Sinking current for clamp diodes in arduino project

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Joined Sep 12, 2008
I have an arduino to be used as a voltage monitor. I set the AREF on the chip to EXTERNAL so I can use the DAC over 0-3V as opposed to 0-5V. By doing this i can use the remaining 2 volts as grey area for clamp diodes to protect the input pins. (Using an external AREF voltage of 3v, input voltages exceeding AREF will only saturate the DAC at the highest value. The chip wont be damaged unless the input voltage rises above the boards operating voltage (5v in my case).

I have set up a circuit on a breadboard I drew it out in the attached image. The test I did in the picture was done with a power supply that has multiple outputs (12v and 3.3v in my use). They share an internal GND connection but are sourced from separate coils in the power supply secondary stage. I could be wrong but I believe that is the reason it works well with that particular power supply.

Now the Voltage monitor I am trying to build has a single polarity 12V "power brick". The 'output' GND connection on the power supply is tied to earth ground. Inside the unit I am building i have the 12v go directly to the Vin pin of the arduino and also to a LM7805 regulator with filter caps and all that for powering LCD logic, ect. I would like to use the clamp circuit with this setup but I need a way to sink the current from the clamp diodes. Thinking out loud i have considered connecting the cathode of the upper diode to the +5v or +3.3v arduino pin but I believe these are linear regulated therefore really shouldnt be sinking current.

Any suggestions?



Joined Aug 21, 2008
Depending upon the load you have on your 3.3 volt power supply, there is a good chance that you can get enough current through your 240 ohm resistor (35 ma!) for the signal you want to clamp to 3.3 volts to pull the 3.3 volt power supply positive. In this situation I add a zener diode across the 3.3 volt (or whatever voltage) power supply. The zener's voltage, including tolerance needs to be high enough to be sure that it won't draw current from the 3.3 volt supply but will clamp the voltage at a level that is safe for the controller.

Why are you using a 240 volt resistor? In the input resistance of an AVR A-to-D is about a trillion ohms (unless zapped) and work best with about a 10k ohm source resistance.