# Stable voltage divider circuit

Discussion in 'Electronic Design' started by Alan Adrian, Apr 23, 2006.

I'm using a Basic stamp to parse out a GPS string and send a beep into my
motorcycle intercom in time to warn me of the info screen on said GPS
showing me the details of an upcoming turn

I intend to use the microcontroler to read the temperature and also to
monitor Bike voltage via and ADC.

voltage down to fit... A resistor voltage divider seems to be the thing..
but is it going to be pretty stable (I'd like to keep a reasonable accuracy
of say.... .05V I know... overkill, but I want to do it if I can...) in the
temperature range of less than freezing to 40 deg C?... .is there a better
way?

Al...

2. ### T. AtkinGuest

Sure there is a better way.
I'm not sure if you need a reference voltage or a regulator but you can use
a TL431 to get 5.00V. The TL431 is cheap yet very precise and stable, I use
to read 4.99V with power supplies using it as a reference.

Tom

4. ### Guest

There are better ways, but you don't need them. Regular metal film
resistors have temperautre coeffiicients of 50ppm/C. Two combined give
a worst case error of +/-100ppm/C which is 4,000ppm or 0.4% over 40C,
almost exactly the resolution of your 8-bit A/D converter (one part in
256).

It you wnat to do better, you can buy +/-15ppm/C metal film precision
resistors off the shelf from broad-line electronic distributors like
Farnell, who also stock thin film precision dividers which offer ratio
temperature coeffiicients down to about +/-5ppm/C.

For the sort of divide ratio you'd need, the dividers cost about \$5
apiece - about twice what you'd pay for a pair of precision resistors.

5. ### Fred BloggsGuest

It depends on the bike voltage and the scaling factor of the attenuator
to make it fit the full scale A/D range. The A/D range is fixed at 5V
and measured with approximately 0.02V maximum error. If your input
scaling is S then your scaled reading error will be S*20mV, making
maximum allowable S=2.5 for maximum battery voltage range of
2.5*5=12.5V. If this is not good enough, then you will have to scale and
offset so that final reading is of form S*Vm+OFFSET where S<=2.5 and
OFFSET= BATT,max-S*5.00. In any case, be sure to low pass and clamp the
battery voltage making its way to the A/D input.