Wheatstone bridge=20

Bob Ammerman <picram@roadrunner.com> wrote:

>>So I have a board that I designed with a 10K
>>thermistor, but its feeding a 3.3V micro (and that is fixed...cant change=
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>>this
>>part). They have come back and asked for
>>a better resolution.
>
>>Now a thermistor, it simply generates a voltage
>>as a divider with a source impedance of course. As the temperature change=
s,=20
>>the voltage that
>>is generated across it changes, and the resolution
>>of what you can measure is going to be dependent on the source voltage as=
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>>well
>>as the ADC resolution.
>
>>Sorta thinking....using a PIC running at 5V to
>>read the thermistor, and then either using a PWM output (with RC filter) =
to=20
>>regenerate
>>a voltage of controlled steps over the temperature range they want to=20
>>measure
>>(pretty sure its 0 to 100F) for the ADC in their part to measure.
>
>>Any other possible clever ideas?
>
>Perhaps...
>
>Go ahead and use +5V at the top of your voltage divider. If you pick the=20
>source resistor so that the voltage out of the divider never exceeds the=20
>3.3V of the PIC over the range of possible values of the thermistor you'll=
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>be Ok. If you are concerned that the voltage will jump up if the thermisto=
r=20
>is disconnected or reaches a higher resistance than you thought, then you=
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>can include a schotky diode from the middle of the voltage divider to the=
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>+3.3V rail.
>
>-- Bob Ammerman
>RAm Systems
>
>--=20
>http://www.piclist.com PIC/SX FAQ & list archive
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