I was trying to make a signal inverter using HD74HC.
Signal input is 2.5 V negative. What surprised me is that HD74HC still works without being powered (Vcc +5V)!
I checked the output signal (without Vcc), it shows lower output voltage, 1V.
My question is that why without power it still works and what else will it affect if not powered?
Thanks.

 

why without power it still works and what else will it affect if not powered?

It's powered by the input voltage going through the internal protection diode that is connected from the input to the Vcc pin.
It will not work properly if you have a low input that requires a high output.
Always use it properly powered. It's not ok to operate it unpowered.

Signal input is 2.5 V negative.

You should never apply a negative voltage to a logic gate.

 

It's powered by the input voltage going through the internal protection diode that is connected from the input to the Vcc pin.
It will not work properly if you have a low input that requires a high output.
Always use it properly powered. It's not ok to operate it unpowered.

You should never apply a negative voltage to a logic gate.

Sorry, it is H to L signal to be converted to L to H signal.

 

You should never apply a negative voltage to a logic gate.

You can and should....if your aim is to break the device

 

I was trying to make a signal inverter using HD74HC.
Signal input is 2.5 V negative. What surprised me is that HD74HC still works without being powered (Vcc +5V)!
I checked the output signal (without Vcc), it shows lower output voltage, 1V.
My question is that why without power it still works and what else will it affect if not powered?
Thanks.

My best guess is you are using your voltmeter incorrectly and measuring voltage drop across it rather than actually measuring the output voltage. If you measure between input and output when you are supplying negative voltage with your positive probe on the output and negative probe on the input and read 1v when input is -2v than you have essentially dropped -1 volt. Aka your output is actually -1 volt rather than 1v. And never run any IC, logic gate, etc.... without properly powered as one you can and will damage the device, and 2 the behavior will not only be incorrect but also unpredictable and therefor useless.

 

What surprised me is that HD74HC still works without being powered (Vcc +5V)!

You're powering the device through it's inputs or outputs and that's a NO-NO. The input protection diodes are there for ESD, not power. Run 10mA through any one of them and that device would no longer be considered reliable (electromigration in the metalization would be a typical cause of failure).

 

Which IC did you used?
74HC04, 74HC14, 74HC4049?