I have written about how one can use the TL431 IC to build adjustable voltage regulator. Now another application usage of TL431 is constant current source and sink circuit. TL431 is adjustable zener diode and perhaps you know that zener diode are often used to set constant voltage or reference voltage. In fact, zener diode based voltage regulator are the simple voltage regulator one can build. An improvement of zener diode voltage regulator is to add a transistor to the circuit. I covered this in the note simple series voltage regulator. The TL341 IC can be salvaged from older PC PSU(Power Supply Unit). TL431 can be used in power supply circuits such as inverter or regulators.
The following diagram shows circuit schematic of how the TL431 can be configured as a constant current source.
Here TIP41 is used as the pass transistor whose base is connected to the the TL431 cathode, the emitter connected to the ref pin of TL431. The desired constant output current \(I_o\) value is set using the series resistor R2 which is calculated using the following equation.
\(I_o = \frac{V_{ref}}{R_2}\)
that is,
\(R_2 = \frac{V_{ref}}{I_o} = \frac{2.5V}{0.1A} = 25 \Omega\)
Similarly the value of the resistor \(R_1\) can be calculated using the following equation:
\(R_1=\frac{V_{IN}-V_B}{I_{KA}+I_B}\)
where,
* \(V_B \approx V_{REF}+V_{BE} \approx 2.5\,\text{V}+0.7\,\text{V}=3.2\,\text{V}\)
* \(I_{KA}\) = TL431 cathode bias (design for ~2–5 mA; ≥1 mA at all times)
* \(I_B = \frac{I_O}{\beta_\text{forced}}\) for the TIP41 (pick a conservative \(\beta_\text{forced}\) like 10–20)
For \(I_O=100\,\text{mA}\):
* If we assume \(\beta_\text{forced}=20\Rightarrow I_B=5\,\text{mA}\) and choose \(I_{KA}=2\,\text{mA}\) → total \(=7\,\text{mA}\):
* \(V_{IN}=12\,\text{V}$: $R_1=\frac{12-3.2}{0.007}\approx \boxed{1.26\,\text{k}\Omega}\) (use 1.24 kΩ or 1.3 kΩ).
So in this way we can calculate the resistor value for building constant current source using TL431 IC.
