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MichaelN wrote:Typically you just use the regulated supply voltage with a resistor to limit the current and a diode to stop the capacitor discharging when power is removed.So there isnt a special charging circuit like for charging Li-Ion Batteries (Im thinking of the Solar Charger from Adafruit)? Or is that overkill?
Mee_n_Mac wrote:The polarity on you cap is wrong but other than that, yup that's the concept. How long are you expecting the cap to run the RTC for ? Which RTC chip ?Just noticed the polarity issue. I did that last night while in a hurry.
viskr wrote:500 nA worst case would run down a 1F cap to 0 in about 25 days, so probably useful life of 15-20 days.Viskr what calculation did you use to find the length of the charge?
Mee_n_Mac wrote: 30 days isn't going to happen. At least not in size smaller than a battery and it's circuitry.Thanks for the link. According to that calculator, a 3.3V capacitor at 1F would potentially run for about 39 days.
Viskr what calculation did you use to find the length of the charge?Basically i=C*dV/dt, yes I know the current into a device won't normally be a current source, but we're doing back of the envelope calculations here, and it won't look like a resistor either
viskr wrote:Assuming C = 1F, 500nA, gives you 1V decrease every 2,000,000 seconds. That's 23 days, to 2V, another 1/2V to 35 days. Pretty close to the calculator mentioned above, but this is just a rough number.You won't get anywhere near that in practice, as the leakage current will be MUCH higher than 500nA. Manufacturers rarely seem to quote it, but leakage current for supercaps can be tens or even hundreds of microamps.
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