3

I made an accurate voltmeter using an Arduino Nano as the microcontroller.

What I want now is to add a reverse polarity protection using an op-amp with a gain of 1 without changing the actual accuracy it has.

In the image I show both my actual circuit and the circuit I designed for reverse polarity.

enter image description here

What rail-to-rail op-amp would you suggest for this circuit?

JRE
  • 67,678
  • 8
  • 104
  • 179
marcosbc
  • 414
  • 2
  • 11
  • It looks like you accidentally swapped the inputs on the amplifier. If you are already using an amplifier, the [resistors](https://imgur.com/a/LeZJsDa) may be higher. It puts less strain on the measured circuit and also helps in protection. You can use any CMOS RRIO amplifier, all of its inputs are protected. No extra protection is required. – csabahu Oct 06 '20 at 20:02
  • @csabahu for example if I choose the AD8546 RRIO amp (https://www.analog.com/media/en/technical-documentation/data-sheets/AD8546_8548.PDF), what resistors should I choose in order to keep the input voltage in A1 on the same range as now? so the accuracy keeps the same – marcosbc Oct 06 '20 at 20:20
  • The amplifier will be good and the resistors may be the same as I showed earlier. So e.g. 220k and 682k. You can choose bigger ones yourself. (There's a link to 'resistors' in the previous comment.) – csabahu Oct 06 '20 at 20:27
  • @csabahu, I recently see the image of resistors, thanks! My last question is about impedance. Arduino ADC inputs impedance have to be <= 10K ohms. With the resistors you show me in the image, do I have to worry about input impedance on the ADC? – marcosbc Oct 06 '20 at 20:33
  • The output impedance of the amplifier is close to zero, so it will be much better than it was at 10k. – csabahu Oct 06 '20 at 20:38
  • I only showed the positive circuit, the [negative](https://imgur.com/a/D90a64O) will look like this. – csabahu Oct 06 '20 at 21:02
  • @csabahu thanks so much! – marcosbc Oct 06 '20 at 21:15
  • I also show a [third](https://imgur.com/a/DmKRaFs) version. :) There are no separate positive and negative. 2.5V corresponds to zero at the ADC input. What is below is positive, what is above it is negative. – csabahu Oct 06 '20 at 21:42

1 Answers1

1

You don't need an operational-amplifier as it will add some inaccuracy due to OPAMP input bias current.

Instead, use a Schottky diode parallel to 10K resistor and ground.

If the reverse voltage is applied, the voltage across the ADC pin will be at max -0.3 to -0.5V which is safe for Arduino.

Deepak
  • 896
  • 2
  • 8
  • 3
    Tip: "op-amp" is short for "operational-amplifier". It's not an acronym or initialisation so it doesn't get capitalised. – Transistor Oct 06 '20 at 19:08