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Sep 18 2020

TL431 Shunt Regulator: How to Test and Replace

I Description

  • How to check if TL431 is broken?
  • If TL431 is really broken, then how to replace it?

These two kinds of problems will be discussed in this blog.

tl431

Figure 1. TL431

I Description

II Introduction

2.1 TL431 Brief Introduction

2.2 TL431 Pinout

III How to Measure

IV How to Test TL431

V Replaceable Models

VI Conclusion

FAQ

II Introduction

2.1 TL431 Brief Introduction

TL431 is a 2.5~36V adjustable shunt regulator.

With its excellent performance and low price, it can be widely used in single-chip precision switching power supplies or precision linear regulated power supplies. In addition, TL431 can also form a voltage comparator, power supply voltage monitor, delay circuit, precision constant current source, etc.

2.2 TL431 Pinout

The symbol of TL431 is shown in the figure 2. It has three poles A, G, and K respectively (marked A, K, R on some schematic diagrams). A and K are the positive and negative terminals of the Zener diode, and the G pole is the sampling terminal.

tl431 pinoutFigure 2. TL431 Pinout

(1) How to determine A and K poles

According to the schematic diagram, the A and K poles can be judged by measuring the diode with a multimeter. When measuring, set the range to RX1K gear. When the black pen is connected to the A pole and the red pen is connected to the K pole', the resistance is in a conducting state (the resistance of a common silicon diode).

In addition, interchange the test leads, if the resistance is infinite, the pin connected to the black pen is the A pole, and the other pin is the K pole.

(2) How to determine G pole

Set the multimeter's range to Rx10k, connect the black pen to the K pole, and the red pen to the A pole. At this time, the meter should have no indication. When touching the black pen with one hand and the G pole with the other hand, the pointer should swing greatly. When this condition is met, the pin touched by the hand is the G pole.

III How to Measure

(1) Measurement of forward and reverse resistance of zener diode

The multimeter range is set to Rxlk, the black pen is connected to A pole, and the red pen is connected to K pole. At this time, the forward resistance of the Zener diode is measured. To measure the reverse resistance elbow, the range should be set to Rxlk. The data measured with the MF47 meter is: the forward resistance is 6xlkΩ, and the reverse resistance should be infinite.

(2) Measurement of the forward and reverse resistance of G pole and A and K poles

The multimeter range is set to Rxlk, the black pen is connected to the G pole, and the red pen is connected to the A pole. The resistance should be 35xlkΩ. The resistance of interchangeable test leads should be 10xlkn. Connect the black pen to the G pole and the red pen to the K pole. The resistance should be 11×lkΩ. If the test leads are interchanged, the resistance should be infinite.

(3) Measurement of the forward and reverse resistance of K pole and A, G pole

The multimeter range is set to Rxlk, the black pen is connected to the K pole, and the red pen is connected to the G pole. At this time, the resistance is infinite. Exchange the test leads, the resistance should be 11×lkΩ. Connect the black pen to the K pole and the red pen to the A pole, and the resistance should be infinite. If the test leads are interchanged, the resistance is 8xlkΩ.

IV How to Test TL431

As shown in the figure is the circuit tested by TL431. For the power supply, it is a 0~20V maintenance power supply.

We connected an ammeter between the K pole and the power supply. This is done in order to clearly observe the changes in the current of the K pole as the voltage of the G pole changes. We also connect a voltmeter between K and A. In this way, we can clearly observe the changes in the output of the TL431 with the power supply.

Before the test, adjust the potentiometer to near the middle value. Then, use a digital meter to measure the K pole-to-ground voltage and adjust the voltage output of the maintenance power supply. At this time, it can be found that the voltage between the K pole and the ground has only two states: one is about 2V (low level); the other is equal to the power supply voltage (high level).

tl431 circuitFigure 3. TL431 Circuit

Then how to judge whether TL431 is normal?

For the on-line TL431 power supply error comparator, the external maintenance power supply can be used to detect. Connect the maintenance power supply to the sampling point of the TL431, and when the voltage is higher than the nominal voltage, the TLA31 will be turned on and the K-pole voltage will be low.

That is to say, when the power supply voltage increases, the TL431 is turned on, so that the diode of the photocoupler is turned on, so that the transistor is in a saturated state, and the turn-on time of the primary power switch is finally shortened (reducing the duty cycle). In this way, the output voltage is reduced. If the maintenance voltage is reduced, the TL431 will be cut off, the K pole voltage will be high, and the diode of the photocoupler will be cut off, which will make the Triode in the cut-off state, and finally control the increase of the turn-on time of the primary power switch of the transformer (increase the duty cycle). Increase the output voltage. The closed-loop voltage stabilizing circuit of the switching power supply uses the TL431 on or off two states to adjust the duty cycle of the switch to control the stability of the output voltage.

When measuring the multimeter, if the resistance between the poles of the lC is normal, the TL431 can be judged to be normal. When using the maintenance power supply power-on test, in the case of changing the power supply voltage, if the TL431 pole to the ground has two changes of high and low levels, the TL431 can be judged to be normal.

tl431Figure 4. TL431

V Replaceable Models

When TL431 is damaged, if there is no replacement of the same model, it can be directly replaced with KA431, μA431, LM431, YL431, S431, etc.

TL431 suffix letters indicate product level and operating temperature range.

C is commercial product (-10℃~+70℃);

I is an industrial product (-40℃~+85℃);

M is military product (-55℃~+125℃).

VI Conclusion

In summary, it is the introduction of how to test TL431 and its replacement.

TL431 has a wide range of applications. It can be used as a precision reference voltage source, and can also be used to replace a regulator tube to form a parallel adjustable regulated power supply. It can also be used as a constant current source and voltage detection circuit. In addition, in the switching power supply, TL431 can also be used as a simple error amplifier.


FAQ

  • What is the use of tl431?

The TL431 is a "programmable precision reference" and is commonly used in switching power supplies, where it provides feedback indicating if the output voltage is too high or too low. By using a special circuit called a bandgap, the TL431 provides a stable voltage reference across a wide temperature range.

  • What is tl431 transistor?

The TL431 is a regulator diode whose output voltage can be programmed by changing the value of resistors connected to it. It acts almost like a Zener diode except for that the voltage rating of this IC is programmable. It is commonly used to provide negative or positive voltage references.

  • How does a shunt regulator work?

The shunt regulator or shunt voltage regulator is a form of voltage regulator where the regulating element shunts the current to ground. The shunt regulator operates by maintaining a constant voltage across its terminals and it takes up the surplus current to maintain the voltage across the load.

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