Repaired Harbor Freight digital caliper

Well, let's dive a bit deeper into the contents. The CAS number identifies the exact ingredients. We have: CAS % by weight Aliphatic Hydrocarbon 64742-47-8 45-50% That would be:

Low odor paraffinic solvent, Dearomatized kerosine, Deodorized kerosene. In other words, lamp oil as used in a kerosene lamp.

Petroleum Base Oil

I've seen them only new, and decided that in my shop conditions, they would not work very well. :-)

O.K. Do you know the maximum voltage that the solar panel is likely to produce? And the voltage drop on the LED? I know that silicon diodes are typically between 600 mV and 750 mV. Also, any clues as to the maximum voltage that the calipers can tolerate long term?

[ ... ]

And -- they are supposedly not replaceable according to the manual. :-) (You've got to cut the package apart to get to them.) There is a web page describing how someone opened one up and set a holder for two AA cells outside the package. I'm really tempted to go for the induction charger when I finally have to dig into mine. But it is significantly less expensive than auto-darkening ones from MSC -- to the point where three HF ones match the cost of one from MSC. :-)

Enjoy, DoN.

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

Here's a sketch:

D1 .--|>|--+-----+----> (+) to caliper + | | | .------. + | | | | --- --- | ---- | --- \ / ~~>

| | C1| --- | ---- | | | LED (red) | | | | '------' | | - | PV | | '--------+-----+---> (-) to caliper

C1 - A capacitor to power the caliper during momentary outages. Optionally a super-cap., e.g.

o.asp?number=3DG13133 (Note1: The PV will take HOURS of bright light to charge a super- cap, so you might want to precharge the cap, then let the PV just float it.) (Note2: Super-caps have significant electrical leakage. If your cap is too leaky, it'll never charge, and you'll be disappointed. I haven't measured the above-linked PAS920 to see if this is the case.)

PV -

or, stolen from the cheapest solar-powered calculator you can find. Walmart and the dollar stores have some good candidates. LED - shunt-regulates the PV output to a safe voltage. D1 - 1n4148, prevents PV from draining a super-cap C1 when dark, possibly not needed. (Depends on PV panel's dark leakage current.)

To set the voltage you'd choose an LED with a forward voltage of, say

1.6-1.8v. Three ordinary small-signal silicon diodes in series (e.g. 1n4148) wouldn't be a bad choice either. With surface-mount parts, the ckt can be tiny.

If the voltage is too high it's not a matter of "long-term," the caliper will die instantly. I don't know what that voltage is, but I'd easily wager one of *my* $10 calipers that 2v is okay, and I'd wager one of *yours* that 2.5v might be okay too. ;-)

-- Cheers, James Arthur

snipped-for-privacy@yahoo.com wrote: ...

...

Doncha' need a current limiter on the LED?

Bob

Not if the PV cell's rated SC current is less than the max LED current rating.

Nup. It's a 'shunt regulator'.

--Winston

(...)

Next time, advise jump on it before someone else does.

I have two of those. They just keep working without any issues. Well, except for turning off in low-light situations. I just charge it up with the flashlight and it works just fine. 'Way better than having to run to the store for a $5 battery! They are fine tools and have my highest recommendation.

--Winston

Nope. These little PV panels barely manage 1 or 2 mA even in sunlight, 20uA under fluorescent. The LED's good for 10x the PV's max. output.

I dragged an old calculator panel out[*] and connected it to one of those 0.6F super caps. It's charging the cap 1mV/8s, with no load.

[*] this panel is 10x55mm--much larger than the one I referenced. I have several of the smaller ones, but they're all still working hard in $1 Walmart calculators.

It's pretty impractical (clunky, delicate), but fun.

-- Cheers, James Arthur

Yes, but I got one of the HF's and hacked it up with a Dremel tool, to mount to the lathe.

Can't do that with a Mitutoyo!

-- Cheers, James Arthur

Isn't the capacitor's rated current MUCH more than the LED's? Bob

The wiki ckt has a current-limiting resistor in series with the voltage source. Not so the posted ckt.

Bob

(...)

And hold one's head up in public, that is. :)

--Winston

It *is* in the posted circuit. Sort of. :)

As James mentions, the internal resistance of the PV cell is the current-limiting resistor.

--Winston

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.

Bob

Hey, that's one hell of an idea! :)

Jamie

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. In order for C1 to produce a current large enough to endanger the LED or the caliper, it would have to be allowed to charge significantly above the 'zener point' of the LED. It just cannot.

--Winston

What has that to do with it? The voltage is clamped. What are you missing?

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