# LM393 help

#### shortbus

Joined Sep 30, 2009
10,049
Hi, been away for quite a while and just starting into electronics again. My question is about the Vcc and input voltages on theLM393.

Does the combined input voltages on one of the comparators have to be the same as the Vcc or slightly lower? I've looked and looked on Google and can find no answer, not even in the data sheets. My circuit will have 12VDC for the input, does that mean I need to keep the combined input signals below the 12Vof Vcc? I'm going to measure the voltage of ~100VDC and know I need a voltage divider to lower that, but don't know what voltage is usable when combined with a variable reference voltage. Thanks for the help.

#### crutschow

Joined Mar 14, 2008
34,831
can find no answer, not even in the data sheets.
You just didn't know where to look (below):
So the input should not be greater than 2V below the supply voltage.

#### Michal Podmanický

Joined May 11, 2019
112
Your variable refference voltage should be at least 2V lower then Vcc. The measured voltage can go above Vcc.
However, I would set the refference to about Vcc/2 (not very close to Vcc-2V limit).

#### shortbus

Joined Sep 30, 2009
10,049
You just didn't know where to look (below):
So the input should not be greater than 2V below the supply voltage.

View attachment 322605
I saw that but didn't understand it, still don't. Does it mean the combination of both inputs or each input has to be below Vcc? I have never designed a circuit or have any training in electronics.

#### shortbus

Joined Sep 30, 2009
10,049
Your variable refference voltage should be at least 2V lower then Vcc. The measured voltage can go above Vcc.
However, I would set the refference to about Vcc/2 (not very close to Vcc-2V limit).
So then each input is considered separate from the other, not combined. I saw where they called it common mode somewhere for the inputs and thought it meant combined voltage. See the reply to crutschow.

#### crutschow

Joined Mar 14, 2008
34,831
Does it mean the combination of both inputs or each input has to be below Vcc?
Common means both together, not added.
Since the comparator only switches states when one input is slightly above or below the other, the CM voltage limit means that normal operation of the circuit requires that both inputs be no higher than Vcc-2V.

Make sense now?

#### shortbus

Joined Sep 30, 2009
10,049
Thank you for the help.