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Three-terminal regulators are commonly used in instrument circuits. In this article, CHANGHUI INSTRUMENTS shares methods for testing three-terminal regulators, to help instrument technicians quickly identify the faults in these regulators during maintenance work. 1. Basic knowledge of three-terminal regulators. Commonly used three-terminal regulators in instrument circuits include the positive voltage supply 78×× series, the negative voltage supply 79×× series, three-terminal adjustable regulators, and precision voltage references. Among them, three-terminal voltage regulators are the most widely used; here, only the 78×× series will be introduced. The 78×× series has a maximum output current of 1.5A, and it features built-in current limiting protection, overheat protection, and overvoltage protection, ensuring stable and reliable operation. It has only three pins, making it very convenient to use. Figure 1 shows a typical application circuit for the 78×× series three-terminal voltage regulators. http://yunrun.com.cn/upload/201712/07/201712071115150543.png Figure 1: Typical application circuit for the 78×× series of three-terminal regulators. For example, the manual for the three-terminal regulator 7805 describes its features as follows: Features ◆ Output current exceeding 1A ◆ Internal thermal overload protection ◆ No external components required ◆ Protection for the output transistor ◆ Internal short-circuit current limit ◆ Available in aluminum TO-3 package Voltage Range ◆ LM7805C: 5V ◆ LM7812C: 12V ◆ LM7815C: 15V When selecting the three-terminal regulator 7805, technicians should pay attention to these ten technical specifications; the meaning of each of these specifications is not explained in this article. ①Output voltage and output current ②Linear regulation rate ③Load regulation rate ④Quiescent current ⑤Change in quiescent current ⑥Output noise voltage ⑦Output reactance ⑧Ripple suppression ratio ⑨Short-circuit protection current ⑩Minimum input voltage required to maintain output 2. Detection methods for three-terminal regulators: A three-terminal regulator can be evaluated by using a multimeter to measure the resistance values between its pins as well as to check its voltage regulation performance. ①Measure the resistance between the pins: Set the analog multimeter to the “R×1k” setting. Connect the black probe to the ground terminal of the voltage regulator, and use the red probe to touch the other two pins in turn to measure the forward resistance between those pins. Then, connect the red probe to the ground terminal and use the black probe to touch the other two pins in turn to measure the reverse resistance between them. If the forward resistance value between the pins is a fixed value, while the reverse resistance value is infinite, then the three-terminal regulator is functioning properly. If the forward and reverse resistance values between any two terminals are both very low or close to 0, it can be concluded that the internal components of the three-terminal regulator are damaged ； If the forward and reverse resistance values between any two terminals are both infinite, it indicates that the integrated voltage regulator is damaged due to an open circuit ； If the measured resistance value is unstable and changes with temperature, it indicates that the thermal stability of the integrated voltage regulator is poor. ②Measure the regulated voltage at the output terminal. Depending on the output voltage of the three-terminal regulator, set the multimeter to the appropriate DC voltage range, such as “10V” or “50V”. Then, a DC voltage is applied between terminal 1, the input terminal of the voltage regulator under test, and terminal 2, the ground terminal, and the output voltage at terminal 3 of the voltage regulator is measured. It is possible to determine whether the voltage regulator is functioning properly based on the output voltage value. This method is also applicable to online measurements, and it makes it easier to observe the voltage stabilization effect. The voltage applied to the input during testing should be 3V higher than the nominal output voltage, but it must not exceed the specified maximum input voltage. Source: http://yunrun.com.cn/tech/2546.html