**Fig. 1 TL431 current source and current sink schematics.**

# TL431A Constant Current Source Working Circuits Demo

by Lewis Loflin

Follow @Lewis90068157

For the background on TL431 CCS circuits see TL431A Based Current Limiter Constant Current Source Circuits.

This page concerns the use of the constant current source configuration Fig. 1 A. The NPN transistor used in the specification sheet example, with little useful information, has been replaced by a TIP120 NPN Darlington transistor. This produces better output characteristics than an NPN.

The current formula is 2.5 / R. Figures 2 through 5 below illustrate the test set up and the calculated and measured results for an R value of 15, 47, and 150 Ohms.

The test load is a white power LED that operates at a measured ~2.8V. Notice the smaller value of R the brighter the LED due to higher current. Notice the heat sink on the LED.

The TL431 cathode and Ref pins control the Base-Emitter current of the TIP120. The resistor R is connected between Ref and the anode. This controls the TIP120 base current from the TL431 Cathode-Ref pins. The 4.7K resistor supplies the cathode current for the TL431. This can be dropped to 3.3K or 2.7K.

The measured voltage across R ranges from from 2.4V to 2.5V. The power value for R in watts is the current X resistance. For example, a 10-Ohm value for R produced 250mA: 250mA X 2.5V = 625mW. Use a 1 or 2 watt resistor.

**Fig. 2 TL431 power constant current source test setup.**

**Fig. 3 TL431 power constant current source test with 15-Ohm resistor. Current = 166mA.**

**Fig. 4 TL431 power constant current source test with 47-Ohm resistor. Current = 53mA.**

**Fig. 5 TL431 power constant current source test with 15-Ohm resistor. Current ~17mA.**

The current outputs closely follow the calculated values. A 10-Ohm resistor for R produced 250mA. A 5-Ohm for R started to reveal some problems. The TL431A is a low power device while the TIP120 Darlington is a much higher power device. This was stressing and starting to heat the TL431 above 400mA.

**Fig. 6 TL431 power constant current source test setup 2.**

## Current Boost

A new test setup is shown above. The TL431A CCS circuit was built on a PC board. Screw block are used for power input, power output, for current set resistor R, and for an additional NPN transistor Q2. The original circuit above used the TL431A to drive a TIP120 Darlington power transistor. Now that TIP120 drives Q2 a higher power NPN pass transistor.

The input voltage is 19.5V and the test load of a 10-Ohm 10-watt resistor. A cheap Harbor Fright voltmeter measures the current.

**Fig. 7 TL431 constant current source power demo schematic.For a larger image of above click tl431_power_ccs.jpg**

The revised schematic is shown in Fig. 7. A 5-Ohm value for R produced 500mA as expected. A 2-Ohm value for R produced 1.136A. A 1-Ohm value for R produced only 2A max. I would limit R to no lower than 1.5-Ohms.

At 19.5V, ~2.5V is dropped across R, 2.5V across the 10-Ohm load. Then 14.5V is dropped across Q2. To cut down on wasted power reduce Vin from 19.5V to 10V.

While Q2 is a 2SC3281, it can be any NPN power transistor such 2N3055.

- Experiments with TL431 Shunt Regulator
- TL431A Precision Current Regulator Circuits
- TL431A Based Current Limiter Constant Current Source Circuits
- TL431A Shunt Regulator Circuits

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- TL431A Constant Current Source Working Circuits Demo

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**Web site Copyright Lewis Loflin, All rights reserved. If using this material on another site, please provide a link back to my site.**