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I have a real time clock that requires 2.5V to 3.6V, 35 microamps. I have a vehicle battery at a nominal 12V that could actually range from 10V to 15V for the sake of this question. What kind of circuit would I use to power the RTC? I'd like to maximize the efficiency so as to not put any more drain on the battery than necessary, and minimize parts count/cost.

Is a voltage regulator (maybe some sort of high efficiency one) the answer? Or can something simpler be done involving a voltage divider or zener diode or something?

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  • \$\begingroup\$ If you look for a regulator, keep in mind that many need a minimal load \$\endgroup\$
    – PlasmaHH
    Commented Dec 7, 2016 at 21:52
  • \$\begingroup\$ An alternative could be to just use a lithium coin cell (CR2032 for example) to power that RTC. That has the advantage that if the 12 V is disconnected the clock will keep the correct time. \$\endgroup\$
    – Bimpelrekkie
    Commented Dec 7, 2016 at 22:00
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    \$\begingroup\$ If this is a lead-acid battery then the self discharge current of the battery will be far more than the 35uA of the RTC. Expect the battery to loose 5% capacity per month for a good battery at room temperature. BTW 35uA sounds a lot for a RTC, most are well below 1uA. \$\endgroup\$
    – Steve G
    Commented Dec 7, 2016 at 22:02
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    \$\begingroup\$ At 35 microamps a simple linear regulator will do fine. Because of what Steve said it's probably not worth worrying about efficiency at all. (Even if the battery didn't self-discharge, how much would you care about having 2500 years of battery life instead of only 600 years?) \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Dec 7, 2016 at 23:07
  • \$\begingroup\$ What is the application here? Are you trying to put this RTC in a vehicular app using the hot battery bus as your backup source? \$\endgroup\$
    – ThreePhaseEel
    Commented Dec 7, 2016 at 23:24

2 Answers 2

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Connect a 180K resistor and a 3V zener (in reverse) in series (with the zener connected to ground). Place a 0.1 uF electrolytic capacitor across the diode and draw your output from there.

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I would simply use a linear regulator like LM317 (not sure) but only if I can't use a simple coin cell.

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  • \$\begingroup\$ The LM317 is a floating part, so it's no bueno -- you need to draw 10mA from its output just to get it to regulate! \$\endgroup\$
    – ThreePhaseEel
    Commented Dec 8, 2016 at 0:48

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