Question

why is it important to check the dc voltage from a power supply before connecting it to a logic circuit ?

Answer 1

The gates are mostly ICs that can work at a maximum of 5V, not more than that. They usually see their input as HIGH or LOW signals. Mostly the voltage between 4.5-5V is taken as High, and the voltage between 0-1V will be taken as Low. So, if you can't check the dc input before applying to the logic circuits, whether the supply is in between these two peaks or not, there may be a chance of other voltage instead of 5v or 0v. In that case the IC's cannot understand its input so, it can't work. For higher DC voltages the IC's are damaged. So we need to check the supply before giving the logic circuits.