I have a 12 inch digital caliper that I bought about 15 years ago. It's still accurate but had started eating batteries about two years ago. I use the SR44 batteries in it. I thought that maybe bad battery contacts were to blame but that's not the problem. In frustration I soldered a Duracell AAA alkaline battery to the caliper with wires so I know that bad contacts can't affect the thing now. But the AAA battery only lasted about 6 months before the display started flashing again. Could it be that something else is dirty in the thing? It used to run two years on one SR44 battery. I know that it uses capactance to read the scale. Could a change in capacitance cause the power usage to climb? Thanks, Eric
So, just solder a tiny switch in one of those wires going to the AAA.
It's probably not worth the effort to try and find out why it's draining batteries faster now, given the low cost of a new unit.
Chances are if I tried opening up the electronics section in one of those trying to find a leaky capacitor or maybe some dried up coffee, I'd f*ok up something and end up smashing it to bits with a BFH to make sure I didn't decide to waste more time on another try at it later.
I'd suspect that some components' values have drifted with age, or a capacitor that has developed internal leakage (also likely in a component with a PN junction.. transistor or zener diode). These would be fairly difficult to locate without having an identical unit for comparison. Many surface mount components' values are marked on the components, and some aren't.
A minimum equipment list would be a lighted magnifier lamp, ohm meter, some method to desolder (and replace) components and an ESR meter.
If you've ever seen the circuit board in an LCD digital watch or clock, you might've noticed that there appears to be a blob of epoxy on the board. That's where the integrated circuit is located, and obviously isn't replaceable. Many ICs contain internal clock/timing and voltage regulator circuits that are affected by the components that can be seen outside of the blob.
It's somewhat surprising that the caliper is still functional. Connecting a battery with a higher current rating, such as the AAA (roughly equivalent to replacing a failed 1A fuse with a 3A) could stress a failing component into the breakdown range, but it seems as though that hasn't happened yet.
As others mentioned, you can try cleaning the PCB in alcohol (eg, 90-95% ethyl (Everclear), or if you don't have that,
91% isopropyl with no added oils). Distilled water is ok too, followed by a few hours in a warm oven or above a light bulb. The other suggestion about adding a switch is practical if you don't want to disassemble the caliper.
An SR44 has about 150mAh capacity, and to last a year must discharge at no more than 10 to 20 uA (microAmps). It has a fairly flat discharge curve, as shown near the beginning of
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contrast to alkaline cells, where voltage drops off faster (relative to total capacity). In other words, even though an alkaline AAA is rated at 1250 mAh, or more than 8 times as much as the SR44, you might only be able to use 3 or 4 times as much power out of the alkaline AAA before the low-voltage flashing begins. To run the AAA down in 6 months, there probably is between 100 and 200 uA of leakage, which amounts to a leakage path of about
10K ohms, which offhand sounds rather low unless the caliper has been dunked in coolant, sugar water, etc. If the caliper is clean and has 100-200 uA leakage, then a transistor probably has gone bad, and adding an external switch is about the only reasonable choice aside from replacing the whole thing.
If you could find a "Titanium Carbon Lithium Rechargeable Battery" as mentioned near the end of above link, it would be worth trying. Unfortunately, these seem to have really low energy density - only
20% as good as NiMH - but the voltage vs discharge curve starts a little higher and is flatter than for alkalines.
Won't happen either if the voltage is no higher. It will only draw what the resistance of the circuit will allow. Could have a battery the size of Rhode Island, and as long as the voltage is right, no harm done.
Of course, for an instrument that's operating normally, the battery/supply current delivery doesn't matter.
For a device where the current consumption has increased due to a failing/faulty component(s), connecting the device to a higher current source will increase the likelyhood of more rapid failure. The OP indicated that the normal battery life is significantly shorter.
There are special current-limiting power supplies that will interrupt or crowbar to a safe mode when a specific level is attained, to prevent further damage to components. A cheap alternative is to use a series light bulb to limit current to an appropriate level while troubleshooting, unless one chooses to find faults by following/chasing smoke signals.
A lithium AAA cell (Energizer L92) will last considerably longer than an alky because it has a flatter discharge curve. A CR123 lithium cell with a surfacemount 1.5v micropower LDO regulator chip would last a very long time.
I too have a spindle mounted digital indicator that eats batteries. And now the little plastic slide cover is worn out. Don Foreman made a solution to my problem, a "battery" that is really a wall wart power supply lead. I plan on doing this "when i get time"
But the button battery is ALSO capable of providing enough short circuit current to fry the circuitry. A good 357 battery is only 9 ohms Thats 150ma capacity at 1.35 volts. About SEVEN times the current required for a 20ma device.And the caliper draws 20 microamps, not 20 milliamps, iirc. Definitely more than enough juice to fry defective micro-circuitry.
It might be a semiconductor failure, but more likely, a capacitor that has decided to leak. Look for one between the battery contacts, likely a surface mount chip device. Probably designed in to save the electronics from spikes during battery replacement, so instead of just destructive removal, I'd try to wire in a small tantalum electrolytic or something. WOnder how an LR44 sized supercap would do?
On Sat, 27 Jan 2007 09:21:33 -0800, Eric R Snow scribed:
Jueesbus...
15 year old digital caliper and your wondering why it went south? Do yourself a favor and just get a new one... eBay or Harbor, J&L, Rutland, MSC... they all have hundreds to choose from and will cost less than the strife of trying to figure out a new battery design...
I bet that this "old crap" is much better than the Chinese crap you suggest. Just touching the cheapos is enough. I'd invest time to get that old iron back running again, it's worth it.
Thanks to all of you who have responded so far. I never thought I'd get so many good responses. I just measured the voltage and it measures 1.468 volts. This seems too high for the calipers to start flashing. So it's probably a bad component. I'll check to see if it's a cap that I can replace. If not then I'd like to try the lithium battery and the LDO regulator that Don suggested. If that doesn't pan out then a switch is in order. I don't like the switch as much though because it means that zero needs to be set every time. Watch for more posts on the calipers. Thanks again, Eric R Snow
I have had several apart. If you can change the batteries in a common digital watch, you can take one of them apart quite easily for cleaning. Really no intricate stuff in there to worry about.
If I understand this right it sounds like you still have decent voltage at the battery but the scale is saying the battery is bad.
If this is the case get a decent magnifier and carefully study the PCB area around the battery (one may have leaked sometime ago) looking for traces that show a bit of corrosion. If you find a suspicious spot you can either scrape, solder bridge, lay small piece of wire (individual strand from some small gauge wire works pretty well) along the trace or try measuring the voltage on either side first to verify the problem.
And if you are really attached to this scale and don't feel capable of doing this I would be willing to look it over. No guarantees, but I work cheap for people I like/respect.
Greetings Leon, I suspected at first that corrosion of the contacts was the problem. However, when I used the AAA battery for power it was soldered to wires which were in turn soldered to the caliper circuit board. So that's not the problem. There is some corrosion visible though. I wonder if it is making a high resistance short somewhere which makes the display flash when the battery drops to 1.47 volts. Thanks, Eric
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