dual power rtc options?
Erik Friesen
Posts: 1,071
Does someone know a decent hardware circuit that would emulate a typical rtc chip on the market in this respect that -
When main 3.3 reg power goes down on the main processor(ie prop, pic , etc), a lithium battery comes on line to keep a rtc on the processor alive.
When main 3.3 reg power goes down on the main processor(ie prop, pic , etc), a lithium battery comes on line to keep a rtc on the processor alive.
Comments
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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.
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· -- Carl, nn5i@arrl.net
Or you could choose a regulator that isn't bothered by backfeed from an emf that's below its own -- and then you'd need only one diode.
But I'd use two diodes in order to preclude any backflow current into the regulator; not because of any worry about the regulator, since I can't imagine any regulator's being damaged by the current available from a lithium cell; but rather because that way I'd be sure not to waste the limited energy available from the lithium cell.
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· -- Carl, nn5i@arrl.net
-Phil
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· -- Carl, nn5i@arrl.net
I vaguely remember Beau suggesting a forward diode in the ground leg of a three-pin regulator to boost the output by 0.6V. 'Seems like a workable idea...
-Phil
almost 0v drop. this is how my dual power supply module keeps the high efficiency
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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.
Erik, you didn't say that you were feeding an RTC chip, and I assume you are feeding something else, because if you were feeding an RTC chip you'd just use its backup-battery input and wouldn't ask this question.
If you can stand, say, 50 ma reverse current backfeeding into the regulator from the backup cell, you could just use three 1N4001 or the like.
If you need absolute cutoff in the regulator output line, you could instead use a depletion-mode FET with its gate and drain tied to the regulator output and its source feeding the circuit.· Under normal operation, the gate, source, and drain would be at nearly the same voltage and the FET would be conducting (remember, it's a depletion-mode FET).· When the regulator is not providing power, the drain and gate would be at zero volts and the source would be held up by the backup battery,·turning the FET off.· Probably you'd need a diode between the regulator output and the drain, but connect the gate directly to the regulator output, to prevent the battery backfeed from holding the FET gate up.· After the FET gate falls to zero, of course, there will be no battery backfeed at all, which is the idea.
You'd still need a better reference for your ADC, but -- trust me -- you'll need that no matter what you do.· Regulator outputs make crummy references in the best of circumstances.
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· -- Carl, nn5i@arrl.net
Post Edited (Carl Hayes) : 4/14/2009 1:32:39 AM GMT
needed it to run a low current device for a long time then run a high current high voltage for a short time.
it is actually a really cheap system for charging batteries as it was ment. I am handling 100 amp with no voltage drop. how much are diode that can take .7v drop at that current
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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.
You can buy an assembly at most auto parts houses, consisting of two big diodes and a heat sink, nicely potted to protect it from the various crud under the hood of a vehicle.· Such an assembly is called a battery isolator, and last time I bought a 150 amp isolator it was about $40 and came with twenty feet of #8 copper wire, ten feet of #14, a circuit breaker, and connectors.· The forward drop may, for all I kinow, be larger than 0.7 volts, but it doesn't matter because (in a proper installation) the vehicle's voltage regulator feedback connection is moved from the alternator output to the battery itself, causing the alternator to increase its output voltage by precisely the needed amount.· Some vehicle makes have regulators that don't allow that, but in most of them you can move the feedback (sampling) connection.
Of course you could build all this yourself, perhaps more inexpensively, perhaps not.
Works like a charm.· I've used two of them for years.
Your application may not be automotive, and the regulator you use may not be capable of remote-sensing the voltage on the other side of a diode (instead of at the alternator or regulator·output); so your apparatus may be the best approach for you.
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· -- Carl, nn5i@arrl.net
Post Edited (Carl Hayes) : 4/14/2009 4:34:20 AM GMT
diode cheap this ic is more efficient which do you need? buck boost is good way to deal with different voltage input
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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
· -- Carl, nn5i@arrl.net
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.