Electronics > Beginners
Comparator - Odd behavior in circuit
t1d:
I built this circuit on a breadboard. See schematic. I substituted a LED and 1K resistor to represent the e-load.
I used an op amp tester to verify that the comparator functions properly. This schematic is provided, also.
I have verified that all of the proper voltages are present on the comparator’s physical pins.
The voltage of +IN is one volt greater than the voltage of -IN, so OUT should go high. It does not. Please remember that this same chip tests as good.
I have done all of the typical troubleshooting that I know to do. I am stumped.
Thoughts and suggestions, please and thank you.
floobydust:
LM339 has open-collector output, so you need pullup resistor there to source current. I'd also add a little hysteresis resistor so it doesn't oscillate.
t1d:
Thank you, floobydust, for the amazingly fast response. I will have to google your instructions, to understand what needs to be done, but I very much appreciate you pointing me in the correct direction.
t1d:
Okay, floobydust... Open Loop means no feedback loop... Got it.
I think this is what you want for the pull up resistor, correct?
https://www.electronics-tutorials.ws/opamp/op-amp-comparator.html
So, the comparator output is a sink, not a source. The pull up resistor holds the pin high, until the sink pin turns on and grounds out the MOSFET gate voltage, correct?
Thank you for your help and support.
t1d:
--- Quote from: floobydust on December 27, 2019, 06:11:10 am ---I'd also add a little hysteresis resistor so it doesn't oscillate.
--- End quote ---
Umm... To add hysteresis, don't I use an inductor? If so, what value? If not, please explain what you mean. Thank you.
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