A few weeks ago, we’ve made this battery charger and protection circuit that was
cutting the voltage when this was reaching 4.2V. To make that circuit I was using this ZENER diode,
the TL 431 but I never explain you this component works and what more it could do.
This is actually a variable reference ZENER diode and it can be used in so many circuits.
Today I will show you a few circuit examples that you can make with this component and you will see
how useful it can be for so many applications. Fixed value current limiter, voltage reference
or variable voltage reference, it can be used as a undervoltage or over voltage protection
or as a delay timer and much more. So stay till the end of the video and learn everything about
this component and see all the examples I have to show you. So, let’s get started.
What’s up my friends, welcome back. This here is the TL 431 and this is a normal ZENER diode.
A normal ZENER diode could be connected like this.
The normal ZENER diode usually has a voltage reference that we can find in the datasheet.
If this reference voltage is for example 5V, what this component will do in this configuration,
is to allow the needed amount of current to pass towards ground, in such a way that
the output will be regulated at 5V. But if you want a different voltage at the output,
you need to change the diode by one that has that reference voltage. That’s why we use the TL431,
because this one has a variable regulation voltage that can be changed on the third pin.
So let’s see the first circuit with this component. As you can see, contrary to a
normal diode which has 2 pins, this one has 3 pins, anode, cathode and the reference voltage.
So from that circuit we can make this one here. The problem is that the TL431 could consume a
maximum of only 100mA, with more than that, it will burn out. And the voltage reference
could be from 2.5V up to 36V. So with this circuit right now,
since the reference is connected at the output, the value will be the minimum voltage of the TL431
which is 2.5V. To limit the current and not burn the IC I have this 330ohms resistor at the input.
So, the current that passes through the IC is 29mA if the input is 12V.
Here I have this circuit mounted and as you can see the input is 12V.
But if I measure the output, it is stable at 2.5V. This was the first example. But now,
what if I want the output to be different that 2.5V. Let’s see the second example.
Well, instead of connecting the reference pin at the output, we add a voltage divider. Now
the output value would be equal to 2.5V multiplied by 1 plus the ratio between these two resistors.
So for example if the resistors are both of 10K, the output would be 2.5 multiplied by 2,
so it would be 5V. Changing the value of these resistors you can get any output from 2.5
up the 36V but have in mind the output can’t be higher than the input, obviously.
I have this same circuit mounted on my breadboard. But now, for the third example, instead of adding
fixed value resistors, let’s add a potentiometer. Now we can change the output value just by varying
the potentiometer value. Here I have this simple circuit mounted on my breadboard.
As you can see, using the potentiometer I can change the value at the output.
This output is not affected by the input. As you can see I change the input but the output
is stable at the same value since the IC is making the regulation by consuming more or less current.
But for all these examples, the current was limited to under 100mA using that
input 330ohms resistor in order to protect the IC. What if now, we want to connect at this regulated
output a load that needs 300mA for example. Since the circuit can only give up to 100mA,
well we wouldn’t be able to supply our load. That’s why we have this next example.
For the fourth example we have this constant current limiter using the TL431.
In this case, the IC will control the voltage at the base of a transistor
in such a way that the passing current value is always the same value. Let’s see how. For example,
if we have a supply of 8V and we need to limit the current at let’s say 500mA. I
add a 15 ohms resistor in series with the load. As you can see, now only 500mA are flowing. But the
problem with this setup is that the current value will change with the input voltage value.
Look, as I increase or decrease the input, the current value also changes. What
we want is a fixed value current limiter. For that we use this circuit.
The current value is fixed and is equal to this formula where V ref is 2.5V.
And to calculate the R1 value we use this formula where V plus is the input supply,
Io is the output current, and HFE is the gain of the used transistor
which in my case is a TIP31 and this one has a gain of around 30 as mentioned in its datasheet.
I mount this circuit on my breadboard with a resistor of 15 ohms. As you can see,
I turn on my supply and connect my load and there are around 180mA of current flowing.
And if I change the input, this time, the current value is stable. It won’t change
and that’s exactly what we wanted. The excess of power is wasted inside the transistor
so make sure you add a heat dissipator and also make sure you know the limits of that transistor,
otherwise you might burn it out. Ok, let’s see the next example circuit and this is
called an undervoltage protection. We’ve seen something similar on the battery charger circuit
where the IC will stop the charging process if the voltage was above 4.2V.
But in this case, we want to turn off the output if the voltage is below a certain voltage.
We will use this circuit and the cutoff voltage is equal to 2.5V multiplied by 1 plus the ratio
between these two resistors. For example, with 2k and 1K, the cutoff voltage would be 7.5V.
When the input is below that voltage, the TL431 will power off the transistor
gate so the load is not powered. I mount this circuit on my breadboard.
I connect a load at the output and supply the circuit at 3V but the load is still OFF. I start
increasing the input voltage and when I get above 7.5V, as you can see, the load is now powered on.
And if I get back to below 7.5V, the load is once again OFF. That’s how you can make an undervoltage
protection circuit. You also have the circuit from the previous video for the battery balancer
which is pretty much the same. Check that video in order to learn more.
Ok, one final circuit I have with the TL431 for the sixth example is a delay
timer. This would be the circuit. Using this capacitor, the IC can regulate the current
flow in such a way that it will control the time it takes the capacitor to charge or discharge.
The delay is given by this formula where V ref is once again 2.5V.
By changing the capacitor or resistors values, you can control the delay time.
I mount this circuit on a breadboard and supply it at 12V. After X seconds, time the LED turn on.
I push the button and once again, after X seconds, the LED turns on.
Here I have the same circuit but with a smaller capacitor value. Now the delay time
is only X seconds. So as you can see you can use the TL431 as a voltage reference,
also as a variable voltage output, you can use it as a shunt regulator and control the
current value, you can use it for undervoltage or overvoltage protection and charging batteries,
use it in power supplies and also use it as a delay timer for any of your projects
where you need these applications. I hope that now you know more about this IC and this will
give you ideas for your circuits. If you have learned something mew, consider supporting
my work and giving me a like or comment below. Thanks again adn see you later guys.