Repaired Harbor Freight digital caliper

I get my LR44's and CR2032's here:

Orders > $20 ship free. I use more CR2032's.

Oh ... right ... yeah. Dope slap for me. Bob

(...)

We are all here to learn. :)

--Winston

Unfortunately, most of us have to use a digital vernier, because we can't SEE the real vernier!

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?

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.

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@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

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.)

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.

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.)

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.

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.

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

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.

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I found this, which calculates and measures caliper battery life:

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.

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?

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.

"Solar calculators may not work well in indoor conditions under ambient lighting as sufficient lighting is not available."

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.

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.

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.

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).

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

Overview of CDE super-caps:

Some interesting notes on charge time and lifetime near the bottom.

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.

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.

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@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Trying the same calc using the super-cap formula from Pg 6 of:

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@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Yes, good site. I linked to it earlier in this thread.

There aren't any ICs with low enough Iq, at least not that I know of. I used discrete transistors.

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.

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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.

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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.

Windup calipers--that's cool!

-- Cheers, James Arthur

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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.

I calculated the caliper as being a constant-current drain on the super cap, then applied Q=3DCV. Actual current drain drops a tad with falling Vdd, so my approximation is probably slightly conservative.

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

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?

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From the graph on the front page, it looks like n =3D ~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

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@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

A step-down regulator/converter could be made from a Microchip = PIC18LF14K22=20

which has a =

quiescent current of 34nA and an operating current of about 10 uA at 1.8 =

VDC.

And it may be even more efficient to use a low power linear regulator = such=20 as the TPS71501

which has 3.3 uA=20 quiescent current. If the input voltage is, say, 2 VDC and the output is =

1.6=20 VDC at 12 uA, the overall efficiency is (1.6*12)/(2*15.3) or almost 63%. =

Even at 3 VDC input it is 42%.

Paul=20

Yes, but, in the caliper context, who's going to turn the switcher off, how, and when?

See the problem?

-- Cheers, James Arthur

22 h 1.6

The LED shunt regulator saves the 2uA, so it's simpler, cheaper, and even more efficient.

-- Cheers, James Arthur

When the photocell voltage output too low to power the calipers?

A timer. Caliper runs for 120 seconds and then shuts off. I have a few small battery operated devices like this that have no OFF switch. Just punch any button, and it turns on. Wait a while, and it turns off. My Central Tools "Storm" 3C301 cheapo electronic calipers has this feature.

The unintelligible owners manual proudly highlights this feature on the cover. Apparently, previous versions did NOT turn off automatically. Oddly, it still has an on/off push button, even though it will turn on if the jaws are moved.

Hmmm... the catalog says that it ships with an SR44/357 silver oxide battery. Yet, the one I purchased had alkaline LR44 batteries.

Nope.

I still think that wind up power would be more interesting.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558