So I am trying to clamp a higher voltage analog input than that which can be tolerated by Arduino (Uno/Nano) via a Zener diode. Problem: Zener diodes in my inventory are all 5.6V, not 5.1 or less as I would have liked. Must I go shopping?
I would think so. I would go for at the most 4.7V, and don't forget you need a series resistor with the input to drop the voltage across to limit the current through the Zener.
More on the topic here
https://www.ednasia.com/downloads/schaffner_wp_en_202109_1/
Cant you just protect the analog input with a large resistor, and let the internal clamping diodes do the protection?
You should be aware that using a zener - depending on the source resisitance - will affect your readings.
@johnerrington I was actually thinking of that. Why all fuss about clamping when one has to in any case use a current limiting resistor. So basically Arduino analog input can take any voltage value as long as the current is in limits?
I use 10K for R2.
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And what are those limits?
I hear its 1mA for Nano and Uno. At 12V power supply, 12K current reducing resistor should make it 1mA. If we use a higher value, say 20K, it should be even safer?
Where do you here that?
Zeners are not good for input clamping. Use a TVS diode.
Can you tolerate some series input resistance in your input circuit? What is the source signal (the one you're inputting)?
Page 258, under absolute maximum ratings, check the row "Injection current at VCC = 5V".
The source signal is is from a TCRT1000 pulse sensor.
That datasheet is for the automotive grade IC
The UNO uses the commercial grade IC and there is no specification for injection current.
How high is this voltage you are trying to clamp?
it is 12V. Have I fried something up giving it 1mA 
?
I won't post any more until I see a schematic of your circuit.
Phrases like, "giving it 1mA" mean very little outside the context of the whole thing in front of you, which we can't see.
I asked about the sensor because the bandwidth limits the series resistance that you can use. If you can add one, it makes overvoltage and EMI suppression orders of magnitude easier and better, in combination with the other clamping.
Again, I see no circuit from you, but it's sometimes useful to add Schottky input clamps before the MCU input as well.
When you reference things, please provide links or post inline. We don't have time to Google for you. Please read this posting guide if you missed it:
Power the TCRT1000 from 5V.
Then the problem goes away.
It is in the application note concerning mains control of the brightness of a light bulb using phase control modulation.
It is on my machine somewhere and I am trying to find it. Sorry I posted too soon by mistake.
Next to the
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Not sure where you get 1mA from.
12V means there is 7V above the 5V supply. 7/12000 give 0.58mA.
There seems to be confusion between voltage and current. If the resistor is connected to 12V at one and and the pin at the other that does not expose the pin to 12V because the pin is not connected to 12V, it is connected to a resistor. Some current will flow through the resistor. Calculate the voltage across the resistor as a result of that current. Subtract that voltage from the supply voltage then you have the voltage the pin is exposed to. However, it's the current that matters. If the data sheet says 'absolute maximum 1mA', then do not feed 1mA into the pin! Feed some lower amount of current in.
OK it is in AVR182: Zero Cross Detector
I have attached the PDF of it.
Atmel AVR182 Zero Cross Detector.pdf (95.1 KB)