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John S
Guest
Thu Nov 24, 2011 12:50 am
On 11/23/2011 4:56 PM, Bob Engelhardt wrote:
Quote:
John S wrote:
On 11/23/2011 2:05 PM, Bob Engelhardt wrote:
Doncha' need a current limiter on the LED?
Not if the PV cell's rated SC current is less than the max LED current
rating.
Isn't the capacitor's rated current MUCH more than the LED's? Bob
What has that to do with it? The voltage is clamped. What are you missing?
Ron D.
Guest
Thu Nov 24, 2011 2:03 am
I get my LR44's and CR2032's here:
http://www.infinitelights.com/alkalinewatchbatteries.html
Orders > $20 ship free. I use more CR2032's.
Bob Engelhardt
Guest
Thu Nov 24, 2011 3:38 am
Winston wrote:
Quote:
This circuit, yes?
D1
.--|>|--+-----+----> (+) to caliper
+ | | |
.------. + | |
| | --- ---
| ---- | --- \ / ~~
| | C1| ---
| ---- | | | LED (red)
| | | |
'------' | |
- | PV | |
'--------+-----+---> (-) to caliper
Under no circumstances would the voltage across
the LED (and capacitor) go above, say 1.8 V
because the LED turns any additional
voltage into current. ...
Oh ... right ... yeah. Dope slap for me. Bob
Winston
Guest
Thu Nov 24, 2011 5:19 am
Bob Engelhardt wrote:
Quote:
Winston wrote:
This circuit, yes?
(...)
Quote:
Under no circumstances would the voltage across
the LED (and capacitor) go above, say 1.8 V
because the LED turns any additional
voltage into current. ...
Oh ... right ... yeah.
We are all here to learn. :)
--Winston
Jeff Liebermann
Guest
Thu Nov 24, 2011 11:20 pm
On Wed, 23 Nov 2011 06:45:46 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:
Quote:
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.
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.
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?
<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.
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.
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.
Happy Day of the Turkeys.
--
Jeff Liebermann jeffl_at_cruzio.com
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Guest
Fri Nov 25, 2011 3:30 am
On Nov 24, 5:20 pm, Jeff Liebermann <je...@cruzio.com> wrote:
Quote:
On Wed, 23 Nov 2011 06:45:46 -0800 (PST), dagmargoodb...@yahoo.com
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.)
Quote:
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.
Quote:
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.)
Quote:
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.
Quote:
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.
Quote:
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
DoN. Nichols
Guest
Fri Nov 25, 2011 4:16 am
On 2011-11-23, Bob Engelhardt <bobengelhardt_at_comcast.net> wrote:
Quote:
Winston wrote:
Bob Engelhardt wrote:
The wiki ckt has a current-limiting resistor in series with the voltage
source. Not so the posted ckt.
It *is* in the posted circuit.
Sort of. :)
As James mentions, the internal resistance of the PV
cell is the current-limiting resistor.
I meant the capacitor voltage source. Is its internal resistance a
sufficient current limiter? I'm not familiar with super caps, but the
common ones that I am familiar with will supply huge currents, momentarily.
The capacitor gets its voltage from the PV cell. Assuming that
you don't put a switch between the LED and the cap (there is none shown
in the schematic), the cap will never charge high enough to be able to
damage the LED, because the LED will have already clamped the maximum
voltage based on the current limit of the PV cell. Not sure what would
happen with the PV cell close to an arc welding process like a TIG -- it
depends on the internal resistance of the PV cell and the peak voltage
which the PV cell can produce with such excessive illumination.
Enjoy,
DoN.
--
Remove oil spill source from e-mail
Email: <BPdnicholsBP_at_d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. |
http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
Jeff Liebermann
Guest
Fri Nov 25, 2011 6:46 am
On Thu, 24 Nov 2011 18:13:15 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:
I found this, which calculates and measures caliper battery life:
<http://www.davehylands.com/Machinist/Caliper-Batteries/>
Quote:
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.
True. However, switching regulators usually have some manner of load
shedding when the supply voltage is insufficient. Below that
threshold, the current drain is usually in nanoamps.
Quote:
(I've made a study of designing microwatt switchers, from scratch.
It's possible, but wholly inappropriate here.)
You're ahead of me. I've never designed anything in that low power
class. Different world. Can you point me to a suitable (or close to
suitable) regulator chip?
Quote:
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.
