Protecting the AD pins

I've been playing around with measuring the voltage in an automotive environment. I've tried several ways but I've blown up a few A/D inputs on the arduino....

So here's my latest shot:

Any comments? Precision is not essential; I am looking at fairly gross differences in voltages. I am more concerned about protecting the inputs on the arduino without using an op-amp to isolate it.

(I'm planning to use Schottky diodes; I think that's the right symbol. The web cad I used doesn't have the symbol I am used to.)

Should work OK. R3 isn't doing anything useful so could be eliminated.

So you have a resistor divider then a zener to clamp down...

The zener on the vcc line serves what purpose? ... zeners are slow which could be why you keep blowing adc ports ? Buy a coupke TVS diodes instead to protect the arduino. clamp it to say 16v on the input making sure the resist divider does it job

They're schottkys, not zeners. Not sure if I used the right symbol. The webcad software I used to create the diagram was limited in its library.

Any suggestions for what specific diode I should use? I can clamp it at 16 or higher. I'm not interested in any voltage above about 15.5.

I guess you can use the tvs either end? Eg 5v on the input to arduino, 16v on the input/resistor divider.

Yes sorry I'm often mixing zener with schottky, they look similar to me, what's the purpose of the schottkys? I'm curious

The schottkys are fast acting and low forward drop, so any voltage above about 5.3 gets dumped to the Vcc rail, and any voltage below -0.3 gets dumped to ground. I put them after the voltage divider so I don't have to big current handling in the diodes.

I have not tried this particular circuit; I found that zeners don't protect arduinos in noisy automotive environments.

And r3 is there to limit current to the ad pin if all else fails.

cptdondo: And r3 is there to limit current to the ad pin if all else fails.

Well it's then much too low a value as if and when the ADC internal clamping diodes conduct they are rated at less then a milliamp before damage can result.

There is no such protection that covers 'if all else fails'. ;)

retrolefty:

cptdondo: And r3 is there to limit current to the ad pin if all else fails.

Well it's then much too low a value as if and when the ADC internal clamping diodes conduct they are rated at less then a milliamp before damage can result.

There is no such protection that covers 'if all else fails'. ;)

A 1ma blowable fuse? :)

Here is another option to consider.

Been used effectively so far without any blown A/D ports in a battery monitor device.

SnipImage.jpg

the design is farily good, i believe the most important problem is :what kind of diodes you use.