TL431 power constant current source schematic.
Fig. 1
Click for larger view.

Power TL431 Constant Current Source

See YouTube video TL431 Power Constant Current Source Demo 1

The goal of this tutorial is the teach circuit theory. This is also an inexpensive way of creating a constant current source for circuit prototyping.

This tutorial explores how power divides in a series circuits. This also explores using power transistors to boost current output from a basic TL431 constant current source circuit.

Fig. 1 is the TL431 power constant current source schematic.

A 10-Ohm resistor R set the series current by controlling the bias current through the Q1 and Q2 base-emitter-base-emitter circuit. Q1's emitter current (Ie) is the base current for Q2.

These three PN junction drop ~1.8V.

The Q2 emitter current (Ie) minus a few milliamps Ik in the TL431 is most of the circuit current. Ik is disregarded in the following example.

The R voltage is rounded off to 5V.

The series current I is fixed by R (10-Ohms) with the formula 2.5V / R = 2.5V / 10 = 250mA.

Fig. 2 TL431 constant current source test setup for 250mA.

TL431 constant current source test setup for 250mA, input voltage 19.5V.
Fig. 3

TL431 constant current source test setup for 250mA with a Vin of 19.5V. The current I is set by a 10-Ohm resistor R.

The sum of all power drops will add back the Vin input power.

The input voltage Vin divides mainly across the Q2 pass transistor, the 10-Ohm current set resistor R, and the output load. The total power input is 19.V * 250mA = 4.874W.

Q2 voltage drop is 14.5V with a current of 250mA. The transistor dissipates 3.625W.

Current set resistor R always drops ~2.5V * 250mA = 625mW. The 10-Ohm load resistor is identical at 2.5V at 625mW.

~75% of the power is wasted across Q2.

TL431 constant current source test setup for 250mA, input voltage 8V.
Fig. 4

The circuit in Fig. 4 is identical to Fig. 3, except the input voltage is 8V. The overall power input from Vin is 2W.

R and load are fixed, the power dissipation in Q2 drops 750mW or 37.5% of the Vin power.

In summary excess power is dissipated across Q2. Reducing the input voltage to just above the required voltage (Vload + VR + 3V) reduces wasted power.

TL431 constant current source test setup for 1.25A, input voltage 19.5V.
Fig. 5

Fig. 5 is heavier current test setup. R is changed to 2-Ohms for a current of 1.25A. The load resistance is now 5-Ohms.

The output voltage on the load is 6.25V at ~7.8W.

The R voltage is still ~2.5V at 1.25A for 3.125W

The Q2 collector-emitter voltage drop is 10.75V at 13.45W with a Vin of 19.5V.

TL431 constant current source test setup for 1.25A, input voltage 15V.
Fig. 6

Fig. 6 is identical to Fig. 5 except the input voltage is 15V. This reduced the Q2 collector-emitter voltage 6.25V at 7.825W.

Vin could have been reduced to 12V reducing Q2 collector-emitter voltage to 3.25V at 4W.

Live test revealed the circuit produced 2A with a 1-Ohm R value, not the expected 2.5A. Limit R to no lower than 2-Ohms for 1.25A. The circuit wasn't tested for 1.5-Ohms for 1.67A.

The wattage rating for R is minimum 5-watts.

Fig. 7 LM7525-Adj variable power supply circuit used in these test circuits.

For information on Fig. 7 see LM2575 Simple Switching Voltage Regulators.

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