[SOLVED] lm339 positive input difference detection ?

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plouf

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Hi

i am testing a LM339, i see that it described to detect up to power supply with a few mV offset
however iwth a 5 Volt input i see that detection is triggered when i drop negative input to around 3,9 volt (and positive to 5)

while detection from 0 is nearly instand 10-50mV and triggered

do i uderstand something wrong about lm339 limits ?
 

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Review common mode input voltage range in LM339 datasheet. It's 0 to Vcc-1.5V. If you operate LM339 with 5V supply, it doesn't work with 3.9V input.
 
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You can always, if you have to use LM339, divide the input down into the inputs to get
the CM range in spec. But easiest is to use a RRIO comparator (or RRIO OpAmp) if speed
not a problem.


Regards, Dana.
 

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However because final project will eun with 14-16 volt input i see no rail-to-rail with than input (all rr are up to 5v) so voltage division is only option ..
 

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plouf said:
However because final project will eun with 14-16 volt input
Click to expand...
Unclear. What is comparator supply and input voltage?

Anyway, once you understood the problem, you'll surely be able to choose components and design your circuit after reading datasheets.
 

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thanx everyone,
original query was why lm339 does not detect positive rail which answer my mistake (tahnx)

best solution imho is voltage divider which should work perfect for this
 

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