Electronics > Projects, Designs, and Technical Stuff
Clock Battery Monitor Circuit
bitogre:
--- Quote from: Ian.M on July 18, 2020, 10:10:52 pm ---Looks like a mess. The original circuit would work as long as the JFET's pinch-off voltage was less than the lowest battery voltage of interest, but great care would have to be taken *NOT* to forward bias the gate-channel junction by taking the gate above the battery voltage.
As there's a 12V supply available, it would make more sense to use a N-MOSFET as the ADC isolator, source to the ADC, and use the main battery voltage (after the usual automotive transient filtering and protection circuits) to pull its gate well above Teensy Vcc when you want it on and down to 0V when you want it off. A BSS170 would do well enough, with another one for the Teensy to pull its gate down if you want to be able to turn it off when the main 12V supply is present.
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I cannot be sure that the gate voltage would not exceed battery voltage without adding additional circuitry. So, to ensure that, it would likely be just as complex as the BJT solution.
The 12V supply is only available when making measurements. But it is not available when I need the ADC Input isolated. In fact, the reason the Teensy is off is because that 12V had been removed.
bitogre:
--- Quote from: james_s on July 18, 2020, 10:11:00 pm ---What's the input impedance of the ADC? I'd have thought it would be very high, but maybe they've got some external circuitry on there?
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According to Page 60 and 61 of https://www.pjrc.com/teensy/IMXRT1060CEC_rev0_1.pdf, the input impedance should between 5k ohms and 30k ohms but I am pretty sure that is while power is applied to the Teensy 4.1. When I measured is with my multi-meter while not powered, it was about 1500 ohms. According to https://www.pjrc.com/teensy/schematic.html, there should be no additional circuity on the pin I was measuring relative to ground.
Peabody:
There probably is a way to make this work, but I wonder if your time might be better spent on some other part of your project. If you disconnect the RTC module from your circuit when your RV goes into storage, the coin cell will keep it going for a year, or even a lot longer. Not to mention larger batteries, or even two coin cells in parallel. And even in the worst case, it's not the end of the world if you have to reset the RTC at some point. I just got a DS3231 RTC, and the datasheet says when it's battery powered and inactive except for timekeeping, it draws 1µA. You're looking at months and months.
bitogre:
--- Quote from: Peabody on July 18, 2020, 11:24:39 pm ---There probably is a way to make this work, but I wonder if your time might be better spent on some other part of your project. If you disconnect the RTC module from your circuit when your RV goes into storage, the coin cell will keep it going for a year, or even a lot longer. Not to mention larger batteries, or even two coin cells in parallel. And even in the worst case, it's not the end of the world if you have to reset the RTC at some point. I just got a DS3231 RTC, and the datasheet says when it's battery powered and inactive except for timekeeping, it draws 1µA. You're looking at months and months.
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I was hoping it would be a bit easier than it is currently looking like it might be. I am currently not planning on using a DS3231 RTC as the Teensy 4.1 does have a built in RTC (not as many features and draws more current at 250uA but less work to use). Also, was not planning on putting this some place where it is easy to access for replacing the battery but I am starting to rethink that plan. I do have WIFI D1 mini - Data logger shield which does have a DS1307 that I could use for this project for the RTC.
Ian.M:
As long as nothing else is holding the nom. 12V rail up, and the MOSFET doesn't get overheated in storage, this will do:
Leakage current should be under 1uA. It will go up a bit at seriously elevated temperatures, so don't store your RV in a tin shed in Death Valley! A 2N7000 should also work.
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