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The working principles of a diode

PCB design is a difficult problem to solve without the right tools. The two main reasons are the inability. One is to view the circuit board design accurately and remove parts precisely. As a result, much of PCB design is stuck in trial and error on a process with no clear-cut solution.

All these problems are because diode detection circuits are analog from one device. Therefore, understanding how they work and putting them to work is the most fundamental element in effective PCB design today.

This means that there is no absolute method for designers to find the right type of diode circuit. Therefore, we must create a method by understanding some basic principles of applying diodes.

The first thing necessary in our process is a basic understanding of what a diode does.

What is a Diode?

Fleming discovered the diode. The story goes that he was trying to build a radio receiver. He was having trouble getting his amplifier to work. Also, the frequency produced did not sound right. He played around with the parts until he found a solution. It was that reversing the direction of current flow produced a sound. The sound was so sweet and different from anything else he had heard before. It must be something special.

So it is that Fleming discovered what we now know as the diode (or rectifier). He found that a certain kind of charge could flow in one direction in simple terms. The diode was a galvanic action.

Don’t get this wrong – some have tried to use the word “rectifier” incorrectly to mean something else. But it’s a case of “the devil, you know.” It wasn’t until many years later that we understood the physics behind the diode.

For now, let’s think of a diode as a special kind of resistor. It is a resistor that allows current flow in one direction but blocks it in the other. It doesn’t matter what kind of diode it is. They all work the same way, so they still come with the symbol.

So having said that, you might be wondering what good a diode could do. After all, there are more than enough resistors out there already, and they’re all pretty cheap.

The difference is in the way we produce them. A resistor consists of coiled metal wire around a ceramic form. We solder the wire with another wire to form a circuit (i.e., loop). When current flows through this circuit, it encounters more resistance. The resistance is less if it flowed in the opposite direction, just as Fleming found.

Vacuum tube diodes

Instead of a wire and a ceramic form, we have two-wire or metal foil loops. So, if you took one loop, rolled it up, and passed current through it, you could focus the charge onto one point. Then, if you place this point near a wire or loop, the current will move from the metal wire to the metal loop. This was an exciting discovery. It meant that you could gather current on the metal loop and pass along many wires at once.

So you can see that a diode has two loops of metal foil. You roll or place one in the other in a certain way. There are several ways to do this, and we’ll explain these as we go along.

Once you place the electron flow on the metal loop, it moves from one end to another. This meant that you could pass current from one end of the diode to another without any problem. It is similar to what an ordinary wire would allow.

Solid-state diodes

When the diode came to being, they made all the parts out of wires, loops, and tubes. When solid-state materials came into play, the same thing seemed to happen. The voltage was able to pass through one way but not the other.

In reality, a solid-state diode works quite differently from its earlier cousin. They are two semiconductor diodes joined together with metal. We make one of P-type material and the other out of N-type. The name has nothing to do with the type of metal. It is short for “p” and “n,” respectively, which are the elements that make up these diodes.

We call them this because they are a p-n junction or semiconductor diode.

The bottom line is that we can make a diode from various materials. But specific shapes and forms give them their unique properties.


The word diode comes from the Greek for ‘two.’ Of course, this is a bit of an overstatement since a diode only has one loop at the most. There are no two loops. But it made sense at the time, and it’s still used today in describing the essential parts of a diode.

Today we have many kinds of diodes. But unfortunately, they have many different names.

That’s because each name describes a specific thing, depending on how we make it. So, we will first look at the most common ones. Then we look at similar but still very different from one another.

How does a diode work

Diodes are one of the essential components of electronic circuitry. We find them in almost every electronic device. They control current flow through specific circuits. Diodes may also convert direct current into alternating current. They also control the flow of electricity in electric motors and dynamos.

Parts of a circuit

The diode is an electrical component consisting of a p-n junction semiconductor material. They have relatively high resistance to the flow of electricity. We mark the diode with an arrow pointing to the current flows.

The action of the diode affects the load or circuit. The load, in our case, consists of a light bulb. The diode allows electricity to flow from the positive side to the negative side. This happens when you connect it in series with a load that can handle this electrical load.

The diode is a two-element semiconductor device consisting of an anode and a cathode. The diode conducts electricity in the direction of the arrow. However, it will not allow the current to pass in the direction of the arrow’s bend.

The anode is a positively charged terminal that produces a low resistance path between it and the cathode. The negative charge on the cathode spreads evenly around it. This allows electricity to pass easily through this region. It then produces a low resistance path between it and the anode.

In the U-shaped part of the symbol, there is a negative charge. As current flows through this region, it encounters an extended area. There is a lower resistance than if it were to flow in the direction of the arrow. It creates a low resistance path between these two regions. With an anode and cathode on either side, electricity can flow in both directions.

Diodes do not allow electricity to pass through them easily. Instead, electricity must push through the semiconductor material. The material forms the diode until it moves across the other side. As a result, electricity will move from low resistance to high resistance until it balances. We call this the voltage drop.

