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Comparator

Chloe
Prodigy 70 points
Other Parts Discussed in Thread: LM239

Hi

 

I bought a comparator LM239N and I am trying to use the comparator to drive the gate of a mosfet.

I supplied 5V to the comparator (Vcc) , and then for the inputs had a range of values from 2-6V on the V+ and 0-6V on the V-. The output of the comparator did not appear to change even when V- was greater then V+.

I then put a 4 kohm resistor from the Vcc to the output, and did get a small voltage at the output but it did not really change with input voltage. What am I doing wrong?

Thanks

C

 

 

 

  • 0
    Genius 5430 points

    Chloe;

    The LM239 comparator has open-collector outputs; you must use a pull-up resistor to a positive voltage that is high enough above your MOSFET's gate threshold so that it turns on.

  • 0
    TI__Guru**** 151206 points

    Hello Chloe,

    The pull up output resistor is required for the LM239.  For a valid output, at least one input must be between 0V and 3V (VCC-2V). 
    When V- (inverting input) voltage is greater than V+ (non-inverting input) voltage the output will be low voltage (near zero)
    When V+ (non-inverting input) voltage is greater than V- (inverting input) voltage the output will be high voltage (near VCC)

    Regards,
    Ron Michallick