How to Test Diode Polarity and Condition with a Multimeter
① Zener diode.
A voltage regulator diode is a type of diode that operates in the reverse breakdown region and has the function of stabilizing voltage. The measurement of its polarity and performance is similar to that of ordinary diodes, but the difference is that when using the Rxlk mode of a multimeter to measure a diode, its reverse resistance is measured to be very high. At this time, when switching the multimeter to the Rx10k mode, if the multimeter pointer deviates significantly to the right, that is, the reverse resistance value decreases significantly, then the diode is a voltage regulator diode; If the reverse resistance remains basically unchanged, it indicates that the diode is a regular diode, not a voltage regulator diode. The measurement principle of a voltage regulator diode is that the internal battery voltage in the Rxlk range of the multimeter is relatively low, which usually does not cause breakdown of ordinary diodes and voltage regulators, so the measured reverse resistance is very high. When the multimeter is switched to Rx10k mode, the battery voltage inside the multimeter becomes very high, causing the voltage regulator diode to experience reverse breakdown, resulting in a significant decrease in its reverse resistance. As the reverse breakdown voltage of ordinary diodes is much higher than that of voltage regulators, ordinary diodes do not break through and their reverse resistance remains high.
② Light Emitting Diode (LED).
Light emitting diode is a special type of diode that converts electrical energy into light energy. It is a new type of cold light source commonly used in devices for level indication, analog display, and other applications. It is often made of compound semiconductors such as arsenide and phosphide. The emission color of light-emitting diodes mainly depends on the material of the semiconductor used, and can emit four types of visible light: red, orange, yellow, green, etc. The casing of a light-emitting diode is transparent, and the color of the casing indicates its emission color. Light emitting diodes operate in the forward region, and their forward conduction (turn-on) operating voltage is higher than that of ordinary diodes. The larger the applied forward voltage, the brighter the LED will emit light. However, it should be noted during use that the applied forward voltage should not exceed the maximum operating current of the LED to avoid burning out the tube. The detection method for light-emitting diodes mainly uses the Rx10k range of a multimeter, and its measurement method and performance judgment are the same as those of ordinary diodes. But the forward and reverse resistance of light-emitting diodes is much larger than that of ordinary diodes. When measuring the forward resistance of a light-emitting diode, a slight light emission phenomenon can be observed.
③ Photodiode.
Photodiode, also known as photodiode, is a special type of diode that converts light energy into electrical energy. Its casing has a window embedded with glass for easy reception of light. The photodiode operates in the reverse working region. When there is no light, photodiodes, like ordinary diodes, have a very small reverse current (generally less than 0.1 uA) and a high reverse resistance of phototubes (over tens of megaohms); When illuminated, the reverse current increases significantly and the reverse resistance decreases significantly (from several thousand ohms to tens of thousands of ohms), that is, the reverse current (known as photocurrent) is proportional to the illumination. Photodiodes can be used for measuring light and can serve as an energy source (photovoltaic cell). It is widely used as a component in optoelectronic control systems. The detection method of photodiodes is basically the same as that of ordinary diodes. The difference is that there is a significant difference in reverse resistance between illuminated and unlit conditions. If the measurement results are not significantly different, it indicates that the photodiode is damaged or not a light-emitting diode.
