Problem in Comparator circuit

[vipul]

comparator circuit

Hi everyone,

I am designing circuit for Battery monitoring in which i want to switch off battery using relay when battery voltage will go below lower referance volatge.So i have used Comparator LM358 to compare input and referance voltage.When out put is open,Comaprator gives out put high(=Supply voltage of LM358) when input voltage is more than ref voltage and output is low(0.7V) when input voltage is less than ref voltage.

But when i am connecting transistor(which is used to drive relay) to comparator output,it decreas gradually when input voltage goes below instead of instant drop to 0.7V and viseversa.

So somebody please tall me what changes should i make to my circuit...

 

[BABESH]

cutoff battery circuit 5.4 v

Hi Vipul,

what r u trying to do,
is it switch ON transistor when battery voltage is <5.4V?
where is battery, the one shown as 12V?

--Babesh

 

[vipul]

design lm311 comparator calculate

Hi Babesh,

Yes that is Battery which is shown as 12 V..
Actaully i want to turn on transistor when Battery voltage is less than 10V..

 

[BABESH]

why zener diode is used in comparator circuit

probably that may not be the good option,

As i understand that u may want to have a discharge cutoff circuit.

If you ON transistor, current will be consumed to switch on relay coil, which will further drain current from battery,

do you think it is good option...

--babesh

 

[vipul]

lm358 low voltage disconnect zener

hi Babesh,

I know.....
But my acual problem is when i am connecting transistor or MOSFET to the output of LM358,the output of LM358 is changing gradually instead of abrupt change which i am getting if the output is open as i discussed priviously...

-Vipul

 

[millwood]

battery monitor divider zener

with the resistor setting, the non-inverting terminal is getting Vdc*10k/(10k+22k). so for any Vdc < 18v, the voltage on the divider is going to be less than 5.4v, and your comparator will always output low.

for it to trigger, you will have to recalculate the two resistors and swap the inverting / non-inverting termianls.

BTW, you don't need the pull-up resistor.

 

[vipul]

lm311 battery monitor

Hi Millwood,
sorry by mistake ..
the correct circuit is attached...

I think opamp inverting and noninverting inputs are right so why u told to swap it...

and as i told earlier my comparator function is working fine when nothing is connected to LM358 output...but not when i connect transistor or MOSFTE to it..

-Vipul

 

[BABESH]

lm358 comparator

Hi Dude,
it seems MilliWoiod is correct, when battery Voltage is Less than 11.3V O/p is turnning Low as per design. I think u need to recalculate resistors to have desired cutoff limit.

then in practicle you might have connected opposite..

Are you testing with powersupply or the exact battery?

Try remove 1K pull up and check..

--Babesh

 

[vipul]

my comparator circuit will not work

Hi Babesh,

As per circuit on inverting input there is fixed referance 5.4V using zener diode but actually it is 4.65V practically..

And on Noninverting input the voltage is initially 5.73V at 12 V battery voltage and decreasing as the battery discharge...

when the battery voltage is nearby 9.7V,voltage at non inverting input will be about 4.65.
Out put of Opamp will be high uptill now..

But when Battery voltage decrease from 9.7V,voltage at noninverting input will decrease
from 4.65 and output will low...

I am using battery but testing the circuit using potentiometer connected to battery..and supply to circuit is given through 10K pot...
To check the output at different voltage i am changes the pot value...

-Vipul

 

[millwood]

comparator not working

just do a series of calculations of the voltage on the non-inverting and inverting pins, under different power supply voltage, and ask yourself if that behavior is what you want out of the circuit.

Added after 3 minutes:

Hi Babesh,

As per circuit on inverting input there is fixed referance 5.4V using zener diode but actually it is 4.65V practically..

And on Noninverting input the voltage is initially 5.73V at 12 V battery voltage and decreasing as the battery discharge...

when the battery voltage is nearby 9.7V,voltage at non inverting input will be about 4.65.
Out put of Opamp will be high uptill now..

But when Battery voltage decrease from 9.7V,voltage at noninverting input will decrease
from 4.65 and output will low...

I am using battery but testing the circuit using potentiometer connected to battery..and supply to circuit is given through 10K pot...
To check the output at different voltage i am changes the pot value...

-Vipul

your problem is that you ask questions about a circuit, and tell people the behavior of a different circuit.

how could anyone other than you know what else to go on in helping you other than the circuit you have posted?

You have to post a schematic that exactly describes what you are playing with, or it will be very difficult for others to help you.

