Repaired Harbor Freight digital caliper

Actually -- there *are* some "solar powered" ones -- by Mitutoyo IIRC. They would probably be excellent used daily in a well-illuminated shop. In my shop, often dark for days at a time, until a project lures me there, the replaceable batteries are a better choice.

I did recently get an auto-darkening welding hood from Harbor Freight which is solar powered, and based on what I have read about them dying if not used regularly, I've put mine on a folding workstool facing out the window so it sees daylight to maintain the charge. I'll proably eventually have to perform surgery and replace the rechargeable cells in there. There is a temptation to provide a connection for an AC-powered trickle charger so I can store it more conveniently. What would be particularly nice would be an induction coupled charger like those for electric toothbrushes. Just put it on a stand and expect it to be fully charged when I come back.

If the charge is good enough to work on the first strike, it should work fine for the rest of the day, because it will be getting a charge boost from the arc -- close enough to vigorous sunlight. :-)

Enjoy, DoN.

itutoyo

I saw a solar-powered Mitutoyo at a flea market and was intrigued, but resisted. Something about having a glass window on a machine tool close to all that hard steel just didn't sit well--it brought up memories of scratched and broken watch crystals.

OTOH, I paid $10 for most of my HF calipers. Those you can take chances with. Using the 8x25mm solar panel from a $1 calculator, a super capacitor for storage, and an LED as a regulator diode was my notion. $2 in parts, $500 labor ;-).

I fitted one to the lathe carriage--best thing I ever did. Removable. I fitted another to the tailstock ram. With it you can bore to 0.002" depth every time without even trying. Magic.

rbor

it

Sounds like a LiIon cell. If so, those can't be allowed to go dead, as you've surmised.

-- Cheers, James Arthur

Still, there's no benefit to turning off the display, in this case. = Might=20 just as well have it display "OFF".

I bought a high-end headlamp from Coleman a few years ago

while I'm not using it. There's no easy place to install a switch.

Paul

ed.

re

I measured some ordinary NiMH cells' self-discharge, 1,600mAH, @ 1.6mA IIRC. The high-capacity rechargeables are wickedly worse. I've got one set that won't hold a charge much over two weeks, no kidding, even brand-new. Self-discharge current on the order of 5-7mA.

There are low-self-discharge NiMH that hold a charge much longer, sometimes up to a year. Highly recommended. Ray-O-Vac Hybrids, Sanyo Eneloop, and Duracell has some too.

-- Cheers, James Arthur

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.

You use the LED as a shunt regulator.

tm

itions,

nel is

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.

PV -

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

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

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