Hi, I know people love using 431's, and I have an application where one would be useful, and cost effective, but i might be overthinking it....
Powered from a single lithium battery, i'd like an LED to come on when the battery voltage gets to 3.3V and below. Off when the batt voltage is above that.
Note: no need to worry about leakage or under voltage shutoff, as the B+ input to this circuitry is switched. 2.8V Under Voltage cutoff

i was thinking of using the 1.24V reference devices, to give more headroom. (let me know if i'm overthinking that part)

this is where i'm at:
1.24V at T10 is 3.306V at the + rail(T7). *R6 is there incase i need some resistance there
when the battery is >3.306V, D3 will be ON, therefore D4 will be OFF due to the voltage across it not being enough for it to be on. Current through D3 will be limited by R7
When the battery is <3.306V, D3 is OFF, D4 will be ON. LED current limited by R7

is my thinking above correct?
any alternate way of doing it?
do I need to use the low reference voltage part? or would the 2.5V ref be ok to use (divider altered)?

thanks in advance guys
Alex

 

Sounds like it might work, though the LED will be off if the battery falls to much less than 2V.

Try simulating it.

 

Sounds like it might work, though the LED will be off if the battery falls to much less than 2V.

Try simulating it.

the battery won't get any lower than 2.8V due to the under voltage cut off protection, so this won't be an issue
i will get LT Spice installed and give it a try

 

the battery won't get any lower than 2.8V due to the under voltage cut off protection, so this won't be an issue
i will get LT Spice installed and give it a try

If you already have a low voltage cutoff, it should be able provide the signal for the LED. You should not need a separate circuit.

 

i don't think that is a good approach. it draws too much current for something powered by battery.

 

i don't think that is a good approach. it draws too much current for something powered by battery.

this circuitry will not be powered always, only when the 'device' is turned On. the device current consumption is around 5A, so the few mA this circuitry consumes is negligible really.
unless i've misunderstood your comment?

 

One word(?): ICL8211

They're obsolete, but plenty of them are still on the new component market. 1.5 to 30V supply, adjustable threshold voltage, 7mA current-limited output for LED at the threshold voltage, and minimal idle current consumption. I'd also experiment with the LED current, because many times, you can use far less than the rated current to get acceptable light output.

 

this circuitry will not be powered always, only when the 'device' is turned On. the device current consumption is around 5A, so the few mA this circuitry consumes is negligible really.
unless i've misunderstood your comment?

then you are ok. i did not realize battery or load size... i suppose mention of "battery powered" and "3.3V" made me assume tiny battery.

 

Using the TL431 for Undervoltage and Overvoltage Detection Application Report SLVA987A

a random guess for TL431 at Falstad's circuit simulator https://tinyurl.com/2dylwkoo

device	V.ref	I.ᴋ.min @ V.ᴋᴀ = V.ref	I.ᴋ.rec.max (I.ᴋ.abs.max)
TLV431 ( 0 … 70°C / –40 … 85°C / –40 … 125°C )	1.24 V	55 … 100 μA	15mA (20mA)
LMV431 ( 0 … 70°C / –40 … 85°C )	1.24 V	55 … 80 μA	15mA (30mA)
TLA431 –40 … 85°C TLA432 –40 … 125°C	2.495 V	150 … 200 μA	100mA (150mA)
TL431 ( 0 … 70°C / –40 … 85°C ) TL432 –40 … 125°C	2.495 V	400 … 1000 μA (much likely above ≥ 1400 … 1800 μA 4 stability/speed)	100mA (150mA)

 

Welcome to AAC.

You might consider a voltage supervisor like the TI TLV8xx. They are designed to provide a reset at a certain battery threshold, require very few external parts, and can source or sink 20mA which is perfect for an LED.

It seems like it might be a nice solution.