I need a circuit that turns on an led when the voltage of a 1s lipo goes less than 3.3v. Can someone please suggest a circuit that can accomplish this?
Thanks

 

A V comparator with a V ref. on - input, maybe LM385Z, 1.235 V @ 10 uA, or LM336Z, @2.5 V., + battery thru adjustable V divider to + input. Similar ckt. was published on AAC recently, but I do not remember where.

 

a comparator like tl431 would be a great choice here.

 

Here's the LTspice simulation of a simple circuit using the TL431.
It turns the LED on at a battery voltage of ≈3.29V.
Changing R3's value will vary this voltage.
If the LED you select stays on slightly at voltages above 3.3V, add a diode (1N4148 or similar) in series with the LED.

 

Here's the LTspice simulation of a simple circuit using the TL431.
It turns the LED on at a battery voltage of ≈3.29V.
Changing R3's value will vary this voltage.
If the LED you select stays on slightly at voltages above 3.3V, add a diode (1N4148 or similar) in series with the LED.

View attachment 118749

Can you please explain how this circuit works?

 

The TL431 is being used as a comparator.
When the voltage at the control input becomes higher than 2.5V, it starts to conduct.
So the resistor values for the voltage divider consisting of R2 and R3 are selected so that the control voltage is 2.5V at 3.29.V.
Thus the TL431 is OFF at any voltage below that and the LED is ON.
Any voltage above that, the TL431 is ON, which shunts the current away from the LED and it is OFF.

 

Here's the LTspice simulation of a simple circuit using the TL431.
It turns the LED on at a battery voltage of ≈3.29V.
Changing R3's value will vary this voltage.
If the LED you select stays on slightly at voltages above 3.3V, add a diode (1N4148 or similar) in series with the LED.

View attachment 118749

Tried it. The LED is on all the time.

 

I too tried the circuit and found that the anode to cathode voltage only dropped to about 1.9 volts when the TL431 was conducting whic was enough for a red LED to still be lit. The forward volyage of a silicon diode is series with the LED made it work as designed. I think the fact that the circuit would always draw about 4 mA could be a problem if it was connected to the battery all the time.

Les.

 

I don´t think that a TL431 is a good choice for battery undervoltage sensing as it will needlessly draw a lot of current. There are micropower comparators and undervoltage detectors that will do the job, quite possibly in a single part.

 

I too tried the circuit and found that the anode to cathode voltage only dropped to about 1.9 volts when the TL431 was conducting whic was enough for a red LED to still be lit. The forward volyage of a silicon diode is series with the LED made it work as designed. I think the fact that the circuit would always draw about 4 mA could be a problem if it was connected to the battery all the time.

Les.

Ah, bad choice of LEDs on my part then.

 

I don´t think that a TL431 is a good choice for battery undervoltage sensing as it will needlessly draw a lot of current. There are micropower comparators and undervoltage detectors that will do the job, quite possibly in a single part.

It depends upon the application and how much the load draws compared to what the TL431 circuit takes.

There certainly are other devices to do the job but how many are as cheap as a TL431 chip?

 

The problem is you'll forget about it, and the LED will drain your battery too much.

 

The problem is you'll forget about it, and the LED will drain your battery too much.

I would assume the low voltage detection circuit would be switched with the circuit power.

 

I don´t think that a TL431 is a good choice for battery undervoltage sensing as it will needlessly draw a lot of current. There are micropower comparators and undervoltage detectors that will do the job, quite possibly in a single part.

That's my opinion too - the TL431 isn't exactly "micropower".

Maxim probably do off the shelf battery management chips.

 

It's rather a tradeoff between power and cost/availability.