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[SOLVED] LM339 Output Always on

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necrow

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For my project, I need to evaluate two voltages and output a voltage depending on which one is higher. To accomplish this, I'm using two comparators on the LM339, one with V1 at the + and V2 at the - and one with V2 at the + and V1 at the -. I connected all unused pins to ground, the GND pin to ground, Vcc to 2.0V, and made V1 2.0V and V2 2.5V. I connected a buzzer from 9v to output on each, and they both went off. I'm confused because only one of the buzzers should go off. Any ideas where I went wrong?
 

karthick1987

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Hey,

You might want to connect Vcc to a higher voltage. Look at page one in the datasheet, it says Supply voltage must be atleast 1.5V more than the common-mode voltage. Also be careful when using comparators going close to the rails. Some only work between Vcc-1.2V and GND + 1.2V. Check it out.

Hope it helps
K

- - - Updated - - -

If you look at the schematic you will see that there is a two stage differential amplifier. So you loose about 1.5V in the drop from Vcc. Hence make the power supply atleast 2V more than your input voltages

- - - Updated - - -

View attachment LM139_239_339_Quad_comparator.pdf

Heres the datasheet
 

necrow

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I used 9v and it worked, feel like an idiot but thanks!!
 

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