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Topic: Zener Diode affecting Voltage divider (Read 2 times) previous topic - next topic

123tcpip

I have a voltage divider measuring  a car battery
Volts +-20k <> 10K - Ground with a ATtiny85 ADC connected to the junction of the 2 resistors. (with a 100nf cap on the input to the ADC to ground)

Everything works fine but I added a 5.1Volt Zener to stop any spikes and this is dragging  /affecting the ADC reading down from 1023 for 15v (5v at the pin) down to 1014 for 15V.
If I remove the zener and just measure the voltage with a fluke scope it also affects the reading, i just tried a basic DVM and this doesn't affect the reading so I am assuming its a capacitive issue rather than the voltage being dragged down.

I've built two prototypes and both do the same.

Has anyone else had a similar issue, I did try searching but couldn't find any answers.

Rob


LarryD

Show us a schematic diagram, I'm old.
The way you have it in your schematic isn't the same as how you have it wired up!

LarryD

The way you have it in your schematic isn't the same as how you have it wired up!

123tcpip

I guess showing the Zener would be useful, FYI I'm using .4w resistors

jackrae

#4
Jan 07, 2013, 06:25 pm Last Edit: Jan 07, 2013, 06:28 pm by jackrae Reason: 1
Where is the diagram  -  I saw it for a second then it disappeared

What I did see was no ground connection between the divider and the arduino

Edit : Oops it's back.
can only suggest that the zener is tripping in slightly "early".  Although it is specified as 5.1 volts there is usually a +- tolerance of around 0.1 to 0.2 volts

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