N-Type Silicon

The N-type diode is more commonly available, as it is easier and cheaper to produce. N-type silicon diodes are also very common in most electronic devices.

An N-type diode has a positive plate (anode) and a negative cathode. In an NPN transistor, we connect the base of the transistor to the anode of the diode. Current can then flow from the positive plate to the negative side, but not in the other direction.

The N-type diode is also known as an anode diode. It is a semiconductor device that conducts electricity in one direction only. As with any semiconductor device, electrons will flow with enough voltage to push them through the material. In this case, it is silicon.

We make the N-type diode from silicon, adding impurities in a gas or metal. The positive side of the diode (anode) produces a low resistance path for the current to flow.

NPN transistors are more common. PNP transistors are essential in applications that need rectification.

P-Type Silicon

Trivalent impurities, such as phosphorous, aluminum, and titanium, are more common. We add them to silicon. These impurities will increase the number of charged carriers in the semiconductor material. As a result, current will flow from the negative side to the positive side, instead of vice versa.

The P-type semiconductor is a cathode diode. We also call it an electron valve or rectifier diode. These diodes have a positive plate (anode) and a negative cathode.

We make the P-type diode from silicon that contains impurities in gas or metal.

When adding boron to Si, the impurities move through the silicon more readily. This creates a positive charge on the slice of silicon. This encourages electrons to flow in this direction. It also produces a low resistance path between the slice of silicon, anode, and cathode.

Electrons can move easily through this region when we apply a voltage across the silicon-boron-silicon slice. However, the voltage drops in value when the electrons reach the cathode. As a result, the current is insufficient to flow in either direction. Instead, it produces a low resistance path between it and the anode.

The P-type diode is more commonly available, as it is easier and cheaper to produce.

We manufacture both types of silicon diodes to have “N” or “P” symbols for identification purposes.

Typical diode characteristic

1. Small voltage drop across the diode: usually from 0.5V to 1.5V across the silicon-silicon junction of the diode

2. Large current: usually from 100µA to 2mA across the silicon-silicon junction of the diode

3. Low resistance: typically less than 10 ohms

4. Selectively high impedance in PNPN circuit, where Γ is very high (typically 100 Megohm) when the temperature is high and low, Γ may be as low as 0.5 ohm

5. Special characteristic: low ohmic losses on the reverse power supply

A diode can rectify a voltage in the same way as a rectifier valve. But it has an inverted arrow to show that it supplies the output in the direction of the arrow. It can also isolate negative feedback circuits from positive feedback. They are present in amplifiers. For example, a positive voltage applied to one plate of a PNPN diode causes the other plate to become negative. This is useful in circuits where positive feedback is present. A good example is the amplifier circuits.

The symbol for a diode shows the cathode or the “K” symbol. A line connects it to the anode or “A” symbol, but not that the arrow bends.

The Role of Diodes

Diodes are helpful in almost every electronic project. Diodes allow electricity to flow from the P side to the N side but not from the N side to the P side. When we place a diode in series with the load when powering a circuit, the diode allows the current to flow. It flows through while preventing the backflow of current. This allows a single power source to supply power to a circuit. However, it stops the reverse flow of electricity if there is an interruption of power.

1. Rectification

Diodes are helpful in AC-to-DC converters, power supplies, and rectifiers. They convert alternating current to direct current. The current direction always changes when electricity is alternating current. Soo diodes can maintain a constant current when the voltage changes. In addition, diodes have the property of only allowing electricity to flow in one direction. So, if the AC flows in one direction through a load, the diode prevents electricity from flowing in the other direction. This is what we call rectification.

2. Radio wave detection

Diodes can detect radio signals. Rayming PCB & Assembly place the diode in series with a receiver circuit and tuned it to the frequency we would like to receive. When the antenna receives a radio wave, it passes through the diode. This allows the current to flow only in one direction. This current then creates a voltage across the whole receiver circuit. The receiver can change this voltage when sound waves are present. This is what we call detection.

3. Voltage control

Diodes are often helpful in controlling voltage in circuits. For example, we connect a high voltage battery to a circuit. If you flick the circuit’s switch on or off several times, it will charge and discharge the battery. We call this ripple conditioning the voltage in the circuit. Diodes used in this process are bistable diodes. We feed this diode from the battery through a resistor and a diode bridge. The diode that allows current flow only in one direction is an NPN or PNPN (for positive-negative). We use the PNPN or NPN as an amplifier. This is because we can make its gain high by using a transistor. We amplify the voltage across its collector-emitter junction.

4. Current conversion

Diodes can convert the current from one form into another. For example, we place diodes in series with the input and output of a DC-to-DC converter. They convert a larger current to a small current. Diodes convert the current from AC to DC with a rectifier circuit. When we apply no voltage from the outside, it charges a capacitor by AC flowing from the AC source to the capacitor and the ground. When the voltage across the capacitor reaches a critical value, a diode turns on. The current cannot pass between them because the diode is reverse biased at this time. This generates a negative pulse at the point when it starts conduction.

