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'[EE]: Battery backed operation'
2001\01\31@233537 by Nicholas Irias

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I want to implement battery backed operation, wherein a PIC based device
ordinarily runs from a 9v to 12v wall wart, and switches to a 6 volt battery
pack (AA alkalines) when the external power supply is unavailable.

My initial plan is to use diodes between each of the two power sources and
my 5v regulator.  I would then use a resistor, a 5v zener, and perhaps
decoupling capacitor so that one of the PICs input pins could monitor the
external power supply.  In the event that external power is lost, program
logic would invoke a low power operation mode (dont try to run relays, LEDs,
etc until external power returns).

Can I expect this scheme to work flawlessly, or did I miss something?

Incidentally, the application is a PIC based temperature monitor/heater
controller for my wellhouse.  I want it to keep the water temperature in the
hydropneumatic tank at 38 to 39 degrees, which is just below the temperature
that the water comes out of the ground.  I also want it to log the amount of
time during which power is down, and the minimum air and water temperature
during those incidents.

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'[EE]: Battery backed operation'
2001\02\01@052953 by Simon Nield
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nicholas:
>logic would invoke a low power operation mode (dont try to run relays, LEDs,
>etc until external power returns).

>Can I expect this scheme to work flawlessly, or did I miss something?

sounds fine to me. a couple of ideas:

if you are not expecting long power outages (i.e. you only need to stop running the relays and leds
and so on and not actually start putting the pic into sleep) then how about simply having two
seperate power rails? run the leds and relays from the mains-derived power supply and the pic from
both through diodes...
a nice tweak to that circuit might be to use NiMH batteries for your backup supply, have a diode
from there to the pic and a diode from your mains-derived power rail to the pic too. then add a
suitable resistor across the diode to the batteries to trickle charge them when there is mains power
present... you need to have a higher voltage on the mains-derived rail than on your batteries for
this to work, somewhat over one diode drop more.

regards,
Simon

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2001\02\01@113656 by Mike Mansheim

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>I want to implement battery backed operation, wherein a PIC based device
>ordinarily runs from a 9v to 12v wall wart, and switches to a 6 volt
battery
>pack (AA alkalines) when the external power supply is unavailable.

If it is acceptable to only see when external power is plugged in, jacks
are available that close (or open?) contacts when the wall wart plug is
inserted.  This doesn't tell you anything about the "validity" of the
power, however...

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2001\02\01@131839 by Dan Michaels

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Nicholas Irias wrote:
>I want to implement battery backed operation, wherein a PIC based device
>ordinarily runs from a 9v to 12v wall wart, and switches to a 6 volt battery
>pack (AA alkalines) when the external power supply is unavailable.
>
>My initial plan is to use diodes between each of the two power sources and
>my 5v regulator.  I would then use a resistor, a 5v zener, and perhaps
>decoupling capacitor so that one of the PICs input pins could monitor the
>external power supply.  In the event that external power is lost, program
>logic would invoke a low power operation mode (dont try to run relays, LEDs,
>etc until external power returns).
>
>Can I expect this scheme to work flawlessly, or did I miss something?


Not quite flawlessly. When operating off the 6v battery, you'll
get a v.drop across the diode, leaving only a couple of tenths/volt
overhead on the 5v regulator. You can use schottky diodes and
LDO v.reg, but you will still have pretty marginal overhead.
Batteries discharge a little below 6v, and wham. This may be ok,
if the batteries only supply maintenance current, rather than
operating current.

2 possible improvements - use a 5th AA, and use a power jack
with internal switch, as Mike Mansheim suggested, to eliminate
the diode.

- danM

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2001\02\01@152041 by M. Adam Davis

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Your AA batteries will go to 3.6v before you've extracted all the power
from them that you can, rechargeable batteries will go lower.

I would suggest you design your circuit to run off 3.3v.  Use a low
dropout voltage regulator from the battery pack and other power supplies.
Depending on your current draw you may want to put a resister in series
with the other power supplies so getting 12v down to 3.3v won't fry your
regulator.

You might even design your circuit to operate over a voltage of 3.3 to
5v.  Hook the batteries to the circuit with a smallish resister.  Put the
external supply through a 5v regulator - no need to use diodes.  When the
power supply is on it'll trickle-charge the batteries to 5v through the
resister (maybe use a zener here to limit the voltage to 4.8?), and power
the circuit.  When it is not on the circuit will power itself from the
batteries.  You'll need to make sure brown-out protection is working.  You
might even want to have a free A/D determine what the input voltage is and
initiate lower power modes when the voltage goes below 5 (ie, on battery).

Now, this isn't the best way to charge a battery, but as a stand-by power
source it would work fairly well.

-Adam

Nicholas Irias wrote:
{Quote hidden}

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