How to select Zener & Resistor?

[udhay_cit]

Dear friends,
I need to measure a DC voltage range from 0-30V using microcontroller ADC. For that i am using potential divider value of R1=12K, R2=1.5K. My MCU operating @ 3.3V so with this resistor combination i can get 3.3V output @ 29.7V input. I am using 3.3V zener to protect overvoltage.

I can get 3.3V output for 29.7V input, without using zener diode... But if i connect zener diode the voltage never reach 3.3V for even 35v-40v range.


I know the zener will start conduction little bit before reach the Vz, But how can i avoid this problem... Please give me the solution...

Notes: R9=150E is not involving in this circuit & it is optional for 4-20mA current measurement, so please don't consider this...

Thanks & regards
Udhay

 

[IanP]

Dear friends,
I need to measure a DC voltage range from 0-30V using microcontroller ADC. For that i am using potential divider value of R1=12K, R2=1.5K. My MCU operating @ 3.3V so with this resistor combination i can get 3.3V output @ 29.7V input. I am using 3.3V zener to protect overvoltage.

I can get 3.3V output for 29.7V input, without using zener diode... But if i connect zener diode the voltage never reach 3.3V for even 35v-40v range.
View attachment 82565

I know the zener will start conduction little bit before reach the Vz, But how can i avoid this problem... Please give me the solution...

Notes: R9=150E is not involving in this circuit & it is optional for 4-20mA current measurement, so please don't consider this...

Thanks & regards
Udhay

You will have better protection than just single Zener if you use 2 diodes connected between input pin of the MCU and [Vcc and GND] as shown on the attached picture.
Make sure to select diodes with Vf(max)<0.3V, [eg. Schottky diodes]...
:wink:
IanP

 

[udhay_cit]

Yes, i agree thank you very much for the solution...