Decisions, decisions, and more decisions. Polycrystaline has a cost
advantage and is more efficient than single layer thin-film. Well, if
I wanted to go cheap, I would use amorphous cells and mold them into
the plastic case. For small solar cells, the cost of monocrystaline
isn't all that much more (i.e. most of the cost is in packaging and
handling) but won't work well with indoor lighting. So, I guess
thin-film is the least disgusting.
<http://en.wikipedia.org/wiki/Solar_powered_calculator>
"Solar calculators may not work well in indoor
conditions under ambient lighting as sufficient lighting
is not available."
Quote:
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.
I used 1000uF elsewhere in my calcs, but slipped here and used 100uF
instead. Sorry.
I think you might be a bit too conservative. 5ua leakage is high.
Most of the spec sheets I've skimmed show 1-2ua for a typical 1F 5.5V
super-cap.
Quote:
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 alternative is to lose approximately 0.3V in a series Schottky
diode. That's about 20% of the power budget, which is probably too
much.
Quote:
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.)
Ok. You've sold me. I was trying to see what could be done with
commodity electrolytic caps. Also, super-caps fail to appreciate high
humidity, which may become a problem.
Quote:
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.
Yep. However, I screwed up. The discharge load is:
1.5VDC / 15uA = 100K ohms
However, the charging ESR is much less.
3.0VDC / 2ma = 1.5K
It will certainly be higher a lower illumination levels. Checking my
junk cell under random room lighting conditions, and again scaling for
size, I get:
0.333 * 0.55v / 0.02mA = 9.2K
I don't have a small thin film panel to test. (I have 90watt panel,
but that's a bit much for scaling to caliper size).
Quote:
Alternatively, an electrolytic works, but gives a caliper that quickly
quits if you accidentally shadow it.
Not if you do exactly like it's done with a calculator. When the cell
is shaded, it runs on battery. A silver-oxide battery holds:
1.5v * 150 mA-Hr = 22.5 milliwatt-Hrs
and will deliver most of that before the voltage drops to unusable
levels.
The super cap will deliver (very roughly):
1.5v * 15uA * 4Hr = 90 microwatt-Hrs
Quote:
There are much smaller supercaps--0.02F--used in cellphones. That's
another option / compromise. Leakage should be better too.
Overview of CDE super-caps:
<http://www.cde.com/catalogs/EDL.pdf>
Some interesting notes on charge time and lifetime near the bottom.
Quote:
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.
There has to be a chip in the calipers anyway to count pulses, run the
display, and deal with the push buttons. Adding a power management
feature does not add much real estate or complexity. However, if
you're thinking of a retrofit, I suspect something could be done with
a separate switcher chip.
Quote:
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.
In the late 1960's, I designed and built a paging receiver, that
produced the message output on a 1/4" wide roll of paper tape. Battery
power to the mechanics for such a portable device was impossible. So,
I went to a wind up coil spring mechanism. I've been somewhat of a
fan of spring power ever since.
Quote:
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?
Agreed. It would be like a digital watch, which typically has a 10
year battery life. However, the solar cell is still a problem because
of the dark current (reverse leakage). An isolating Schottky diode
can reduce that, but then the solar cell would need to be about 20%
larger to compensate for the added loss.
Another problem is that it would be no fun. Windup calipers offer a
far more entertaining problem to solve.
--
Jeff Liebermann jeffl_at_cruzio.com
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Jeff Liebermann
Guest
Fri Nov 25, 2011 7:29 am
On Thu, 24 Nov 2011 21:46:48 -0800, Jeff Liebermann <jeffl_at_cruzio.com>
wrote:
Quote:
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.
Trying the same calc using the super-cap formula from Pg 6 of:
<http://www.cde.com/catalogs/EDL.pdf>
t = C delta V / I
t = C[V0-(i*R)-V1] / (i+iL)
where:
t: Back-up time (sec)
C: Capacitance of Type EDL (Farads)
V0: Applied voltage (Volts)
V1: Cut-off voltage (Volts)
i: Current during back-up (Amps)
iL: Leakage current (Amps)
R: Internal resistance (ohms) at 1 kHz
For this example, I'll use a 0.1F (type F) 5.5V 100 ohm cap.
The low end of the tolerance range might drop this to 0.08F.
V0 = 2.0V, V1 = 1.4V, i = 15uA, iL = 2uA
Plugging in:
t = C[V0-(i*R)-V1] / (i+iL)
t = 0.08F[2.0V-(15uA*100ohms)-1.4V]/(15uA+2uA)
t = 2800 sec = 47 minutes.
Not bad.
