Comparator Questions

Thread Starter

DC_Kid

Joined Feb 25, 2008
794
i need to learn more about comparators.

is the output of a comparator a complimentary stage, can it source and sink current? what is this state of the output when no power is on the device, is it hi Z?

the reason i ask is because i need to drive a fet switch and depending on how the comparator output functions i may or may not need a intermediate stage between comparator and fet, etc. one aspect of the circuit is if the comparator dies the fet gate gets pulled high via a pull-up resistor, etc. its a "fail-safe" feature, etc.
 
Last edited:

beenthere

Joined Apr 20, 2004
15,808
Not to be smart, but some do and some don't. You really need to do parametric searches for devices, or at least get data sheets to see how the output of any comparator is handled.
 

SgtWookie

Joined Jul 17, 2007
22,201
Beenthere's right.

Most of the common comparators (like LM111/211/311, LM339, LM2903, etc) have open-collector outputs; you need a pull-up resistor in order to see any output from them.

Comparators such as the LM339, LM2903 etc. won't sink a lot of current. You'll need to use a driver if you're trying to operate at more than a few hundred Hz, particularly if you're using a high-capacity MOSFET; as they can have hefty gate charge requirements.
 

Thread Starter

DC_Kid

Joined Feb 25, 2008
794
i thought some did and some did not, just thought i would check.

the comparator will run real low Hz, probably 1/10 - 1/200 Hz. i have "adapted" the AD595 into a set point cooling system with hysteresis. the final switch is a IRF1404 and it has big gate charge.
 

SgtWookie

Joined Jul 17, 2007
22,201
I have "adapted" the AD595 into a set point cooling system with hysteresis.
That's not a comparator; it's a thermocouple amplifier.

The final switch is a IRF1404 and it has big gate charge.
Indeed it does. You're going to need to use a driver circuit, even operating at low frequencies. Otherwise, it'll spend much of the time in the linear region, dissipating lots of heat.
 

Thread Starter

DC_Kid

Joined Feb 25, 2008
794
That's not a comparator; it's a thermocouple amplifier.
the ad596/597 datasheet uses a comparator as the setpoint mechanism, etc. AD595 is the same unit. 596/597 are optimized for high temp sensing.
 
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