Resitance value of Pot

[gauravkothari23]

Hi all...
i am making a 3.7V low battery indicator ( Circuit Attached).. the led glows when battery voltage reaches 3.2v.. it works absolutely perfect....
i am using a 5k pot to adjust the desired range of operation.... so i have set the range at 3.2v.. i am going to use this circuit in one of my product, so which i need to manufacturer in quantity.... so everytime in every circuit instead of adjusting the pot can you anybody please tell me how can i check the resistance value of pot and replace it with some resistors.
Thanks You.....

 

[pradhan.rachit]

In simulation, you first change the voltage source to 3.3VDC.

Then you find out the voltage across the variable terminal of the pot and ground, lets call this Va.
Current through the divider @ 3.3V = 3.3V/(1k+5k) , lets call this Ia.

Transistor base current = Ib.
The value of lower resistor = Va/(Ia-Ib)
Value of upper resistor = 5k - Rlower

 

[esp1]

hi,
The circuit will not be stable, the smallest variation in ambient temperature will change the switching points
Component tolerances, diode voltage drops, transistor gain will effect the circuit setting point and its operation.

This LTSpice simulation image, shows your circuit at 20C. image A38
If I sweep the circuit temperature from 10C thru 30C the Set point changes, image A39

E

 

[gauravkothari23]

SO... @ESP1.... what can be done in such cases.....
'is it not possible for me to use this circuit

- - - Updated - - -

pradhan.rachit .... values i am getting are...
va = 1.90
ia =3.9836
ib=0.56

 

[Warpspeed]

Replace the first transistor with a TL431 shunt regulator.
This has a very accurate 2.495 volt switching threshold, and you can replace your potentiometer with two fixed resistors.
Try 6K2 and 22K, that should switch at 3.198 volts if everything is perfect.
https://www.ti.com/lit/ds/symlink/tl431.pdf

 

[gauravkothari23]

Replace the first transistor with a TL431 shunt regulator.
This has a very accurate 2.495 volt switching threshold, and you can replace your potentiometer with two fixed resistors.
Try 6K2 and 22K, that should switch at 3.198 volts if everything is perfect.
https://www.ti.com/lit/ds/symlink/tl431.pdf

I have replaced TL431 as shown in attached Diagam....
But the LED does'nt gets off even after adjusting any range using 5K.

 

[Warpspeed]

Remove the two diodes and try it.

The input to the TL431 needs to be able rise above 2.495 volts to turn on the TL431 and shut off the LED.

 

[gauravkothari23]

Remove the two diodes and try it.

The input to the TL431 needs to be able rise above 2.495 volts to turn on the TL431 and shut off the LED.

changing the value still does'nt work....
check the attached diagram...

 

[Easyrider83]

Like this?

 

[Warpspeed]

]Like this?

Yes, that's it !

Try replacing the 3K3 and 10K in the simulation, with 6K2 and 22K.
That will move the switching point down from 3.3v to 3.2v which is what he wants.

 

[Easyrider83]

I choosed 3.3 because of expected voltage drop on battery because of load. 3.2v - discharged unloaded battery. Anyway, this schematic can cause led flashing because of low histeresys value.

 

[Warpspeed]

We could give it some hysteresis by fitting a high value resistor between the collector of the transistor and the input to the TL431.
It probably does not need much.

*edit*

Thinking about it, when the low battery LED switches on, the slight extra current draw will pull the battery down a bit more, so it may not flash.

There may be enough natural hysteresis action for stability.

 

[gauravkothari23]

Like this?

Thanks Easyrider....
But i have even tried using this circuit.... but it does'nt work.... you have stimulated in proteus....
i have a lots of such circuit whcih works perfectly in proteus.. but when i tried building them they dont work at all....
protues is a scrap.....

 

[Easyrider83]

Thanks Easyrider....
But i have even tried using this circuit.... but it does'nt work....

What do you mean by 'does'nt work'? Led doesn't switching on?

 

[Warpspeed]

It should definitely work if all the parts are good and it's properly connected up.

 

[gauravkothari23]

It should definitely work if all the parts are good and it's properly connected up.

no... the led doesn't gets off at any voltage... i have also tried using 7.4v supply.... but still the led stays on even at 7.4v.... have also tried replacing. the components and checked my circuit 3 times....

 

[Easyrider83]

Do you know what for I used that diode? Show me exact that schematic you used.

 

[gauravkothari23]

Do you know what for I used that diode? Show me exact that schematic you used.

Attaching my circuit diagram.... please check it out...
even this schematic does not work in Proteus.... it shows me error...

 

[Easyrider83]

Yes, proteus is scrap, if use it without brain. You forgot to specify a ground in your schematic.

 

[gauravkothari23]

Yes, proteus is scrap, if use it without brain. You forgot to specify a ground in your schematic.

that schematic i built just for your reference....
but for testing i built a actual circuit with reference to your schematic... which does not work....
if you say for using Proteus you need a brain.... then how does a 24v battery makes a led glow without having any resistor in between insteas of giving an error....
i have also tried many such circuit which shows me an error or run perfectly in Proteus... but when i actually built the circuit... the output is something different...