I guess the protective case that most calipers use will need a clear
plastic window to keep it charged. Maybe another window on top of my
toolbox.
--
Jeff Liebermann jeffl_at_cruzio.com
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Gunner Asch
Guest
Fri Nov 25, 2011 10:27 am
On Thu, 24 Nov 2011 22:29:50 -0800, Jeff Liebermann <jeffl_at_cruzio.com>
wrote:
Quote:
On Thu, 24 Nov 2011 21:46:48 -0800, Jeff Liebermann <jeffl_at_cruzio.com
wrote:
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.
Trying the same calc using the super-cap formula from Pg 6 of:
http://www.cde.com/catalogs/EDL.pdf
t = C delta V / I
t = C[V0-(i*R)-V1] / (i+iL)
where:
t: Back-up time (sec)
C: Capacitance of Type EDL (Farads)
V0: Applied voltage (Volts)
V1: Cut-off voltage (Volts)
i: Current during back-up (Amps)
iL: Leakage current (Amps)
R: Internal resistance (ohms) at 1 kHz
For this example, I'll use a 0.1F (type F) 5.5V 100 ohm cap.
The low end of the tolerance range might drop this to 0.08F.
V0 = 2.0V, V1 = 1.4V, i = 15uA, iL = 2uA
Plugging in:
t = C[V0-(i*R)-V1] / (i+iL)
t = 0.08F[2.0V-(15uA*100ohms)-1.4V]/(15uA+2uA)
t = 2800 sec = 47 minutes.
Not bad.
I guess the protective case that most calipers use will need a clear
plastic window to keep it charged. Maybe another window on top of my
toolbox.
(only 2 digits)
http://www.alibaba.com/showroom/solar-power-digital-caliper.html
Quite a number of them.....
One could not be a successful Leftwinger without realizing that,
in contrast to the popular conception supported by newspapers
and mothers of Leftwingers, a goodly number of Leftwingers are
not only narrow-minded and dull, but also just stupid.
Gunner Asch
Guest
Fri Nov 25, 2011 5:06 pm
On Nov 25, 12:46 am, Jeff Liebermann <je...@cruzio.com> wrote:
Quote:
Yes, good site. I linked to it earlier in this thread.
Quote:
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.
True. However, switching regulators usually have some manner of load
shedding when the supply voltage is insufficient. Below that
threshold, the current drain is usually in nanoamps.
(I've made a study of designing microwatt switchers, from scratch.
It's possible, but wholly inappropriate here.)
You're ahead of me. I've never designed anything in that low power
class. Different world. Can you point me to a suitable (or close to
suitable) regulator chip?
There aren't any ICs with low enough Iq, at least not that I know of.
I used discrete transistors.
Quote:
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.
Decisions, decisions, and more decisions. Polycrystaline has a cost
advantage and is more efficient than single layer thin-film. Well, if
I wanted to go cheap, I would use amorphous cells and mold them into
the plastic case. For small solar cells, the cost of monocrystaline
isn't all that much more (i.e. most of the cost is in packaging and
handling) but won't work well with indoor lighting. So, I guess
thin-film is the least disgusting.
http://en.wikipedia.org/wiki/Solar_powered_calculator
"Solar calculators may not work well in indoor
conditions under ambient lighting as sufficient lighting
is not available."
You can scavenge a PV from a cheap solar calculator, as low as $1. I
also linked to a part from Goldmine-elec.com.
Polycrystalline cells put out lots more in bright light, but AFAIK,
all solar calculators (and calipers, for that matter), use the
amorphous (thin-film) cells for the low-light performance. Cost might
also be a factor.
Quote:
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.
I used 1000uF elsewhere in my calcs, but slipped here and used 100uF
instead. Sorry.
I think you might be a bit too conservative. 5ua leakage is high.
Most of the spec sheets I've skimmed show 1-2ua for a typical 1F 5.5V
super-cap.
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 alternative is to lose approximately 0.3V in a series Schottky
diode. That's about 20% of the power budget, which is probably too
much.
I believe the panels put out a high enough overvoltage that the diode
loss doesn't matter--it's only going to get wasted in the LED shunt
regulators any how. I'll check.
MEASUREMENTS
Panel: 4-section 10x50mm panel, from a (retired) TI calculator:
Lighting 1: 1.8V (open), 18.5uA (short-circuit)
Lighting 2: 2.5V (open), 300uA (short-circuit)
[1] Modest indoor light (indirect sunlight, filtering through blinds,
measured from the ceiling bounce).
