operational-amplifier
Given the following schema (I'm using isis proteus)
Vin's amplitude is higher than the max and min supply voltages (+/-10V), which gives this graph:
(Blue: Vin, Yellow: Vout)
When Vin is greater than +Vcc, Vout goes to +Vsat, which is expected;
When Vin is lower than -Vcc, Vout goes to +Vsat: Shouldn't it stay in -Vsat??
I tried it in real life and it gives the same result. Why?
Im curious why you are using aplification. In my opinion simple cell voltage measuerement could look just like this (i drawed it only for 3 cells):
This should give you acurracy probably below 0.5% + resistor tolerance, but you can compensate resistor tolerance in software.
This is how very acurrate cell measurement can be done
This is part of cell voltage measurement circuit from well designed 6 cell battery charger:
You can find full version of this schematic here
LM324 amplifiers were used in very good Turnigy Accucell 6 and some other RC battery chargers in cell balancer circuits.
1-) Is my non-inverting comparator good enough? I mean do I need some extra caps around any nodes? R2 and R3 sets the threshold, but are they okay in values?
You need power supply decoupling capacitors, 100 nF, close to each chip. Those reference resistor values look fine.
Do I need a pull-up resistor from the output to the Vcc?
Figure 1. LM324 innards showing both a pull-up and pull-down transistor.
A quick look at the LM324 datasheet shows that it has strong pull-up to V+ by Q6 and a strong pull-down to V- by Q13. It doesn't need a pull-up resistor. Note, by the way, the Q5-Q6 Darlington arrangement: this can never pull closer than two diode drops away from V+. The arrangement around Q13 is different and the datasheet says Large Output Voltage Swing 0 V to V +/−1.5 V.
2-) I haven't experiment the final circuit and haven yet bought the relay. Do you have any suggestions regarding inrush currents, decoupling caps or any safety issues or anything related?
You'll need a transistor to drive the relay. The LM324 won't be able. Put a snubber diode across the relay coil to prevent switch-off voltage transients blowing the transistor.
3-) And finally, is there a way to separate AC neutral and GND in LTSpice? You see in my circuit it doesn't represent the real life scenario since AC neutral is the same with GND in my schematics.
I have no idea.
And finally the problem was that this type of fan and controller was not able to totally stop rotating unless someone manually switched it of from the controller's mains switch.
Did you try adjusting the MIN speed pot on the controller? You may be able to set it low enough to stall the motor. Watch out for higher than expected stall current but I don't think you'll have a problem with a triac controller as at minimum speed it will be giving very brief pulses of low current to the motor.
Congratulations on getting it all to work.
Best Answer
It's a behavior called phase reversal. It exist on many but not all opamps.
Basically it works like this: If your input signal exceeds the allowed input voltage range some of the internal transistors in the opamp break down. They shouldn't pass current but you're driving them so hard that they turn on.
The output then flips to the other polarity. Depending on the circuit around the opamp this could even lead to a so called latch-up condition where the output stays at the wrong polarity forever.
In general you should never exceed the allowed input range of opamps. If the phase reversal occurs due to an internal transistor breakdown the transistor may take damage and changes it's characteristics. The OpAmp may seem fine afterwards but it is not guaranteed to work within it's specification anymore.
About OpAmps that may latch-up: Never use them for critical control systems. Think about a heater with temperature control. It senses the temperature and controls a heating element. If your OpAmp is one of those latch-up types it is very possible that at extreme temperatures it runs into latch-up, turns on the heater at full power, stays there forever and burns you house down.
The data-sheet of the OpAmp usually states if it has phase-reversal or if it can latch-up. If this is not mentioned in the data-sheet assume that it does.