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What Is a Diode Circuit?

Diodes.
Light emitting diodes act as semiconductors and lights.
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  • Written By: Andrew Burger
  • Edited By: R. Halprin
  • Last Modified Date: 19 November 2014
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A diode circuit is any of a variety of electrical circuits that take advantage of the distinguishing characteristics of diodes. A class of crystalline semiconductors with two terminals, diodes exhibit a strong bias toward carrying an electrical charge "forward" in one direction while all but completely inhibiting it in the other. Diode circuits are commonly used in power supply applications to convert alternating current (AC) to direct current (DC) and to tune TV and radio receivers. They are also used as analog and digital logic switches, as capacitors to temporarily store and increase electrical charge, in surge protectors to prevent voltage spikes from damaging equipment, and as sensors to detect light and to produce light. Besides rectifier diodes, other common types include light emitting diodes (LEDs), varicap diodes, and Zener diodes.

Diodes were the first semiconductor electronic devices to be invented. Used widely in the electronics industry, they are usually made of silicon, though germanium is used as well. The electrical resistance of a diode circuit is minimal in the forward direction, from the anode to the cathode, hence the term "forward bias." Silicon diodes, for instance, have a 0.6-0.7 volt voltage drop, the threshold point, when carrying current in the forward direction. A relatively high minimum voltage must be reached for current to flow through a diode in the reverse direction. It is these properties that make diode circuits very useful in a wide variety of electronic devices.

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In a diode circuit, a diode may be connected to any of a wide variety of other electrical or electronic devices — capacitors, resistors, transformers, power supplies, etc. — depending on the application. Diodes in circuits may be arranged in series or in parallel. An initial application of a diode circuit, one still in widespread use today, is the switching of analog signals. In the early days of digital computing, diode circuits were used to perform the digital logic operations AND and OR.

Of the many different types of diodes used in circuits, LEDs produce light of visible and non-visible frequencies when current passes between the electrodes. Varicap, or varactor, diodes are used to tune radio and TV receivers. Another type, the photodiode, detects light. They typically operate in reverse bias and are used to generate electricity and in solar photovoltaic cells. Zener diodes also operate in reverse bias and are used widely in power supplies to regulate voltage by producing a stable reference voltage.

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David09
Post 3

@hamje32 - That’s a decent analogy but it’s not perfect. If I understand the article correctly, it’s not that voltage can’t under any circumstances flow in the reverse direction. It’s that it has to be very high to do so.

Apparently there is some threshold you have to reach for that to happen. At any rate it’s been a long time since I looked at circuit diagrams.

One thing I remember is that the symbols for the diodes used arrows to represent the direction of the current flow. Clearly you need to know which way the current is flowing if you’re going to use them effectively in your application.

hamje32
Post 2

@everetra - It seems like the laser diode circuit acts like a check valve in one sense. A check valve is a mechanical device to allow water to flow in one direction only.

The valve closes so that water cannot flow back into the same direction. In the same way, the LED circuit has a “forward bias.” Current will flow in one direction and not the other.

I can see how this would be useful as a safety protection mechanism. By preventing circuits from flowing the opposite direction you can prevent voltage kickbacks, so to speak. I don’t know if that’s a technical term but that’s how I visualize it.

everetra
Post 1

I remember the first time I saw an LED diode circuit in operation. It looked like a string of Christmas lights, except that the LED bulbs had a more permanent quality to the lights, as if they could just stay on forever. The LED didn’t flicker on and off.

This was in the early days of LED electronics many years ago. Years later, LED technology has pretty much become a staple in so many applications. They have LED televisions nowadays which supposedly deliver the ultimate in color and picture quality, each LED light representing a pixel on the screen.

So it’s a lot better than LCD and even plasma technology. Of course the technology keeps changing. Soon LEDS will become passé in television too.

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