device with a wind up key, piezo pressure, body heat, kinetic magnetic
generator, etc. *I kinda like the idea of a wind up caliper.

Windup would be fun--steampunk.

The "real" solution is to design the caliper to draw less current in
the first place, like Mitutoyo and Starrett. If you've done that,
solar-powering is a snap, but then, if the battery lasts years, you
don't need solar power, do you?

--
Cheers,
James Arthur