 

[vipul]

6v battery cut off circuit

Ok...
this is the final circuit i am working with...
and its just same as i describe and attached..

I dont khow why u people get so confused...

And i told many times i am getting right output of comparator when there is no connection at output of LM358...only i am getting gradual change at output pin when i am connecting resistor R5 to output pin(As shown in attachment)

Thanks,
Vipul

 

[BABESH]

problem with comparator circuits

dude,
Use variable voltage source instead of pot set-up
it avoids lot of loading effects..

Try to analyse the root cause, than concluding on the output of the system.

For cut off at 10V
use 8.2K and 9.6K voltage divider n/w.
change R3 as 1K instead of 10K, it supplies Izmin required for zener to maintain 5.4V Value.
As you are testing with battery voltage dips gradually and zener reference is not stable so you may be seeing a different result,

Try to modify with above correction, use power supply, Hope it will work.

All the best
Babesh

 

[millwood]

where to find izmin of a zener

Ok...
this is the final circuit i am working with...
and its just same as i describe and attached..

I dont khow why u people get so confused...

and you expect THAT circuit will work? when you change the pot, what will happen to your divider network? what will happen to the rail to the comparator? what will happen to all of that when the relay is activated?

And i told many times i am getting right output of comparator when there is no connection at output of LM358...only i am getting gradual change at output pin when i am connecting resistor R5 to output pin(As shown in attachment)

Thanks,
Vipul

if you still have trouble understanding why the "final" circuit will NOT work, compare it with the ones you had posted before. Calculate the opamp's behavior with the pot in different positions, and factoring the relay.

you only need ohm's law to do that.

Added after 29 minutes:

here is a circuit that does something quite similar to what you are asking.

it uses a zener for reference, and the comparator's output goes low, when the voltage goes below 10v, and it goes high when the battery goes above 10.1v - ie it has about 0.1v of "gap" built into the transition.

 

[vipul]

comparator transistor problem

ok i ll change the circuit as babesh suggest...
and also try the circuit attached by millwood...

i ll update u guys...

Thanks,
-Vipul

Added after 50 seconds:

dude,
As you are testing with battery voltage dips gradually and zener reference is not stable so you may be seeing a different result,

Try to modify with above correction, use power supply, Hope it will work.


Babesh

I am designing this circuit to cutoff battery itself...

 

[BABESH]

12v low battery circuit lm311

Dude,
Still, even if u r making this circuit for battery,
first confirm the circuit with external power supply,
After that u can replace power supply with battery,

Battery is a big problamatic element avoid at initial stages.

---babesh.

 

[umesh49]

6 v battery+low battery cut off+circuit

Vipul,
Did you get your problem resolved yet? I suspect this relay would be an inductive coil. Try experimenting by connecting a purely resistive load instead of relay. I think this problem my be due to inductve nature of relay.

 

[vipul]

comparator circuit -patent

Did you get your problem resolved yet? I suspect this relay would be an inductive coil. Try experimenting by connecting a purely resistive load instead of relay. I think this problem my be due to inductve nature of relay.

I have to use relay to cutoff supply.....

Added after 8 minutes:

Dude,
Still, even if u r making this circuit for battery,
first confirm the circuit with external power supply,
After that u can replace power supply with battery,

Battery is a big problamatic element avoid at initial stages.

---babesh.

I have checked circuit given by millwood with power supply instead of Battery..
And by changing voltage using pot i can successfully cutoff the relay at about 10.5 V..

Now i am testing it using battery and waiting for what is happening to swiching circuit when input voltage is very close to reference voltage...

- Vipul

 

[BABESH]

comparator circuit]

where is pot connected?
In millwood ckt straight decrease variable voltage source.
it should work.
replace power supply with battery allow battery to drain by connecting external load accross battery terminal,
then observe the relay functining.

 

[vipul]

lm358 comparator circuit single power supply

Hi Babesh,

ya i am doing the same thing...

-Vipul

 

[vipul]

lm358 comparator single power supply

hi..

Circuit is working fine when i m decreasing voltage using pot but it is giving problem when battery voltage is decreasing itself ..

Means when i am taking input voltage down to reference voltage using pot the relay trips correctly and only single time ..
But when battery voltage decreasing itself and when the input voltage and ref voltage are same the relay trips continuously..
I think this is because transistor is taking time to go to cutoff region from saturation region when there is 0.7 V at base

So please somebody tell whether i am correct or wrong???
Is there any other component which i can use instead of transistor??