Don't forget to "reconstitute" the actual voltage value, from the value read
from the ADC, by whatever the resistor voltage divider ratio is. I recently
had a circuit that used a 1M from the battery (a 15V LiSO2 battery) and a
100K to ground. This created a "divide by 11" divider to get the voltage
down into the range that the 10-bit ADC could handle. To display the actual
voltage, I multiplied by 11 (and also added in a fudge factor equal to the
tolerance of the resistors).

Douglas Wood
Software Engineer
dbwood@kc.rr.com

Home of the EPICIS Development System for the PIC and SX
http://epicis.piclist.com

----- Original Message -----
From: "Douglas Butler" <dbutler@IMETRIX.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, March 27, 2002 3:44 PM
Subject: Re: measuring battery supply via A/D


> Use a 2 resistor divider, no op amp needed.  Try a 22K from the battery
> to the A/D, and a 47K from the A/D to GND.
>
> Sherpa Doug
>
> > -----Original Message-----
> > From: Kosmerchock, Steve [mailto:Steve.Kosmerchock@RFSWORLD.COM]
> > Sent: Wednesday, March 27, 2002 3:20 PM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: Re: measuring battery supply via A/D
> >
> >
> > Glen,
> >
> > Two diodes would give you a drop of 1.4VDC.
> > So REF V would be 5.8V, still too high. You could use
> > an op-amp and resistor network.
> >
> > Steve
> >
> >
> >
> > -----Original Message-----
> > From: Glen Wiley [mailto:gwiley@GWILEY.COM]
> > Sent: Wednesday, March 27, 2002 1:12 PM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: measuring battery supply via A/D
> >
> >
> > From a software guy to the EE's of the world...
> >
> > I would like to measure the voltage level of my battery
> > power supply (6 NiMH AAA cells) which should start at around
> > 7.2V.  Using the A/D (AN0) pin on a PIC16F872, what is the
> > best way to do this.  My first guess was to insert 2 diodes
> > in series to take the max input voltage down to 4.8V.
> >
> > The goal here is to log the battery supply voltage (along
> > with some other info which is stored in serial EEPROM) for
> > later analysis. This does not have be absolutely accurate,
> > only relatively accurate - this portion of the data will be
> > used to generate data for trending battery life under normal
> > operating conditions for the device.
> >
> > thanks in advance for your time
> > --
> > Glen Wiley
> >
> > Everything should be made as simple as
> > possible, but not simpler. -- Albert Einstein
> >
> > --
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> >
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> >
> >
> >
>
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>

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