[2] 2' from 20W halogen bulb.
So, a 1n4148 drops too much for comfort. A BAT54 drops about 150mV
forward at these currents, and leaks a fraction of a uA at these
temperatures and reverse biases. Or, you could omit the diode and
just let the thing power down in the shade.
Quote:
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.)
Ok. You've sold me. I was trying to see what could be done with
commodity electrolytic caps. Also, super-caps fail to appreciate high
humidity, which may become a problem.
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.
Yep. However, I screwed up. The discharge load is:
1.5VDC / 15uA = 100K ohms
However, the charging ESR is much less.
3.0VDC / 2ma = 1.5K
It will certainly be higher a lower illumination levels. Checking my
junk cell under random room lighting conditions, and again scaling for
size, I get:
0.333 * 0.55v / 0.02mA = 9.2K
I don't have a small thin film panel to test. (I have 90watt panel,
but that's a bit much for scaling to caliper size).
Alternatively, an electrolytic works, but gives a caliper that quickly
quits if you accidentally shadow it.
Not if you do exactly like it's done with a calculator. When the cell
is shaded, it runs on battery. A silver-oxide battery holds:
1.5v * 150 mA-Hr = 22.5 milliwatt-Hrs
and will deliver most of that before the voltage drops to unusable
levels.
The super cap will deliver (very roughly):
1.5v * 15uA * 4Hr = 90 microwatt-Hrs
There are much smaller supercaps--0.02F--used in cellphones. That's
another option / compromise. Leakage should be better too.
Overview of CDE super-caps:
http://www.cde.com/catalogs/EDL.pdf
Some interesting notes on charge time and lifetime near the bottom.
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.
There has to be a chip in the calipers anyway to count pulses, run the
display, and deal with the push buttons. Adding a power management
feature does not add much real estate or complexity. However, if
you're thinking of a retrofit, I suspect something could be done with
a separate switcher chip.
If we're designing it from scratch, we just wouldn't use so darn much
power to start with. Then, a PV panel and a capacitor are all you
need.
Switcher chips just don't do well on 20uA power input.
Quote:
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.
In the late 1960's, I designed and built a paging receiver, that
produced the message output on a 1/4" wide roll of paper tape. Battery
power to the mechanics for such a portable device was impossible. So,
I went to a wind up coil spring mechanism. I've been somewhat of a
fan of spring power ever since.
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?
Agreed. It would be like a digital watch, which typically has a 10
year battery life. However, the solar cell is still a problem because
of the dark current (reverse leakage). An isolating Schottky diode
can reduce that, but then the solar cell would need to be about 20%
larger to compensate for the added loss.
Another problem is that it would be no fun. Windup calipers offer a
far more entertaining problem to solve.
Windup calipers--that's cool!
--
Cheers,
James Arthur
Guest
Fri Nov 25, 2011 5:22 pm
On Nov 25, 1:29 am, Jeff Liebermann <je...@cruzio.com> wrote:
Quote:
On Thu, 24 Nov 2011 21:46:48 -0800, Jeff Liebermann <je...@cruzio.com
wrote:
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.
Trying the same calc using the super-cap formula from Pg 6 of:
http://www.cde.com/catalogs/EDL.pdf
t = C delta V / I
t = C[V0-(i*R)-V1] / (i+iL)
where:
t: Back-up time (sec)
C: Capacitance of Type EDL (Farads)
V0: Applied voltage (Volts)
V1: Cut-off voltage (Volts)
i: Current during back-up (Amps)
iL: Leakage current (Amps)
R: Internal resistance (ohms) at 1 kHz
For this example, I'll use a 0.1F (type F) 5.5V 100 ohm cap.
The low end of the tolerance range might drop this to 0.08F.
V0 = 2.0V, V1 = 1.4V, i = 15uA, iL = 2uA
That cap is 14x10mm, pretty humungous. You don't need 5.5v, so the
'EN' type, at 7x2mm and 0.2F might be a better fit.
Quote:
Plugging in:
t = C[V0-(i*R)-V1] / (i+iL)
t = 0.08F[2.0V-(15uA*100ohms)-1.4V]/(15uA+2uA)
t = 2800 sec = 47 minutes.
Not bad.
I calculated the caliper as being a constant-current drain on the
super cap, then applied Q=CV. Actual current drain drops a tad with
falling Vdd, so my approximation is probably slightly conservative.
Quote:
I guess the protective case that most calipers use will need a clear
plastic window to keep it charged. Maybe another window on top of my
toolbox.
