For all these examples in the previous tutorial, the maximum current the IC can use is 100mA and I was limiting that with a 330ohms resistor at the input in order to protect the IC. What if now, we want to connect at this regulated output a load that needs for example 450mA of current, as a light bulb 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 the next example.
For the fourth example we have the 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. Let’s see how. For example, we have a supply voltage of 8V and we need to limit the current at let’s say 500mA. For that we can add a 15 ohms resistor in series with the load, right? In that way, only 500mA would be flowing. But the problem with that setup is the current value will change with the input voltage value. 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.
In this case the current value will be fixed and is equal to the formula below where Vref is 2.5V. And to calculate the R1 resistor we use the other formula where VIN is the input supply, IOUT is the output current, and HFE is the gain of the used transistor. In my case is a TIP31 and this one has a gain of around 24 as mentioned in its datasheet. I mount this circuit on my breadboard and I want a current limit of 350mA.
I have some 22 ohms resistors. I add 3 of them in paralel to get 7.3 ohms and to whistand more power. Using the formula, with a 7.3 ohms resistor the current limit should be close to 350mA. I turn on my supply 12v. I’ve connected the load and now we have around 350mA of current flowing. And if I change the input, this time, the current value is more or less stable. It changes a bit but it will always stay close to 350mA. The excess of power is wasted inside the transistor so for longer time, make sure you add a heat dissipator and also make sure you know the limits of that transistor, otherwise you might burn it out. So that's how you can make a current limiter with the TL431.
So as you can see you can use the TL431 as a voltage reference at the gate of a transistor and like that control the current flow taht goes to the load. I hope you like this tutorial and maybe you have learned something new. If my videos help you, consider supporting my work on my PATREON or a donation on my PayPal. Thanks again and see you later guys.