How Hysteresis Works in Opamp Circuits
How Hysteresis Works in Opamp CircuitsThe post discusses how hysteresis resistor in opamps works and helps to create a delayed switch ON and switch OFF across its output in response to the difference between its input pins.
enables the user to set the full charge cut off and low charge restoration of the relay apart by some voltage difference. If the hysteresis was not introduced the relay would rapidly switch ON OFF at the cut off level causing a serious issue with the system.
The question was raised by one of the dedicated readers of this blog Mr. Mike.
why are 4.7 zeners being used for the reference voltage? If we don’t want the 12 volts to drop below 11 for discharge,
Is the feed back resistor going to the virtual ground point a 100K resistor? If so, why was this value chosen?
2) Also, I apologize, I forgot to as why are there 4.7 zeners at the bases of the BC 547 transistors?
The red/green indication LEDs; how do they light up? I mean the red
LED is connected through its resistor to the top + rail, connects to
the output of the OPAMP, then goes down in series towards the green LED.
It would seem they would
be both on at the same time, since they are in series, in both circuits. Does it have something to do with the
the OPAMP is off, the top red LED
current is going through the feedback
resistor (thus its “on”)to the virtual ground point? But how does it get
switched off, when the OPAMP has an output? When the OP AMP gets an
output, I can see that going down to the green LED, but how, in that
state, does
the red LED then get switched off?
My Reply
4.7 is not a fixed value it can be changed to other values also, the pin#3
preset ultimately adjusts and calibrates the threshold as per the selected
zener value.
Question
The 100K feedback resistor and pot are creating the hysteresis value
(meaning, the difference between pin 2 and 3 to make the opamp swing
high to its + rail voltage)?
The op amp in this config is always trying
to make pins 2 and 3 arrive at the same value via its feedback resistor,
correct (zero, since the feedback divider is @ 0 and pin 3 is @
ground)?
Ive seen this solar charger controller done without the feed back, just
using several op amps with voltage ref pins and a pot on the other one
just trying to understand how hysteresis is working in this case
In other op amp circuits, they dont use any feed back just use them in
comparator config mode with ref
voltage at invert/non invert pin, and
when one is exceeded, the opamp swings to its rail voltage
What is the feed back doing? I understand the opamp gain formula, in
this case is it 100k/10k x voltage difference of POT voltage
(preset)value and 4.7 zener?
or is this a Schmidt trigger type of hysteresis UTP LTP circuit
comparators Ive seen just use the opamp in saturation, could you explain
why the feedback and gain for this?
Thanks
12volt rail, correct? So, when its preset value according to the POT
wiper is more? than the 4.7zener, we swing the opamp high? still dont
get the 100k feedback and why its used in a comparator circuit
My Reply
I am sure you know about how voltage dividers work?
As soon as the full
charge threshold is detected, as per the adjustment of pin#3 preset the
voltage at pin#3 becomes just higher than pin#2 zener voltage, this forces the opamp
output to swing to the supply level from its previous zero volt….meaning
it changes from say 0 to 14V instantly.
In this situation we can assume now that the feedback is connected between
“positive supply” and pin#3…when this happens the feedback resistor
starts supplying this 14V to pin#3, which means it further reinforces the
preset voltage and adds some extra volts depending upon its resistance
value, technically this means this feedback becomes in parallel with
preset resistor which is set between its center arm and the positive arm.
So suppose during the transition pin#3 was 4.8V and this switched the
output to the supply level and allowed the supply to reach back to pin#3
through the feedback resistor, which caused the pin#3 to a bit more higher say at
5V….due to this pin#3 voltage will take longer to get back to below the
4.7V zener value level because it has been raised to 5V…this is called hysteresis.
Both LEDs will never light up because their junction is connected with
pin#6 of the opamp which will either at 0V or the supply volt which will
make sure that either the red LED lights up or the green, but never
together.
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