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Repaired Harbor Freight digital caliper
"Winston" wrote in message ...
[big snip...] For your 'fixed' installations, you could solder some small 'earphone' wire to the battery contacts and use a huge, cheap external cell. (Huge = AA) I didn't expect my post to generate such a big discussion, but it's been quite interesting. Since I only use my calipers once in a while (it had been years until I once again tried to use them and found out how to fix them), it seems that just removing the cell is best, and that guards against possible damage if it leaks. My machine shop is in an unheated building and the temperature and humidity extremes may make leakage more likely. Otherwise I think we have established that the normal lifetime of a 175 mA-hr cell, even with the typical 10-15 uA of on or off current, still provides well over 1 year of life. Replacing it annually for $0.75 compared to once every 3-5 years of shelf life does not warrant any expenditure of time or materials for a high-tech (or even low-tech) solution. I've already spent an hour or more in this discussion. At my normal consulting rate, that would buy a hundred batteries! But if removing the battery is too annoying, you might be able to cut a thin slot in the battery compartment, and just insert and remove a thin, stiff piece of mylar or similar insulating material to isolate the negative terminal of the battery from the contact. Paul www.pstech-inc.com |
Repaired Harbor Freight digital caliper
On 2011-11-29, P E Schoen wrote:
"Winston" wrote in message ... [big snip...] For your 'fixed' installations, you could solder some small 'earphone' wire to the battery contacts and use a huge, cheap external cell. (Huge = AA) I didn't expect my post to generate such a big discussion, but it's been quite interesting. Since I only use my calipers once in a while (it had been years until I once again tried to use them and found out how to fix them), it seems that just removing the cell is best, and that guards against possible damage if it leaks. My machine shop is in an unheated building and the temperature and humidity extremes may make leakage more likely. FWIW -- this discussion has prompted me to check out the B&S Digital calipers which I have (two, only one of which I have run on batteries, because the proper batteries went to unobtanium before I got the second one (for free) -- each in its own fitted wooden box. The first cost me $15.00 at a hamfest. The original batteries were the 625 Mercury cells -- weird format and of course a lower voltage than most others -- 1.35 V when new. The caliper has a glass optical scale down the groove where the depth gauge lives and the rack gear is on a dial caliper, so it is one w which I trust to be accurate. It used four of those cells. I figured that if the electronics and LEDs (used to illuminate the glass scale) could tolerate 6 V instead of the 5.4 V with the original cells, I could likely use a pair of CR 2032 3V cells. Well ... a bit of work on the electronics bench shows that it will indeed tolerate the higher voltage. It also draws 1.4 mA instead of the 40 uA range that the newer calipers use) so I really plan to add a switch to the battery holder. I think that all I need to do is remove some of the jumper strips which went between cells, and make a new screw-on cover for the new cells to hold them in proper alignment. If so -- it will add two extra calipers to my collection. Enjoy, DoN. -- Remove oil spill source from e-mail Email: | 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 --- |
Repaired Harbor Freight digital caliper
On Nov 28, 8:40*pm, "P E Schoen" wrote:
"Winston" *wrote in ... [big snip...] For your 'fixed' installations, you could solder some small 'earphone' wire to the battery contacts and use a huge, cheap external cell. (Huge = AA) I didn't expect my post to generate such a big discussion, Well, this is a *discussion* group. :-) but it's been quite interesting. Since I only use my calipers once in a while (it had been years until I once again tried to use them and found out how to fix them), it seems that just removing the cell is best, and that guards against possible damage if it leaks. My machine shop is in an unheated building and the temperature and humidity extremes may make leakage more likely. Otherwise I think we have established that the normal lifetime of a 175 mA-hr cell, even with the typical 10-15 uA of on or off current, still provides well over 1 year of life. Replacing it annually for $0.75 compared to once every 3-5 years of shelf life does not warrant any expenditure of time or materials for a high-tech (or even low-tech) solution. I've already spent an hour or more in this discussion. At my normal consulting rate, that would buy a hundred batteries! Of course. The goal is to avoid spending an hour of your time at your consulting rate finding a new silver oxide cell, after you reach for the thing and it's dead. Also, the prevailing rate is $3 per cell (or at least it was, before we all revealed our fave cheaper sources). But if removing the battery is too annoying, you might be able to cut a thin slot in the battery compartment, and just insert and remove a thin, stiff piece of mylar or similar insulating material to isolate the negative terminal of the battery from the contact. That's the ultimate cheap switch. The body, FWIW, is the (+) contact. -- Cheers, James Arthur |
Repaired Harbor Freight digital caliper
On Nov 28, 5:17*pm, Winston wrote:
