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I have a microcontroller with a built in ADC that I am using to read from an input pin (ATMEGA328P to be specific). I want to allow switching of resolutions on the input for our users (0-5V range, and 0-10V range) which i was planning on doing via a voltage divider. The thing I'm worried about is if the user has our unit in 0-5V mode and plugs in a higher voltage by mistake. I want some sort of overvoltage protection on the output of the voltage divider to protect my ADC. I don't care if this is a crowbar circuit that doesn't reset without removing power, anything that protects the unit will do. I'm open to all suggestions. Thanks!

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  • \$\begingroup\$ How about a 5V Zener Diode across the input? Anything above 5V is clamped to 5V. There is of course some tolerance on the actual Zener voltage. \$\endgroup\$ – crj11 Jun 20 '18 at 22:08
  • \$\begingroup\$ Between 5V and 10V is only 1 bit of resolution. You might want to consider keeping the voltage divider in circuit and scaling the value in software, with 2x oversampling if you need the extra bit. \$\endgroup\$ – Bruce Abbott Jun 21 '18 at 0:06
  • \$\begingroup\$ What is the maximum current that an input can take when overloaded with a higher voltage feeding through a resistor? \$\endgroup\$ – Andy aka Jun 21 '18 at 8:49
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You can use a non-inverting comparator op-amp such that the basic configuration for the circuit detects when the input signal, VIN is ABOVE or more positive than the reference voltage, VREF producing an output at VOUT which is HIGH as shown. enter image description here

credits go to : electronics-tutorials.com

With the help of a NPN you can pull the voltage applied to the ADC Pin down to zero by using this circuit " one of my designs "enter image description here

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    \$\begingroup\$ Thanks Ahmed :-) I try to avoid leaving obsolete comments, so I already deleted the earlier ones. Just to summarise: the rule about referencing material from elsewhere is here as part of the help center. \$\endgroup\$ – SamGibson Jun 20 '18 at 23:45
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    \$\begingroup\$ Ok @Sam good job, and thanks again for your support \$\endgroup\$ – Ahmed M.Zahran Jun 20 '18 at 23:47
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Zeners may be a good solution, however in practice a little bit complex circuitry is required to get low capacitance and leakage current:

enter link description here

A good source of information on the subject is on Maxim

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The approach I've most often used has been diode clamps, usually to a convenient power rail or a buffered version of the reference that can sink current. Even if you include other auto-detection circuits for scaling, it can be nice to also have them for ESD protection.

App notes like Ti's tend to show diode clamps (or suggest buffering with an op-amp that has a limited output range), though you have to be careful about leakage currents. Over temperature it can cause a great deal of skew, so BAV199 and CMPD6001S low leakage variants are helpful. If you must use a schottky at least avoid BAT54s (some ADCs behave poorly with slightly out of range inputs).

Edited to add links to diodes.

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