Understanding and Utilizing Diodes in Printed Circuit Boards

Are you wondering what are PCB Diodes or how do they function? Learning about these small yet important components can be quite daunting. Be it difficulty choosing the best diode for your design or problems with the existing ones, you’re not alone.

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We’ll take a closer look at everything you need to understand regarding PCB diodes, from their primary functions down to choosing the perfect type for your needs. We will also discuss test equipment and common problems associated with them and how to troubleshoot.

So, let’s get started.

A diode is a fundamental electronic component with two terminals, designed to allow current to flow in one direction only. This current conduction characteristic, though unidirectional, makes the component important in controlling the direction of the current conduction in many different electronic circuits.

Diodes help to prevent damage to the other circuit components, allowing conduction in the forward direction of current and resisting conduction of the reverse current.

PCB diodes are specifically made for mounting on printed circuit boards so that the diodes can come in through-hole packages suitable for SMT applications as well.

The integration of diode directly into PCBs in this manner makes direct and small, practical circuits electronic. Therefore making them versatile for use in a wide range of equipment, inclusive of power supplies and signal processing.

Rectifier diodes are built to only allow current in one direction; hence, they are essential in converting alternating current, AC, to direct current, DC.

The diodes often find applicability when there is a need to supply power, converting AC from the power outlet into DC.

Rectifier diodes block the backflow of current, therefore protecting a circuit from any damage that could have been realized due to reverse voltage.

Zener diodes are unique in their ability to regulate voltage. They allow current to flow in the reverse direction when a specific voltage threshold, known as the Zener breakdown voltage, is reached.

They are, hence, effectively utilized in voltage control applications found in circuits. A Zener diode is further seen in applications where a consistent voltage delivered is necessarily consistent, such as power supply and voltage reference circuits.

The diode is forward biased when the p-side connects to the positive terminal of a power source and the n-side connects to the negative terminal.

The external voltage reduces the width of the depletion region; hence as a result, the diode allows current to flow through itself.

The forward voltage drop is typically 0.7V for silicon devices and is the energy that must be spent to exceed the barrier.

When the positive terminal is connected to the n-side and the negative terminal to the p-side, the diode is reverse biased.

The external voltage causes the depletion region to increase in width, thus blocking the conduction band of current.

As a result of minority carriers, there might be a small leakage current flowing, although this contribution usually remains relatively small.

If the reverse bias voltage exceeds a certain threshold, known as the breakdown voltage, the diode will conduct in reverse.

This is potentially destructive unless the diode is designed for such conditions as the Zener diode.

So that’s how PCB Diodes work. Now let’s learn how you can choose the right diode for your PCB design.

First, determine what you would like your diode to do in your circuit. Diodes can do various things: rectification, regulating voltage, protection, and switching.

As a simple illustration of this, power supplies incorporate rectifier diodes for converting AC to DC. Zener diodes are used to stabilize the voltage. Protection diodes such as TVS diodes will ensure the safety of your circuit under abnormal voltage conditions.

Schottky diodes are excellent for high-speed switching, and LEDs are used for lamps and displays. Knowing its function helps you in selecting the right kind of diode.

Use a diode with a voltage rating higher than the highest it will encounter in your circuit. This way, the diode does not break down during a voltage spike.

Since the peak value of your AC voltage is 50V, use a diode with a reverse voltage rating of at least 75V to ensure the diode will not experience this voltage corona and break.

Choose a diode capable of allowing the current flow in the circuit without being overheated. The current rating is greater than the maximum expected current.

For a diode, with a peak current in your circuit rated at 2A, choose a diode that is rated at 3A or higher. This way, your diode has a specified rating high enough so that it doesn’t overheat, causing the diode.

Make sure that the diode has enough power dissipation for its working environment. To find the power dissipation, multiply the forward voltage drop by the current.

If a diode has a forward voltage drop of 0.7V and the current is 2A, it dissipates 1.4W. One would need to ensure that the diode is capable of the excess heat, whether from its self-operating heat sinks or additional cooling means.

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Consider how you envision your diode being mounted on your PCB. Diodes are packaged in one of two ways: through-hole (THD) or surface-mount (SMD). Of these two, through-hole diodes are more accessible to replace. But they typically consume more space on the board.

The most important application of surface-mount diodes is to facilitate a compact and automated PCBA. Select package type either for your PCB layout or for your manufacturing process.

It is always necessary to balance how desirable the features and performance of a diode are against the consideration of how much they cost. High-performance diodes will be expensive.

Look at what your circuit requires of the diode, then select a diode that meets those requirements without overspending.

For example, if your design doesn’t require high-speed switching, then you can get a rectifier, but it is cheaper than a Schottky diode. This way, you choose the cheapest diode that will give you good value based on the diode application.

The Zener diode is used in voltage regulation circuits so that a constant voltage can be maintained at the output over some permissible range of output voltage. In this way, it allows conduction in the reverse way when the applied voltage lies above a determined value.

Thus, they ensure voltage maintenance at the same level for different loads.

