Frequently Asked Questions about Diodes and LEDs is another in a series of FAQS about electricity and electronics that are oriented toward Appliance Technicians and HVAC Technicians.
Frequently Asked Questions about Diodes and LEDs
Q. What is a Diode?
A. An electronic component that passes current in only one direction.
Q. What are the two electrical states of a Diode?
A. Forward bias (conducting), and reverse bias (non conducting).
Q. What is a Diode useful for?
A. One of the most common uses for a diode is for rectification – the process of converting AC voltage into DC voltage.
Q. How does a Diode work?
A. When a diode is “forward biased”, or conducting, current passes through it. Due to the nature of its construction, however, there is a voltage drop across it. The most common type of diode (the silicon diode) has an approximately 0.6v drop across it when it is forward biased.
Q. What happens when you connect a Diode backwards?
A. It is reverse biased, and no current flows through it.
Q. Does a Diode also have a reverse voltage drop?
A. Yes. Generally, no current flows through a diode when it is reverse biased. However, there is a limit to how much voltage can appear across a diode when reverse biased, per the specification. Once that limit is reached it starts conducting. A typical reverse voltage drop for a silicon diode is between 50V – 1000V. The reverse voltage drop is exploited in special diodes called the Zener, and mentioned in a subsequent FAQ.
Q. How do you test standard Diodes?
A. You need a tester that outputs a voltage that is in excess of its forward voltage drop. Most multimeters have a “diode-check” function that accomplishes this. It will give you a voltage drop reading when the diode is connected in such a way that it is forward-biased. The black lead connects to the pointed or striped side of the diode in forward-bias mode. When the leads are reversed, you should get an “infinity” reading on the meter since the maximum voltage such a diode check mode will read is about 3 volts. More info on how to check diodes at: https://techcircuit.org/semiconductor-testing-cheat-sheet/
Q. How do you connect a Diode?
A. The arrow in the diode symbol points in the direction of conventional current flow when forward biased. Conventional current flow travels from positive to negative.
Q. What are other types of Diodes?
A. Some other common types of diodes are Schottky, Zener, microwave oven diode, and light emitting diodes (LEDs).
Q. What is a Schottky Diode?
A. It is a special type of diode that has a low forward voltage drop (about 0.2v, instead of 0.6v), and a fast switching action. This makes it ideal for power supply rectification so that less voltage is lost, as well as for high-frequency switching power supplies because of its high-speed. A Shottky diode is tested in the same manner as a standard diode, but will yield a much lower forward voltage drop as mentioned.
Q. What is a Zener Diode?
A. A Zener diode acts like a regular diode, but has a very low reverse voltage drop. This voltage drop is by design, set at a certain voltage (like 5v, 12v, etc). They are connected in reverse, and are used as voltage references and regulation in power supplies, to name only a few applications.
Q. How do you test a Zener Diode?
A. The forward voltage drop of a Zener diode can be tested with the a regular diode-check function of a multimeter. However, the reverse voltage drop must be measured with a current limited (with series resistor, for example) voltage source that exceeds its reverse voltage drop value. For example, a 9v battery in series with a 10k resistor (along with a voltmeter) would be suitable for measuring the reverse voltage drop of any Zener diode rated for about 8v or less.
Q. What is a Microwave oven Diode?
A. It works like a regular diode in that it passes current in only one direction, but has an extremely high reverse voltage drop. Where a typical reverse voltage drop of a silicon diode is between 50v – 1000v, a microwave diode has a reverse voltage of greater than 12,000v. This is achieved by stacking multiple diodes in series in a single package. This is done at the expense of a greater forward voltage drop as well. The typical forward voltage drop of a microwave oven diode is between 6-12v. A microwave oven diode is used, in conjunction with an HV capacitor, in the “voltage doubler” stage of a microwave oven in which the output of the high-voltage transformer is both doubled, and rectified for use by the magnetron.
Q. How do you test a Microwave oven Diode?
A. A microwave diode cannot be tested with the “diode-check” function of a typical multimeter because the forward voltage drop exceeds the test voltage. The forward voltage drop must be measured with a current limited (with series resistor, for example) voltage source that exceeds its forward voltage drop rating (between 6v – 12v). Microwave oven diodes almost never fail as “open”, however. They will generally fail as a short circuit, or leaky (resistance readings in both directions).
Q. What is a light-emitting-diode (LED)?
A. A special class of diode that emits light when current passed through it in the forward-bias state.
Q. How do LEDs differ from typical silicon Diodes?
A. Aside from emitting photons of light when forward-biased, they typically have a higher forward voltage drop. Typical voltage drops for LEDs are 1.5v for standard indicator LEDs and 3v for high-powered LEDs that are used for lighting.
Q. How do you test an LED?
A. For standard LEDs, the “diode-check” function on a multimeter works because its current-limited test voltage exceeds that of their foward-bias condition. However, for high-powered LEDs, many of them have a forward voltage drop that exceeds the diode-check’s test voltage. Some of them can be tested with the diode-check function, and some cannot. For the ones that can’t, a current limited voltage source powered by a 9v battery and a 2.2k ohm resistor will suffice. However, care must be taken when reverse-biasing LEDs. Connecting LEDs backwards can easily damage them.
Q. How do I learn more about testing Diodes?
A. Here is a full article that shows the forward and reverse voltage drops of typical diodes: https://techcircuit.org/how-to-test-leds-zeners-and-microwave-diodes/
A great multimeter for testing diodes is the Klein CL800, it has a test-voltage of 3.0v, which is high enough to test most diodes and many LEDs. Purchasing the Klein CL800 from our link below helps support our goal to further educate Appliance and HVAC technicians in the area of electricity and electronics.
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