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Comparator with Hysteresis problem
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ranthalion:
I'm having an issue with the following circuit.  Channel 2 behaves as intended and turns the output off when the input duty cycle is <10%, but Channel 1 does not turn off.  I found that Channel 1 will turn off if I measure the voltage on the non-inverting input of the comparator.  So, it works if I am measuring the voltage, but not under normal operation? 

What would be the cause of this?  How should I fix it?

I also tried adding a 5.1M resistor to ground to maybe simulate having the meter connected.  That did change the behavior, but it seems to break the hysteresis since the output starting oscillating.





duak:
What are the voltages on U1A-1, U2A-1, -2, -3?

The schematic shows the power pins of U1 are connected twice - are the power pins of U2 connected?
ranthalion:
The appearance of the double connection on U1 is just a side effect of the symbol. The single pin is connected to 12v.  U2 is correctly connected to 12v and GND, but the symbol doens't show it.

That's the part that is troublesome.  The circuit works correctly if I measure voltage.


Duty Cycle0%10%11%10%OpAmp OutputU1A-10.04v   0.95v   0.99v   0.95v   Comparator Output    U2A-10.03v0.03v4.72v0.03vComparator Inverting Input    U2A-20.99v0.99v0.99v0.99vComparator Non-inverting Input    U2A-30.05v0.94v1.01v0.94v
JagV12:
What about J6 ?
ranthalion:
J6 is a jumper to select between 3.3v or 5v PWM input.  I have it configured for 3.3v PWM input.  It configures the gain to appropriately scale the input signal to approximately 0-10v.
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