Taggsladder 2 Posted August 31, 2015 Share Posted August 31, 2015 Hello I want to measure the battery voltage (ballpark measurement) in a car so I thought I do that with a voltage divider connected to a msp430 ADC pin, but I wanted to add overvoltage protection with a zener as in the schematics in the link below. I am using 100k / 10k in the voltage divider and a BZX79C2V7 zener. If I leave the zener disconnected I get the divider to work as expected in a linear fashion, but when I connect the zener I get the correct reading when the input voltage is 10V but if for example I double the input voltage to 20V I get a lower reading compared to when the zener is disconnected and the voltage divider is no longer linear. Measurements without zener: 10V in, 0.9V to mcu pin 15V in, 1.35V to mcu pin 20V in, 1.78V to mcu pin Measurements with zener: 10V in, 0.9V to mcu pin 15V in, 1.25V to mcu pin 20V in, 1.46V to mcu pin Why is this so? http://www.re-innovation.co.uk/web12/index.php/en/information/electronics-information/accurate-voltage-measurement Thanks Kind regards Andreas Quote Link to post Share on other sites
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