# Measure a voltage larger than operated voltage of a comparator

#### terrylai

Joined Oct 11, 2022
7
Hello all,

Recently I am doing project of measuring temperature. The target is a battery operated device. I need to use a comparator to measure supply voltage and sends out a warning LED signal to users if the supply voltage is lower than 3V. The comparator is from the MCU which is operated in 3.3V by four AA batteries (6V) using DC-DC converter. I want to ask would the MCU burn if the 3.3V operated comparator measures the supply voltage which is more than 3.3V (for example 5V). Thanks!

BR,
Terry

#### DickCappels

Joined Aug 21, 2008
9,509
Check the data sheet for the comparator, it is almost certain to say that the input cannot exceed the power supply. This specification is usually referred to as common mode input voltage range.

It may also be listed under Absolute Maximum Limits.

#### Jerry-Hat-Trick

Joined Aug 31, 2022
207
I’m not sure I understand the problem exactly, but could you use the MCU to measure the supply voltage by using a potential divider with two equal value resistors where the central tap is read by an A/D pin of the MCU which would work for a supply voltage up to 6.6V?
If the supply voltage could go higher, adjust the resistor values so the MCU input never goes above 3.3V

#### Ian0

Joined Aug 7, 2020
6,679
Does your DC-DC converter have a pair of resistors which which feed back a proportion of the output to the control circuit? If so, that would be a convenient point to connect to the A/D or comparator input.

#### Papabravo

Joined Feb 24, 2006
19,570
I’m not sure I understand the problem exactly, but could you use the MCU to measure the supply voltage by using a potential divider with two equal value resistors where the central tap is read by an A/D pin of the MCU which would work for a supply voltage up to 6.6V?
If the supply voltage could go higher, adjust the resistor values so the MCU input never goes above 3.3V
It is possible to make a device that WILL tolerate input voltages that are higher than the supply voltage. If that is the case, it will be explicitly stated in the datasheet. It is always safe to make an input voltage be within the stated limits by any means available.

#### BobaMosfet

Joined Jul 1, 2009
2,053
Hello all,

Recently I am doing project of measuring temperature. The target is a battery operated device. I need to use a comparator to measure supply voltage and sends out a warning LED signal to users if the supply voltage is lower than 3V. The comparator is from the MCU which is operated in 3.3V by four AA batteries (6V) using DC-DC converter. I want to ask would the MCU burn if the 3.3V operated comparator measures the supply voltage which is more than 3.3V (for example 5V). Thanks!

BR,
Terry
Learn about voltage dividers (you can scale voltages this way, into a range you can manage).

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3