Reverse battery protection & Battery voltage measurement

[Shark]

Hi guys!

I'm making a small circuit with an ATmega328P and I would like to protect against reverse battery polarity. I will put a Schottky diode in series with battery as the circuit shows. I would also like to have battery voltage measurement directly on the battery, avoiding the Schottky diode, because I will have a boost converter (ommited from the circuit for simplicity).
My question is, do I need to worry about reverse polarity when ADC input pin will see battery ground and GND pin will see 3.0V?

Big thanks

[lewis]

Yes you do need to worry about it. In your circuit, when the battery is reverse connected, the internal ESD diode in the MCU between the ADC pin and ground will be destroyed.

Simple fix! Use a 100K resistor between the battery and the ADC pin. Put another 100K resistor between ADC pin and ground. Put a 10nF or 100nF cap across this second resistor (the ADC will usually have a maximum external impedance spec of 10K or so, the cap allows the voltage at the pin to remain stable during sampling).

The 100K resistors form a potential divider, adjust them to suit your voltage reference. You may want to increase the values significantly to minimise current consumption.

[Andreas]

My question is, do I need to worry about reverse polarity when ADC input pin will see battery ground and GND pin will see 4.5V?

Hello,

The bond wire of ADC input will melt due to input protection diode.
You dont have to worry about: it will not work this way.
You have to limit ADC input current e.g. by a resistor to a safe value (e.g. 1 mA).

with best regards

Andreas

[mij59]

Hi,

Take a look at this reverse protection method.

http://afrotechmods.com/tutorials/2011/12/04/p-fet-reverse-voltage-polarity-protection-tutorial/

Maybe you can omit the boost converter all together.

[Shark]

Thanks for all your replies!
I updated my schmatics with two 100k resistors and a capacitor, is this what you meant lewis?
I will get half of the battery voltage on ADC input (I'll probably use an external reference 2.048V or 2.5V).

[lewis]

That's right, you could even try 220K or 470K instead to minimise divider current and increase battery life. And you can adjust the divider ratio to make the most of the reference if you need greater precision!

Edit - sorry, just noticed I've been repeating myself