A diode is a semiconductor device, typically made of silicon, that essentially acts as a one-way switch for current. It allows current to flow easily in one direction but severely restricts current from flowing in the opposite direction.

Diodes are also known as rectifiers because they change alternating current (AC) into pulsating direct current (DC). Diodes are rated according to their type, voltage, and current capacity.

Diodes are available in various configurations. From left: metal case, stud mount, plastic case with band, plastic case with chamfer, glass case.

What Does a Diode Do?

Diodes have polarity, determined by an anode (positive lead) and cathode (negative lead). Most diodes allow current to flow only when positive voltage is applied to the anode.

When a diode allows current flow, it is forward-biased. When a diode is reverse-biased, it acts as an insulator and does not permit current to flow.

Strange but true: The diode symbol's arrow points against the direction of electron flow. The reason is that engineers conceived the symbol, and their schematics show current flowing from the positive (+) side of the voltage source to the negative (-). It's the same convention used for semiconductor symbols that include arrows — the arrow points in the permitted direction of "conventional" flow, and against the permitted direction of electron flow.

Types of Diodes

Different diode types all perform key functions in an electrical system. Here are some of the most important types of diodes.

• Rectifier Diodes: A rectifier diode converts AC into DC. Direct current flows in one consistent direction, making it easier to control.
• Zener Diodes: Zener diodes are designed to protect electrical systems from overvoltage. They conduct current in reverse whenever the cathode reaches a predetermined threshold voltage.
• Light Emitting Diodes: A light-emitting diode (LED) emits light when forward-biased current flows through it. LEDs are widely used in displays and indicators.
• Schottky Diodes: Schottky diodes, also known as barrier diodes or hot-carrier diodes, are often used in high-speed applications because of their fast-switching capabilities and their low forward voltage drop.
• Photodiodes: Photodiodes produce electrical currents when they absorb photons. They are widely used in sensors and in solar cells.

Diode Ratings and Testing

Every diode has a current and voltage rating, determined by factors like materials and design. Those ratings represent the maximum levels of voltage and currency the diode can tolerate.

Exceeding currency and voltage levels can cause permanent damage to the diode or to the whole circuit. High voltage levels may also cause the diode to short circuit and either allow current to flow in both directions or halt current from flowing in either direction.

Proper diode function can be tested with a digital multimeter (DMM).

Testing Diodes with a Digital Multimeter

There are two methods of testing diodes using a digital multimeter: diode test mode and resistance mode. Diode test mode is by far the more effective means of testing. Resistance testing is less reliable and should only be used if the multimeter does not have a diode test mode.

A digital multimeter's diode test produces a small voltage between the test leads, enough to forward-bias a diode junction. A good forward-bias diode displays a voltage drop from 0.5 to 0.8 volts (for most silicon diodes). The meter will display ‘OL’ when a good diode is reverse-biased. OL will indicate the diode is functioning as an open switch.

The forward-biased resistance of a good diode should range from ohms to 10 Mohms. Thereverse-biased resistance on a good diode, will read OL The diode is bad if readings are the same in both directions.

Applications of Diodes

• Power Conversion: Rectifiers convert AC signal into pulsing DC signal, making it easier to control the flow of power.
• Signal Demodulation: Demodulation diodes are a critical part of radio receivers. The diodes retrieve the original message sent through the airwaves for transmission.
• Overvoltage Protection: Zener diodes are used to protect circuits from unsafe voltage levels. They are also used to protect supply lines and power supply control lines.
• Logic Gates: Diodes play a crucial role in modern computing and digital technology. They make it possible to reinforce binary systems through gates that perform simple logic functions, like and/or/not.

Case Study: Using Diodes with the Fluke 87V Digital Multimeter

Scoreboards often use LEDs in connected series. When one diode fails, all or part of a segment often fails to light.

Solving this problem using legacy tools can be time-consuming: it means setting up a DC power source and resistor to test each LED individually.

Fortunately, the Fluke 87V Industrial Multimeter has a built-in diode test mode, so that technicians can identify the defective LED and fix the problem quickly. The process consists of just four steps:

1. Verify that all power to the circuit is off and that all capacitors are discharged.
2. Set the multimeter to measure either AC or DC voltage, as needed. Turn the dial to diode test mode.
3. Connect the test leads to the diode and record the measurement displayed.
4. Reverse the test leads and record the measurement displayed.

This test pinpoints exactly which diodes are causing the trouble.

Bad diodes, like the ones that brought down the scoreboard, stop current from flowing in both directions. The multimeter display will read OL for both steps three and four when measuring a bad diode.

A shorted diode will have the same voltage reading in both directions (usually in the range of 0.4 volts).

Once the faulty diodes are identified, a technician can simply replace them, and the scoreboard should work like normal again.

Final Thoughts

Though small and simple, the diode is an essential part of modern electrical systems. By controlling the flow and direction of current, diodes enable countless applications of modern electronics.

Fortunately, tools like digital multimeters make it easy to measure diode health and determine when it’s time to replace diodes. This allows technicians to keep circuits in good running order, powering the electrical systems we all rely on.

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