would zener work as pull-down?

#12

Joined Nov 30, 2010
18,210
No, that's a job for a resistor. If accuracy is a specification, you might have to account for the voltage change caused by installing a voltage divider. It depends on the rest of the circuit.
 
When I think of a pull-down, I think of something that is going to pull/force the voltage down to 0 V. A resistor provides this function.

The zener diode will not pull the voltage down to 0 V, but could stop the voltage from going above 5.1 V. Other voltages also possible.
 

Thread Starter

palkapalka

Joined Aug 15, 2015
13
Thank you for prompt responses, guys.
Yes - this is a schematic for overvoltage protection.
Now I have it working without R and D - and ADC reads 4.9V if nothing is connected to P1. If something (battery) is connected the voltage is correct.
Now I want to upgrade it to have overvoltage protection with zener and I also want to avoid the situation with non-zero reading with floating pin1 on P1.
My though was that since zener conducts slightly (reverse current) it would make pin1 on P1 to be at ground level with nothing connected to P1.
 

#12

Joined Nov 30, 2010
18,210
You do over voltage protection with a resistor in series with the input and two normal diodes connecting from the signal line to the +5V power supply line and the ground line.
Then you add a resistor from the input to ground, just enough to drain the leakage current of the ADC input pin. Might be over 100K. Read your datasheet.
 

Thread Starter

palkapalka

Joined Aug 15, 2015
13
You do over voltage protection with a resistor in series with the input and two normal diodes connecting from the signal line to the +5V power supply line and the ground line.
That is another way of doing this.
Then you add a resistor from the input to ground, just enough to drain the leakage current of the ADC input pin. Might be over 100K. Read your datasheet.
This is a very good explanation - thanks. ADC data sheet says 10uA - so 100K works. I will put it parallel to zener:
 

#12

Joined Nov 30, 2010
18,210
Watch that input quality if the circuit is going to run hot. Leakage currents increase with temperature.
 
My though was that since zener conducts slightly (reverse current) it would make pin1 on P1 to be at ground level with nothing connected to P1.
The zener does conduct slightly, reverse current. However, when it does, as you see in your results, the voltage across it is 5V, not 0V as when you have a very small current through a resistor. Your zener diode does not follow Ohm's law like a simple resistor.
 
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