Overvoltage protection using diodes??

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi everyone.

I am trying to deisgn a voltmeter using the Arduino.

I have my voltage divider that will calculate the voltage up to probably 20 volts.

I want to protect the analog input pin of the Arduino by using diodes.

My quesiton is what diode(s) do I need? And how do I wire them?

Thanks.

2

Thanks for your reply.

I am guessing that these 1M ohm resistors will not affect my voltage divider?

Also one will go to the ground the other one will go on the analog input pin?

Thanks.

3

Richard always trots out this not very relevant part of an application note. It's not relevant because it uses a very high voltage and they are not trying to measure it, just detect zero crossings.

You have a series 1M resistor to limit the current to naff all. What you need is external clamping diodes if you are going to have any reasonable input impedance enough to drive the A/D.
See:-
http://www.thebox.myzen.co.uk/Tutorial/Protection.html

I am guessing that these 1M ohm resistors will not affect my voltage divider?

You guess wrong.

4

You will need no protection against high voltage . A modern microcontroller is quite able to look for himself :-) However you need a protection against high current @ voltage above 5V and negative (!) voltage.

The most important thing is to have a around 10k resistor in series with the ADC pin. You most likely have this implicitly in your divider...

You can additionally wire a Zener diode of around 5V from ground to the input pin. They generally do not come at 5V... 5.1V is a value common. If you are a little bit paranoic - as we all are - you should select a lower value. Note that this reduces the input range!

Edit

Sorry, took me one hour to complete this posting (urgent phone call) - so others already said it similarly in the meantime..

5

The 10k is not really safe (it will do in case of 3V - 5V conversion), but the quality of ADC will suffer with much higher values

6

No, they are not :slight_smile:
I think I explained this (and most other things) already four or five times in this forum....
Should stop this... Yes this is no fun any longer...

7

Hi,

Thanks for all the replies.

I found out that the alternators in the cars don't go over 14.6v-14.8v so if I choose my resistors to support up to 20v-25v it should be fine.

My question is, will the normal resistors do the job? Or I need high power ceramic resistors?

Thanks.

8

The resistors I use support up to 0.6Watts.

The alternator's maximum current is 150A and the maximum voltage it produces is about 14.6volts.

So if: P = E^2 / R

Then power will be 0.0005, then I am guessing the resistors are fine?

Thanks.

9

Sorry :stuck_out_tongue:

P = E^2 / R
P = 14.6^2 / 390000
P = 0.00055 watts.

M.

10

I suspect that the internal protection diodes are well capable of sinking 2mA.

From an Atmel data sheet at:-
http://www.datasheetarchive.com/datasheet-pdf/02/DSA0018927.html

It is not recommended that the clamping diodes are conducting more than maximum 1 mA