wrote: (...) Low-current switches can be a bear--the contacts oxidize. *Gold fixes that generally, IIRC. D.C. switches have their own complications, though I suspect ~40 uA wouldn't be a problem, given the proper contact plating and wiping action. I think gold is gonna be problematical if there will be much in the way of capacitor charging 'inrush' needed. TATOO: Look boss, deplate! Deplate! E.s.r. of the cap I linked was 80 ohms. That just isn't a problem. * Most super caps don't tolerate high current well. No problem here. *The low battery voltage and high internal resistance of these tiny cell-phone super caps limits the current nicely. The super cap is in parallel with the low-z bulk bypass caps in the caliper, though. True. A series resistor is needed. *Your pushbutton will have to be properly sized and snubbed. The folks at Palm found out about that much too late. * This mod is dead simple, reversible and does not * * require access to a ground connection. I like some sort of dead-man timer better. *It could be just a low threshold mosfet with a gate cap which you charge, and a *large* discharge resistor(*) so it times out. That guarantees the battery- saving feature even if you forget. I hear LTSpice calling. *:) We need a pass element that has a gate voltage saturation point in the 200-300 mV region. I just don't see a MOSFET in that role, somehow. :) The cell voltage is 1.55v, and it's nearly dead @ 1.2v. A MOSFET with Id = 100uA @ Vgs = 1.2v would be fine. A BSS138 is in that ballpark; there might be better choices. (*) (From the jellybean / junkbox standpoint, the discharge "resistor" might be a reverse-biased rectifier's leakage.) But yes, just adding a real switch is a huge improvement over not having it. A real switch also lets you hold the zero setting over night if you want to. *I sometimes do that with the lathe, if, for example, I'm in the middle of something when it's time to turn in. You *do* need access to the underside of the cell, since that's where the (-) contact is. You can switch the positive side as easily if your interposer board is thin enough. Insert a very thin piece of double-sided printed circuit mat'l, wire a tiny slide switch to both sides of that, and Bob's yer uncle. Yup. that is how we do it. I used very thin double sided stock, though most of the time the 0.062" stuff worked fine. It is great for measuring current too. That's reversible, and if you're a brute, you can even hang the switch outboard by the wires. *That way there's no modification of the caliper needed at all. That's good enough and simple enough that I'll put 'er on the list. I've got maybe a dozen of these (two on the lathe alone), and it'd be nice not pulling the batteries (as I do now). For your 'fixed' installations, you could solder some small 'earphone' wire to the battery contacts and use a huge, cheap external cell. (Huge = AA) Yep. I mentioned that earlier. An Energizer lithium AA or AAA is well suited, voltage-wise. It still needs strapping somewhere...I'll be working in the shop today, so I'll cast an eye toward the lathe and see if there's room. -- Cheers, James Arthur |
Repaired Harbor Freight digital caliper
wrote:
(...) The cell voltage is 1.55v, and it's nearly dead @ 1.2v. A MOSFET with Id = 100uA @ Vgs = 1.2v would be fine. A BSS138 is in that ballpark; there might be better choices. Hopefully! I don't see a gate threshold figure for Id ~100 uA but the worst case (Id=1 mA) gate threshold for that part is 1.5 V. :) http://www.fairchildsemi.com/ds/BS/BSS138.pdf I like an elastomer carbon button driven by a threaded knob that makes contact between the '+' side of the button cell and the caliper cell contact instead: | ---- Caliper | '+' contact ---- | /--------- -- Single sided .---. | |.--------. Flex PCB | | | || | interposer | | | || .. | P | | || || | o | /\/.-.|| || | w |-\/\/| ||| || | e | | ||| || | r | | | | || Caliper | | | | | Button || '-' contact | K | | | | || | n |-\/\/| | | Cell || | o | /\/'-' | || | b | | | || | | | | || | | | | || '---' | | '' | | | | '--------' (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) --Winston --Winston |
Repaired Harbor Freight digital caliper
On Nov 29, 10:05*am, Winston wrote:
wrote: (...) The cell voltage is 1.55v, and it's nearly dead @ 1.2v. *A MOSFET with Id = 100uA @ Vgs = 1.2v would be fine. *A BSS138 is in that ballpark; there might be better choices. Hopefully! *I don't see a gate threshold figure for Id ~100 uA but the worst case (Id=1 mA) gate threshold for that part is 1.5 V. *:)http://www.fairchildsemi.com/ds/BS/BSS138.pdf Id=1mA @ Vgs=1.3V (typical). That's plenty. There are certainly others, but Fairchild's search function doesn't work for me. I like an elastomer carbon button driven by a threaded knob that makes contact between the '+' side of the button cell and the caliper cell contact instead: * * * * * * * * * * * *| ---- *Caliper * * * * * * * * * * * *| * * * *'+' contact * * * * * * * * * * ---- * * * * * | * * /--------- *-- Single sided * *.---. *| * * |.--------. * * Flex PCB * *| * | *| * * || * * * *| * * interposer * *| * | *| * * || * * * *.. * *| P | *| * * || * * * *|| * *| o | */\/.-.|| * * * *|| * *| w |-\/\/| ||| * * * *|| * *| e | * * | ||| * * * *|| * *| r | * * | | | * * * *|| Caliper * *| * | * * | | | Button || '-' contact * *| K | * * | | | * * * *|| * *| n |-\/\/| | | *Cell *|| * *| o | */\/'-' | * * * *|| * *| b | *| * * *| * * * *|| * *| * | *| * * *| * * * *|| * *| * | *| * * *| * * * *|| * *'---' *| * * *| * * * *'' * * * * * | * * *| * * * *| * * * * * | * * *'--------' (created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de) --Winston The knob could drive a screw, compressing a leaf spring contact onto the cell. Or, use a slide mechanism, if you prefer. Viola. -- Cheers, James Arthur --Winston |
Repaired Harbor Freight digital caliper
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Repaired Harbor Freight digital caliper
Steve W. wrote:
You can get VERY tiny slide switches from many sources. Then you install the switch in the spot on top of the caliper with the contact pad. This way you just switch the battery in/out of circuit. They last longer that way. D.C. ratings are really important here. There are really tiny switches available as you say but few will tolerate many activations that include a current spike measuring at multiples of their D.C. capability. --Winston |
Repaired Harbor Freight digital caliper
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