We take the example of a IC 741 battery charger circuit and learn how the feedback hysteresis resistor 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.
1) Hi this circuit is very genius
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 wouldbe both on at the same time, since they are in series, in both circuits.
Does it have something to do with the feedback circuit and virtual ground? Oh I think I may see. So when
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?
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
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 @
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
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?
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
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
Feedback from Mr. Mike
thank you for answering all my questions, especially the one about the
feedback, that seems a bit advanced config so its new for me
would this low voltage set point circuit option work as well;
14 volts on the non invert, 12 volt zener on the invert reference pin.
once the 14 VDC rail dropped to 12, the opamp output trips on. This
would activate the low voltage part of the circuit.
In your case, the 10k pot is just "adjusting", "dividing" or bringing
the 14volt rail to a voltage closer to the 4.7zener?Your still
controlling the 14 VDC, I mean once it goes to 11 VDC etc, you want a
ratio that will swing the opamp high. if you replaced the 4.7 with
another zener value, the pot divider would set up a new ratio, but the
pot is still "following" or in ratio with the rail 14 VDC? Instead of
putting 14VDC on one opamp pin, your dropping it through a divider, but
the ratio is still controlling a small drop from say 14VDC to 11 VDC
through the 10K pot, that will drop to 4.7V?
Im just trying to understand how the circuit closes the "spread" from
11VDC (where we want the low voltage set point to be) and the ref
voltage of 4.7 vdc. most of the comparator circuits Ive seen just have
the ref vdc at pin 2, for example 6 VDC. and a rail voltage of say 12
VDC. Then a pot sets up a divider from that rail of 12VDC, drops to say 6
VDC through the mid point of the divider. Once the voltage at pin 3
approaches the ref 6 VDC @ pin 2, the op amp swings according to its
config, (invert or noninvert)
perhaps where Im messing up is here-
in other circuits Ive looked at, the rail voltage is assumed to be stiff, but in this case, its going to drop
Its that drop (14VDC to 11VDC) upsets the 10K voltage divider ratio? And your using that ratio to ref the
so if you have the 10K pot at its mid position of 5 k, that divider would set the 14VDC at 7 VDC (R2/R1+R2)
if the 14 rail went to 11 VDC, the divider mid position is now 5.5, so
it depends on where the wiper is at, am I starting to get it? we just
adjust the wiper until the 4.7 is in ratio to the voltage divider and
the rail drop we want?
so this circuit is using regular opamp comparator principles, but with
the added affect of hystersis for the low voltage set point control?
Yes you are getting it right.
A 12V zener would also work, but that would cause the opamp to switch between 12V and 12.2V, the feedaback system allows the opamp to switch between 11V and 14.V, that's the main advantage of using a feedback hysteresis resistor.
Similarly in my case, if the feedback resistor was removed, the opamp would start oscillating
frequently between the 14.4V cut-off level and the 14.2V reverting level.
because as per the setting of the 10K preset the opamp would cut off at
14.4V and as soon as the battery voltage dropped by a few milli-volts the
opamp would again switch OFF, and this would go on continuously causing a
constant ON/OFF switching of the relay.
However the above situation would be fine if a relay was not used rather a
transistor was used.
normally what I see in comparators is a fixed voltage like you have @
pin 2, usually through a voltage divider or zener etc, then at pin 3 a
variable voltage from source - pot - ground config with wiper (pot) in
the middel and the wiper will find the set point of pin 2, in your case
4.7 fixed zener voltage and swing the op amp approx to its rails,
according to its config
where its confusing is that the 10K wiper in your circuit is set at 14.4
volts? then that is supposed to trip the 4.7 zener? I don't get the
we first set the upper threshold cut off through the pot by supplying
14.4V from a variable power supply with feedback resistor disconnected.
once the above is set, we connect a correctly selected hysteresis resistor
in the slot, and then begin reducing the voltage until we find the opamp
switching off at the desired lower say 11V.
this sets up the circuit perfectly.
NOW, before confirming this practically we make sure that the battery is
first connected and then the power is switched ON.
this is important so that the power supply is able to get dragged down by
the battery level and begin with a level that's exactly equal to the
battery discharge level.
that's all, after this it's all smooth sailing with the opamp following
the cut off pattern as set by the user.
another important thing is that, the power supply current must be around
1/10th of the battery AH so that the power supply is able to get easily
pulled down by the battery level initially.
yes I was thinking it over and without the hysteresis it wouldn't work.
If I put a 7 zener at pin 2 , set Vin @ pin 3 through a 5k voltage
divider to be 7 volts, and a discharged battery on the circuit, as soon
as the battery got charged to 14 volts, the relay would drop in and pull
in the load, but the load would drop the 7 at the pot down immediately
so the relay would drop out. Without the hysteresis, I can see now why I
wouldnt work, thanks
My Replyeven without a load the battery will never cling to the 14.4V limit and will instantly try to settle down to around 12.9V or 13V. when opamp o/p swings to (+) it becomes as good as the supply rail, which implies that the feedback resistor gets linked with the supply rail, that further implies that pin#3 is subjected to a separate parallel voltage in addition to the presets upper section resistance which is connected with the supply rail. This added voltage from the feedback causes the pin#3 to rise from 4.7V to say 5V...this changes the calculation for the pin3/2 and forces the opamp to stay latched until the 5V has dropped below 4.7v, which happens only when the battery voltage has dropped way down to 11V....without this the opamp would have toggled continuously between 14.4V and 14.2V