Problem with LM339 comparator

Thread Starter

samy555

Joined May 24, 2010
116
Hello

I built this comparator using LM339 but it didn't work



When I add a resistor (R6 = 1K) between VCC and pin #2 (the output pin of the first comparator), it works good



My question is: What is the role played by that resistance?

thanks
 

Bordodynov

Joined May 20, 2015
2,561
This comparator with an open collector. Additional resistor gives current sourcing. The scheme can be simplified. Make R6 = 10kOhm. Remove the resistor R4. The base of the transistor connect to the output of the comparator.
LM339.png
 

Thread Starter

samy555

Joined May 24, 2010
116
The LM339 comparators have an "open collector" output. They require a pull up resistor (R6). Google "open collector" for more info.
Of course the LM339 data sheet will also show that the output is open collector (but who looks at a data sheet? :rolleyes:).
This comparator with an open collector. Additional resistor gives current sourcing. The scheme can be simplified. Make R6 = 10kOhm. Remove the resistor R4. The base of the transistor connect to the output of the comparator.
View attachment 91900
Thank you very much
 
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