Introduction and Basic Summary Diodes

The first diodes to applied in electronics were cat’s whisker detectors used in crystal radios. These comprised a crystal of semiconductor material. The cat’s whisker is simply a bare wire that held in a flexible bracket that touches the semiconductor crystal. By carefully moving the whisker around, at certain points of contact the arrangement would act as a diode, only allowing current to flow in one direction. This property required in a simple radio receiver to detect the radio signal so that it can be heard.


≡ Different types of diodes and their uses


Shows various types of diodes. Commonly, the bigger the diode package, the greater its power-handling capability. Most diodes are a black plastic cylinder with a line at one end that points to the cathode.

The diode on the left an SMD. The through-hole diodes to the right get larger, the higher the current rating.

There are various different types of diodes. Unlike resistors that are bought as particular value resistor diodes are recognized by the manufacturer’s part number.

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The forward voltage, usually abbreviated as Vf, is the voltage across the diode at forward-biased. The DC-blocking voltage is the reverse-biased voltage that if exceeded may destroy the diode. The recovery time of a diode transfers to how quickly the diode can switch from forward-biased conducting to reverse-biased blocking. This is not instantaneous in any diode and in some applications fast switching is required. The 1N5819 diode is named a Schottky diode. These types of diodes have much lower forward voltage and generate less heat.


≡ Current to flow one way


A diode is a component that only provides current to flow through it in one way. It’s a kind of one-way valve if you want to think of it in terms of water running through pipes. In reality, the diode offers very low resistance in one direction and very high resistance in the other. In other words, the one-way valve restricts the flow a tiny bit when open and also leaks slightly when closed. But most of the time, thinking of a diode as a one-way valve for electrical current jobs just fine. There are lots of specific types of diodes, but let’s start with the most common and basic diode, the rectifier diode

Forward-biased Diode
Forward-biased Diode

In this case, the diode allows the flow of current and is considered forward-biased. The two leads of the diode are called the anode (abbreviated as “a”) and the cathode (abbreviated rather confusingly as “k”).

For a diode to be forward-biased, the anode requires to be at a higher voltage than the cathode,

One interesting section of a forward-biased diode is that unlike a resistor, the voltage across it does not vary in proportion to the current flowing through it. Instead, the voltage continues almost constant, no matter how much current flows through it. This varies depending on the example of the diode but is generally about 0.5V.

In the case of we can calculate that the current flowing through the resistor will be: This is only 0.5mA less than if the diode were to return with wire.


≡ Use a Diode to Restrict DC Voltages


Use a Zener diode. If forward-biased, Zener diodes work just like regular diodes and conduct. At low voltages, during reverse-biased, they have high resistance just like standard diodes. But, during the reverse-biased voltage exceeds a certain level, the diodes quickly conduct as if they were forward-biased.

In this fact, normal diodes do the same thing as Zener diodes but at a high voltage and not a voltage that is fully controlled. The difference with a Zener diode is that the diode is carefully designed for this breakdown to occur at a certain voltage (say, 5V) and for the Zener diode to be safe by such a “breakdown.”

Using a Zener Diode to Provide a Reference Voltage
Using a Zener Diode to Provide a Reference Voltage

Zener diodes are useful for giving a reference voltage. Note the slightly different element symbol for a Zener diode with the lit le arms on the cathode.

The resistor R limits the current flowing into the Zener diode. This current is always assumed much larger than the current flowing into a load across the diode.

This circuit is only well agreed to giving a voltage reference. A voltage reference gives a stable voltage but with only any load current. for example when the circuit is used with a transistor as in a resistor value of, say, 1kΩ would, if Vin was 12V, provide a current of The output voltage will remain about 5V whatever Vin is as long as it’s bigger than 5V. To know how this happens, imagine that the voltage across the Zener diode is less than its 5V breakdown voltage.

The Zener’s resistance will, therefore, high and so the voltage across it due to the voltage divider effect of R and the Zener will higher than the breakdown voltage. But wait, since the breakdown voltage is passed it will conduct, bringing the Vout down to 5V. If it falls below that, the diode will turn off and the Vout will grow again. Zener diodes are also utilized to protect sensitive electronics from high-voltage spikes due to static discharge or incorrectly connected equipment.

Protecting Inputs from Over-Voltage
Protecting Inputs from Over-Voltage

Shows how input to an amplifier not required to exceed ±10V can be protected from both high positive and negative voltages. If the input voltage is inside the allowed range the Zener will high resistance and not interfere with the input signal, but as soon as the voltage is passed in either direction the Zener will handle the excess voltage to ground.


≡ Powering an LED


LEDs are similar regular diodes in that when reverse-biased they block the flow of current, but when forward-biased they emit light. The forward voltage of an LED is more than the usual 0.5V of a rectifier and depends on the color of the LED. Usually, a standard red LED will have a forward voltage of about 1.6V

Powering an LED
Powering an LED

shows an LED in series with a resistor. The resistor is necessary to limit
too much current from flowing through the LED and damaging it. An LED worked as an illuminator will usually emit some light at 1mA but usually needs about 20mA for optimum brightness. The LED’s datasheet will tell you its optimal and maximum forward currents. As an example, if in Figure 6-5 the voltage source is a 9V battery and the LED has a forward voltage of 1.6V, you can measure the resistor value needed using Ohm’s Law:

R=VI=91.620=370Ω

370Ω is not a regular resistor value  so you could pick a 360Ω resistor, in which case the current would be:

I=VR=91.6370=20 . 6mA

≡ Reading of the light level


Use a photodiode. A photodiode is a diode that is sensitive to light. A photodiode normally has a transparent window, but photodiodes designed for infrared use have a black plastic case. The black plastic case is transparent to IR and usefully stops the photodiode from being sensitive to visible light. Photodiodes can treat as small tiny photovoltaic solar cells. When illuminated they generate a little current.

Photodiode in Photovoltaic Mode
The photodiode in Photovoltaic Mode

Shows how you can work a photodiode with a resistor to create a small voltage that you could then use in your circuits.

In this circuit, the voltage while explained brightly might only 100mV. The resistor needed so that the small current from the photodiode converted into a voltage (V=IR). Unless any voltage that you measure will depend on the resistance of whatever is measuring the voltage. So, for example, a multimeter with an input impedance of 10MΩ would provide a completely different (and lower) reading than a multimeter with a 100MΩ input impedance.

R1 makes the voltage consistent. The impedance of whatever connected to the output requires much higher in value than R1. If this is an op-amp then the input impedance is likely to be hundreds of MΩ and so will not alter the output voltage appreciably. The shorter you make R1, the lower the output voltage will be, so it’s a matter of striking a balance.

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