Types of Diodes

We categorize diodes depending on their function in a circuit:

1. Zener diode

Zener diodes can control voltage in a circuit. We place it in series with the load and connect its anode to the circuit’s negative terminal. A Zener diode has two terminals called a cathode and an anode. It acts as a short circuit when the applied voltage exceeds its forward voltage drop. A diode used to apply voltage to a circuit like an LED is a regulator diode.

2. Light-emitting diode

The light-emitting diode (LED) converts current into an optical signal. It has two functions in one package:

a. An LED can produce light when forward current flows through it b. Its reverse resistance is very high

A typical application of this phenomenon of LEDs is in digital watches. LEDs are helpful as digital displays due to their low power consumption and long life span. We call a diode in which the junction emits light a laser diode or light-emitting diode.

3. Schottky diode

This special diode prevents corrosion when placed in an electrolytic solution. A good example is an electrolytic capacitor’s dielectric material. A Schottky is typically used to keep the dielectric capacitance constant. A good example is radio circuits, where the capacitive load varies due to varying power supply voltage and frequency. This is a diode made by bonding a metal to a semiconductor. The metal collects the charge carriers when they try to flow back across the diode junction. As a result, the metal becomes depleted of charge carriers. This is because it does not have impurities. So it cannot have electrons flowing through it like a normal diode structure.

4. Esaki Diode

An Esaki diode is a tunnel diode invented in the 1960s by Leo Esaki. It allows current to flow forward and backward between two points. For example, from cathode to anode and from anode to cathode. The Esaki diode allows current flow due to the quantum tunneling effect. It is also known as a split junction diode. The Esaki diode’s function is similar to that of the Zener diodes. They permit charges to flow in only one direction and are commonly used to regulate voltage.

5. Switching Diode

If the diode cannot withstand high voltages and currents, a switching diode may switch current between two points. A switching diode is a special kind of diode used to interrupt currents without damaging the devices it protects. These diodes are helpful in circuits that control electric motors and AC-to-DC converters.

6. Germanium diodes

Germanium diodes are helpful in high-voltage applications and rectifier applications. In very high voltage applications, germanium diodes are essential as rectifiers. Germanium diodes emit light when forward current flows through them. So they are often used to indicate that the voltage is too high. We often use them for detecting waveforms such as radio signals and the voltage of an oscillator.

7. Silicon diodes

Silicon diodes can convert AC into DC. They are a source of current that is not conducted instantaneously from one point to another. However, it flows at an actual current rate for a short time. Silicon diodes are helpful to rectify high-voltage AC currents and sense AC voltages. They also perform frequency conversions in radio circuits.

8. Tunnel Diode

The tunnel diode allows current flow from anode to cathode even with a small forward voltage drop. The forward current of a tunnel diode increases linearly with the increasing voltage.

How To Test a Diode

There are many ways to test a diode. One of the simplest tests is to measure its current-carrying capability. Using the resistance scale, we measure with a multimeter. We set it to measure ohms’ resistance using the “resistance” scale. Be sure to place the black lead on the anode and the red lead on the cathode for this test.

If you have a circuit schematic, consult it to attach the two leads for the correct wiring.


Start your meter by pressing the “Ohms” button and setting it to measure resistance in ohms. Next, measure both sides of the diode using one lead at 160 VAC and 1000 µA. It is just over 10 volts and ten milliamps. Also, we may test the diode with a light bulb. We call this the “flash test.” The light bulb should flash brightly when you connect the diode in series. Next, you should connect the anode to your multimeter’s positive terminal. Then connect the cathode (negative side) to your multimeter’s negative terminal.

Another test of a diode is to measure the voltage drop. Again, we should use the multimeter’s Volts scale to calculate the voltage drop between the diode’s anode and cathode. Now attach the test leads at different points on the two terminals, or you can use separate leads. Next, measure your multimeter’s volts scale at various points between the two terminals. It ranges from 0 VAC to +25 VAC and -25 VAC to +25 VAC with steps of 10 volts.

Between the points, note the number of volts dropped across the diode. With your multimeter’s “diode” or “Volts” mode selected, also measure between one of the test leads and each terminal individually. The voltage measured here should be zero if your meter has the correct wiring. If it is not zero, you have either read a false zero from your meter or have not applied the correct polarity to your meter. Suppose you have applied the wrong polarity, which is unlikely, disconnect your leads and reseat them with the correct polarity. If you can read a zero at some points between 0 and +25 VAC but not others, you made a mistake in your wire connections or applying the test leads.


In conclusion, diodes are electrical components whose only function is to allow current flow in one direction. We use them in many electronic circuits, from blocking voltage spikes to turning lights on and off. In this way, diodes are helpful in microprocessors and other computer chips, such as those that control our television sets. By knowing how they work and the different types of diodes, we can use them more effectively in our electronics designs.




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