Hi guys
I am trying to make the auto ranging ohmmeter shown below, and I was wondering if this circuit would be a good alternative for protection just in case a user accidentally connects a power in place of the resistor to be measured, these powers can be up to 20V DC and up to 15VRMS AC.
Protection yes, but resistance measuring no.
Make the user press a key for measuring. Then check for an input voltage and use a relay to connect or disconnect the device under test.
Thank you very much..
Could I get away with only series resistor and clamping diodes?, the relay method could still be dangerous if the user pressed while power is connected etc..
Yes but the software would have to take into account the resistor in series with the unknown resistor
Then the controller has to detect the voltage and not turn on the relay.
Where does the unknown resistor connect?
To the J54, so one terminal is between the X (after the 100k) and the ADC0 and the other to ground..
Thanks Jim.
Is it because of the connection?, because the diode package might be confusing but they are essentially connected the same as the schematic you shared.
Also can you please elaborate about the 200ohm value?
The 200 ohm resistor will limit the current through the diodes. Your circuit has no current limiting resistor. So when someone applies +/-20V to J54, it will be directly connected to 5V (or GND) through a diode and a very large current will flow untill the diode burns out.
Oh I get it now , the location of the 100k resistor I planned to use for current limiting was wrongly located.
I really appreciate your help thank you so much 
Basically to 200 ohm replaces the 100K in your circuit so you need to remove the 100K
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