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'[EE]:Battery Backup?'
2002\01\30@115122 by Steve Kasm

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Hello All,



I have a circuit I'm working on that has 2 busses. The analogue portion
uses 12v, the digital uses 5vd from a 7805 regulator. The regulator
powers a 16f84 pic and 4 7-seg displays.



In the event of a power failure the only portion of the circuit that I
need to maintain power on is the actual pic chip. This means I can drop
power to the 12v rail, the displays are gated on from the pic through
transistors on the common cathode and for current draw concerns during a
power failure I can leave them off somehow. Can anyone point me to any
design notes on a regulator or circuit that will switch my 5volts to a
9v battery to power the 16f84 chip when the incoming 12v power is
missing?





Regards,

Steve

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2002\01\30@122636 by Eoin Ross

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How about placing a diode in a trace from the 12V to the input of the regulator ... and another from the battery to the input of the reg.

When the the 12V drops the power will be supplied by the battery.

Does anyone see a problem with the reverse current through the battery diode damaging the battery?

>>> .....kcerKILLspamspam@spam@COGECO.CA 01/30/02 11:48AM >>>
<SNIP>
Can anyone point me to any design notes on a regulator or circuit that will switch my 5volts to a 9v battery to power the 16f84 chip when the incoming 12v power is missing?





Regards,

Steve

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2002\01\30@123453 by David VanHorn

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At 12:24 PM 1/30/02 -0500, Eoin Ross wrote:
>How about placing a diode in a trace from the 12V to the input of the
>regulator ... and another from the battery to the input of the reg.
>
>When the the 12V drops the power will be supplied by the battery.
>
>Does anyone see a problem with the reverse current through the battery
>diode damaging the battery?

What reverse current??  The leakage of a diode is negligible.
You might want to use a pair of diodes though, as diodes fail shorted.

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2002\01\30@124500 by Eoin Ross

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I know that it's minuscule - and couldn't see an issue with it myself ... but there are people here with many more years of experience and education on the subject than me - so I am open to learn if my thoughts are misguided : )
>>> dvanhornspamspam_OUTCEDAR.NET 01/30/02 12:36PM >>>
At 12:24 PM 1/30/02 -0500, Eoin Ross wrote:
>How about placing a diode in a trace from the 12V to the input of the
>regulator ... and another from the battery to the input of the reg.
>
>When the the 12V drops the power will be supplied by the battery.
>
>Does anyone see a problem with the reverse current through the battery
>diode damaging the battery?

What reverse current??  The leakage of a diode is negligible.
You might want to use a pair of diodes though, as diodes fail shorted.

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2002\01\30@125513 by Steve Kasm

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This does sound simple, but I have one problem, my circuit has 4 7-seg
led that are gated on from the pic chip using 4 PnP transistors. I will
need to blank these displays somehow during battery backup. Transisers
base to pic pins, collector to ground, emitter to common cathode.

Regards,

Steve

{Original Message removed}

2002\01\30@130634 by Dal Wheeler

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Use the 12v source (before the diode & regulator) to bias another transitor
after the regulator to turn on power to the LED supply --no bias (12 v) no
power to led's.

Say that 7 times really fast...  :)

----- Original Message -----
From: Steve Kasm <RemoveMEkcerTakeThisOuTspamCOGECO.CA>
To: <spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU>
Sent: Wednesday, January 30, 2002 10:54 AM
Subject: Re: [EE]:Battery Backup?


> This does sound simple, but I have one problem, my circuit has 4 7-seg
> led that are gated on from the pic chip using 4 PnP transistors. I will
> need to blank these displays somehow during battery backup. Transisers
> base to pic pins, collector to ground, emitter to common cathode.
>
> Regards,
>
> Steve
>
> {Original Message removed}

2002\01\30@131040 by David VanHorn

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At 12:54 PM 1/30/02 -0500, Steve Kasm wrote:
>This does sound simple, but I have one problem, my circuit has 4 7-seg
>led that are gated on from the pic chip using 4 PnP transistors. I will
>need to blank these displays somehow during battery backup. Transisers
>base to pic pins, collector to ground, emitter to common cathode.

One easy way to do that, run them from the unreg 12V supply.
When that's down, they are blanked.

Also, you get to use "cheap" power for the displays, rather than eating
"expensive" regulated power.

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2002\01\30@143145 by Steve Kasm

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Tried this circuit

It does power the circuit if 12 volt is removed the 9v battery takes
over, the only problem is that it resets the pic processor. The pic is
running a countdown that I need to maintain.

Steve

{Original Message removed}

2002\01\30@143610 by David VanHorn

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At 02:29 PM 1/30/02 -0500, Steve Kasm wrote:
>Tried this circuit
>
>It does power the circuit if 12 volt is removed the 9v battery takes over,
>the only problem is that it resets the pic processor. The pic is running a
>countdown that I need to maintain.

You do need a filter cap on the input of the reg, beyond the diodes.

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2002\01\30@202848 by M. Adam Davis

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Filter cap between the diodes and reg, filter cap after the reg, filter
cap on MCLR, filter cap directly across VCC and VSS for pic.

If it resets after that then tell us what diodes you are using, and more
about the schematic.  Could be a glitch on a pin coming from circuitry
connected to the twelve volt rail.  Any capacitors, inductors, relays,
motors, etc attached to the circuit that are powered through the 12v rail?

Also be careful about powering devices through the pic - when the 12v is
disconnected, could any current be flowing from the pic into the 12v
circuit through i/o pins?

Lastly, I'd suggest monitoring the 12v rail through another pin.  If
it's off you may be able to turn off the LED display by tristating the
pins going to it, or simply not running the display routine.

Check the voltage and current coming off the 9v after turning off the
12v rail.

-Adam

Steve Kasm wrote:

>Tried this circuit
>
>It does power the circuit if 12 volt is removed the 9v battery takes
>over, the only problem is that it resets the pic processor. The pic is
>running a countdown that I need to maintain.
>
>Steve
>
>{Original Message removed}

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