• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

noobie lm339 question

Status
Not open for further replies.

shortbus=

Well-Known Member
Most Helpful Member
my very first post! as a noobie to electronics this will seem silly to most of you. but it's something i can't find a answer to.

with a LM339 comparator, is it O.K. to have a input voltage higher than the supply voltage? example;
+ input 12VDC
- input 5.5VDC
supply(Vcc) 5VDC

any help anyone can give is needed! thank you ,cary
 

crutschow

Well-Known Member
Most Helpful Member
It won't hurt the input, but the device won't work.
 

Hero999

Banned
It won't hurt the input
If you don't put current limiting resistors in series with the inputs the internal protection diodes will burn out causing destruction of the IC.
 

audioguru

Well-Known Member
Most Helpful Member
Why didn't you look at National Semi's datasheet? It says that the positive input voltage may exceed the supply with a max input of +36V.
The LM393 dual comparator is the same.
 

BrownOut

Banned
That's conditional, and the OP's inputs violate common-mode input voltages. Best to just try to keep the inputs within VCC whenever possible.
 
Last edited:

Hero999

Banned
I did look at the datasheet, I looked at the schematic not the technical specification and totally misread it. You're right, the diodes won't be overheated because D1 and D4 feed into PNP transistors which will be turned off and D1 and D2 will protect Q9.
 

Attachments

Last edited:

shortbus=

Well-Known Member
Most Helpful Member
thanks for all the replies. but i still need to know, is it able to work?

to me the schematic of the internals of the ic looks like it doesn't out put any voltage when the output is "high", right? a "high" just shuts off the output transistor and lets the pull up resistor feed to the next component in the circuit, right?

like audioguru said i went back and reread nationals data sheet, on page 7 it says the output doesn't have to be tied to Vcc, it can be anything from +2VDC to +30VDC. what i am doing is trying to use the lm339 to control a stepper motor driver board that needs to have 5VDC to change the motor direction.

so will it work to ;
feed 12VDC to the Vcc pin on the lm339
then feed 5VDC through a 3.3k resistor to the output pin
then to the stepper driver board control pin?

again thank you for your time and answers, cary
 

BrownOut

Banned
If you read the data sheet very carefully, you'll recognize that in your original scheme, you were violating common mode input voltages for that chip. While it's true that you can exceed VCC on an input pin, you can't exdeed it on both pins. The output of the amp will be indeterminate if you do. You can usually divide the input + and - voltages to get them into spec.
 

crutschow

Well-Known Member
Most Helpful Member
so will it work to ;
feed 12VDC to the Vcc pin on the lm339
then feed 5VDC through a 3.3k resistor to the output pin
then to the stepper driver board control pin?
That should work.

If you have 12VDC on the LM339 Vcc pin, then the normal operating range on the inputs would be anywhere between 0V and 12VDC.

Connecting 5V through a resistor on the output pin will give you an output of either 0V or 5V, depending upon the differential input signal level.
 

BrownOut

Banned
Yeah, you're right, the new scheme will work. I was confused because you were originally powering the chip with 5V. Using 12V buys you the wiggle room you need for the input voltages.
 

shortbus=

Well-Known Member
Most Helpful Member
alright! thank you for the help guys!

you guys that know this stuff can't even know how hard it is to try and figure stuff out on your own! most of the time i don't even know how to ask the questions right. for 42 years i was a tool and die maker and working with metal i much easier than electronics.

again thanks for all the help, cary
 
Status
Not open for further replies.

EE World Online Articles

Loading
Top