Yep. Another retro-fit possibility is to fit a supercap in the
caliper, and a lithium-AA (1.65v) in the caliper case that recharges
the supercap when not in use.
That'll last forever (about 10years on the 'AA'), runs for hours per
charge, fits the case easily, and doesn't need a PV or any fancy
circuitry. The PAS920 I linked before costs 5/$1 surplus, from
Goldmine-elec.com.
--
Cheers,
James Arthur
krw@att.bizzzzzzzzzzzz
Guest
Fri Nov 25, 2011 11:31 pm
On Fri, 25 Nov 2011 07:06:57 -0800 (PST), dagmargoodboat_at_yahoo.com wrote:
Quote:
On Nov 25, 12:46 am, Jeff Liebermann <je...@cruzio.com> wrote:
On Thu, 24 Nov 2011 18:13:15 -0800 (PST), dagmargoodb...@yahoo.com
wrote:
I found this, which calculates and measures caliper battery life:
http://www.davehylands.com/Machinist/Caliper-Batteries/
Yes, good site. I linked to it earlier in this thread.
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.
True. However, switching regulators usually have some manner of load
shedding when the supply voltage is insufficient. Below that
threshold, the current drain is usually in nanoamps.
(I've made a study of designing microwatt switchers, from scratch.
It's possible, but wholly inappropriate here.)
You're ahead of me. I've never designed anything in that low power
class. Different world. Can you point me to a suitable (or close to
suitable) regulator chip?
There aren't any ICs with low enough Iq, at least not that I know of.
I used discrete transistors.
There are some pretty good ones, designed for USB applications, but I don't
thing they're quite good enough for this. The TPS6205x Iq is around 5uA to and
in shutdown less than 2uA. You're looking for something an order of magnitude
better than this?
<...>
Guest
Sat Nov 26, 2011 7:28 am
On Nov 25, 5:31 pm, "k...@att.bizzzzzzzzzzzz"
<k...@att.bizzzzzzzzzzzz> wrote:
Quote:
On Fri, 25 Nov 2011 07:06:57 -0800 (PST), dagmargoodb...@yahoo.com wrote:
On Nov 25, 12:46 am, Jeff Liebermann <je...@cruzio.com> wrote:
On Thu, 24 Nov 2011 18:13:15 -0800 (PST), dagmargoodb...@yahoo.com
wrote:
I found this, which calculates and measures caliper battery life:
http://www.davehylands.com/Machinist/Caliper-Batteries/
Yes, good site. I linked to it earlier in this thread.
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.
True. However, switching regulators usually have some manner of load
shedding when the supply voltage is insufficient. Below that
threshold, the current drain is usually in nanoamps.
(I've made a study of designing microwatt switchers, from scratch.
It's possible, but wholly inappropriate here.)
You're ahead of me. I've never designed anything in that low power
class. Different world. Can you point me to a suitable (or close to
suitable) regulator chip?
There aren't any ICs with low enough Iq, at least not that I know of.
I used discrete transistors.
There are some pretty good ones, designed for USB applications, but I don't
thing they're quite good enough for this. The TPS6205x Iq is around 5uA to and
in shutdown less than 2uA. You're looking for something an order of magnitude
better than this?
...
From the graph on the front page, it looks like n = ~35% @ 15uA
output. That's actually very good. Thanks.
My designs were mostly boost topology, so there may be ICs I didn't
consider (plus new ICs I haven't seen). I did some nutty stuff, like
nano-amp oscillators and micro-amp switchers that were roughly 75%
efficient.
--
Cheers,
James Arthur
Jeff Liebermann
Guest
Sat Nov 26, 2011 6:36 pm
On Fri, 25 Nov 2011 21:28:54 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:
Quote:
On Nov 25, 5:31 pm, "k...@att.bizzzzzzzzzzzz"
k...@att.bizzzzzzzzzzzz> wrote:
(...)
http://www.ti.com/lit/ds/symlink/tps62050.pdf
From the graph on the front page, it looks like n = ~35% @ 15uA
output. That's actually very good. Thanks.
That's because of the 12uA typical quiescent current, where the chip
draws about the same current as the caliper load. For equal currents,
that's 50% maximum efficiency. The TPS62054 shows 50% efficiency at
2.7V in and 1.8V out (See Pg 8 Fig 4).
The chips do have a shutdown pin that cuts the quiescent current to
"less than 2uA". Still high, but much better.
--
Jeff Liebermann jeffl_at_cruzio.com
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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