TL431 Precision Programmable Reference operation

Thread Starter

kalemaxon89

Joined Oct 12, 2022
262
I have this circuit that switches an LED on/off depending on whether Vi is greater or less than 10V.
I have never used a "Precision Programmable Reference" (I edited the post because I accidentally wrote tristore, thanks for correcting the error in the replies!).
I don't understand two things:

1) The datasheet says that if Vref<2.495 (in this case it's set by my Vr3) the thyristor conducts between cathode and anode (the LED will light up) .. but what does "conducts " that mean? Do I replace it with a short? I don't think.
In other words:
how do I redesign this circuit when Vref<2.495 and when Vref>2.495
SmartSelect_20230223_145602_Samsung Notes.png

2) Looking at the datasheet I see that the thyristor inside consists of a comparator and a BJT (figure below) ... since it's a comparator, tell me if I am wrong:
if Vr3>Vref i.e. V+>V- then Vo=+Vcc (the thyristor does not lead)
if Vr3<Vref i.e. V->V+ then Vo=-Vcc (the thyristor leads)
What are +Vcc and -Vcc? I would say that -Vcc is GND ... but +Vcc?
(I've replaced the thyristor with its internal model)
SmartSelect_20230223_145626_Samsung Notes.png
 
Last edited:

MrChips

Joined Oct 2, 2009
30,984
TL431 is a precision programmable reference. It is not a thyristor.
Its function is to supply a fixed reference voltage.
 

StefanZe

Joined Nov 6, 2019
191
The datasheet you linked is not the datasheet of a thyristor.
This circuit will give you an analog voltage at the output (the LED) dependent on the resistors R2 and R3
 

Dodgydave

Joined Jun 22, 2012
11,320
Its a TL431 precision zener as said, with the resistor values in your diagram R2/R3 the voltage trip will be 4.7V.
 
Last edited:

LowQCab

Joined Nov 6, 2012
4,218
What is the maximum-Voltage-Range that You want to monitor ?
If the Voltage-Range is too large,
the LEDs should have proper Current-Limiters instead of Resistors.
The LED-Resistors should be calculated with the highest
possible Voltage that the Circuit will experience.
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Voltage Monitor .png
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Thread Starter

kalemaxon89

Joined Oct 12, 2022
262
Its a TL431 precision zener as said, with the resistor values in your diagram R2/R3 the voltage trip will be 4.7V.
Beyond resistive values (maybe think about that later) .. I would like to know if my reasoning is correct:
if Vr3<Vref (see photo of comparator and BJT inside TL431) then Vo = 0V then Vbe<0.7V (=0V) and the BJT does not switch on ... so the current flows from 5V to R1 to the LED switching it on.
Am I wrong?

@LowQCab
@MrChips
 

LowQCab

Joined Nov 6, 2012
4,218
Why do You care ?
Keep it simple.
When the Voltage at the Ref-Pin is below 2.5V, the Transistor is Off.
When the Voltage at the Ref-Pin is above 2.5V, the Transistor is On.
The Transistor Forward-Voltage loss is ~2-Volts. ( possibly as high as 2.5-Volts under maximum-Load )
The Minimum-Current required for operation is 1mA.
The Maximum-Current that can be safely handled is 100mA. ( You might get away with up to ~150mA )
The Maximum-Voltage that can be safely handled is ~37-Volts.
Do not exceed 10mA input on the Ref-Pin.

This device will act like an adjustable-Voltage-Zener-Diode, or a "Fixed-Voltage" Comparitor.

With just these simple rules You can build all sorts of interesting Circuits.

Just for interest ..........
The Output-Transistor is actually a Darlington-Pair ........... ( as if it really matters ).

The actual internal-Circuit is in the Spec-Sheet PDF.
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Thread Starter

kalemaxon89

Joined Oct 12, 2022
262
When the Voltage at the Ref-Pin is above 2.5V, the Transistor is On.
Exactly, I am interested in understanding the inner workings and not seeing it as a 'black box'.

In the case Vr3>2.5V (V+>V-) .. the comparator has Vo=5V-R1*I1 so Vbe>0 (Base-Emitter junction in forward active) and the LED is OFF. But why the LED is OFF? Maybe because the current=5V/R1 all flows in the BJT and not in the LED? Why?

Still considering this case .. what is the value of Vce? How do I calculate it?
I want to know for sure the voltage of the LED.
1677320349400.png
 

crutschow

Joined Mar 14, 2008
34,700
The thing to note is that the minimum cathode on voltage of the TL431 is about 2V, which is the minimum to power the circuit.
That means the LED to ground in your circuit may stay slightly on under those conditions, depending on its forward voltage.

Here's the LTspice simulation of LQC's circuit in post #5 that uses two TL431s to eliminate that problem:

Initially, with Vb (green trace) below 10V, U1 is off with Vd (red trace) high at 2.75V (clamped by the base-collector junction of U2's input transistor), and U2 is on at its ≈2V minimum (purple trace), causing the LED (yellow trace) to also be on.
When Vb increases to 10V, causing the Ref voltage (blue trace) to reach 2.5V, then U1 turns on and Vd goes low
to 2V, turning U2 and the LED off.

All make sense?

1677343402895.png
 
Last edited:

LowQCab

Joined Nov 6, 2012
4,218
"" Still considering this case .. what is the value of Vce? How do I calculate it?
I want to know for sure the voltage of the LED. ""


The Voltage from Collector to Emitter of the Output Darlington-Pair is .........
Power-Supply Voltage when Off,
and ~2-Volts when On.

The LED Forward-Voltage depends upon the particular LED that You have chosen to use.
Look-up the Voltage in the Data-Sheet.
It will change slightly with Temperature.

Are You attempting to find the Forward-Voltage of the LED using a TL431 ?
If You are, this will not provide any useful information.
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Thread Starter

kalemaxon89

Joined Oct 12, 2022
262
"" Still considering this case .. what is the value of Vce? How do I calculate it?
I want to know for sure the voltage of the LED. ""


The Voltage from Collector to Emitter of the Output Darlington-Pair is .........
Power-Supply Voltage when Off,
and ~2-Volts when On.

The LED Forward-Voltage depends upon the particular LED that You have chosen to use.
Look-up the Voltage in the Data-Sheet.
It will change slightly with Temperature.

Are You attempting to find the Forward-Voltage of the LED using a TL431 ?
If You are, this will not provide any useful information.
.
.
.
In the datasheet on page 25 I see this formula ... should I use this?
1677358929504.png
In my schematic Vo=(1+R2/R3)Vref-Iref*R2 because I named the resistors differently
 
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