Diodes - Repair Basics

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Diodes - Repair Basics
Type	Component
Device(s)	Diodes
Difficulty	◉◌◌◌ Easy

This article aims to provide a comprehensive overview of diodes from a repair perspective, detailing their function, types, common issues, testing methods, and replacement considerations.

Diode symbols commonly found in schematics (Figure 1)

What is a Diode?

A diode is a fundamental semiconductor device commonly found in electronic circuits. Its primary function is to allow the flow of electric current in one direction while blocking it in the opposite direction. This property makes diodes useful for rectifying alternating current (AC) into direct current (DC), protecting circuits from reverse voltage, and more.

Diodes have different characteristics found in their datasheets, the most important value when measuring is the forward voltage drop in "Volts"

Diodes are typically identified by the symbols shown in Figure 1 and are usually denoted with the letter D followed by an identifier number. (E.g., D38)

Function

Diodes serve various functions in electronic circuits, with the most common being:

One way path

The most basic function of a diode is to serve as a unidirectional pathway for the flow of electrical current. This is the basis for their use as rectifiers listed below, and in the past they have also been used to implement discrete digital logic known as Diode Steering, using diodes to create AND and OR gates (it should be noted that NOT gates cannot be created with diodes, which makes it impossible to create a computer using only diodes).

Diode-Transistor Logic is an obsolete family of logic ICs that used a combination of transistors and diode steering.

Rectification

Diodes are frequently used to convert AC voltage to DC voltage. They allow the current to flow during the positive half-cycle of the AC signal and block it during the negative half-cycle, effectively converting the signal to a unidirectional flow. Different arrangements of individual or multiple diodes exist to achieve this (such as the full bridge rectifier) each having various advantages and disadvantages.

Voltage Regulation

Various types of diodes are used (or can be used) as voltage references for voltage regulators, this is, generating a stable voltage of a specific value from an unstable or unregulated voltage source. This stabilized voltage is rarely used as a power source of it's own, rather it's used by an associated voltage regulator as a reference to control it's output.

Diodes that are commonly used for such purposes include Zener diodes, which when reverse biased through a resistor (in other words, with voltage applied in the direction where the diode normally would block current) they block current up to a very specific voltage, at which point they start conducting and loading down the associated resistor to maintain this voltage across the diode.

Zener diodes have the disadvantage thay they are not very stable and also produce a lot of electrical noise. Filter capacitors in parallel to them may be necessary in some applications.

Regular diodes have a very stable and low noise voltage drop when forward biased (with voltage applied in the direction in which the diode would normally conduct). Because this voltage is rather low (typically 0.7 Volts for silicon diodes), strings of multiple diodes in series are used for higher quality voltage references.

LEDs can also be used in this manner with the advantage of larger voltage drops per diode (2 to 3 volts) though it is uncommon practice, and mostly used by electronics hobbyists.

Protection

TVS and Zener diodes are often used to protect sensitive components from voltage spikes and reverse voltage by blocking or clamping excessive voltage levels, serving as surge protectors and fly-back diodes in relays and inductive loads.

Photodetection

Photodiodes are used to convert light into electrical signals, making them valuable in various applications, including optical communication, light sensors, and photovoltaic cells in solar panels.

Some diode packages and types (Figure 2)

Types

Diodes come in different physical cases that are either SMD or THT they can look identical but have different properties, outlined below is the different types of diodes.

Full bridge rectifier diode in multiple packages + diagram (Figure 3)

Regular/Rectifier Diodes

Regular or rectifier diodes are the most common type of diodes. Mainly used for converting AC to DC. They come in various package types and sizes.

If used to rectify mains AC to DC, you'll typically find 4 didoes in full bridge configuration in one package.

Schottky Diodes

Schottky diodes are known for their fast switching speed and low forward voltage drop. They are used in high-frequency applications and as rectifiers in power supplies.

Zener Diodes

Zener diodes are designed to maintain a constant voltage across their terminals when operated in the reverse-biased breakdown region. This means Zener diodes conduct backwards at (and only at) a specific *Zener* voltage.

They are used for voltage regulation and are often labeled with their nominal voltage value.

Light Emitting Diodes (LEDs)

LEDs are a specialized type of diode that emits light when forward-biased. They are commonly used in displays, indicators, and lighting applications.

Transient Voltage Suppression Diodes (TVS)

TVS diodes are specialized electronic components designed to protect sensitive electronic circuits from voltage spikes, transient over-voltages, and electrostatic discharge (ESD) events.

They operate by quickly diverting excess voltage away from the circuit they are safeguarding, preventing damage to components.

Internally, they are especially built Shottky barrier diodes, or arrangements of Shottky and Zener diodes.

Varactor Diodes

Varactor diodes, also known as varicap diodes, are used in tuning and frequency modulation applications due to their ability to change capacitance with a varying reverse bias voltage. Rarely used in consumer electronics today.

Photo-diodes

Photodiodes are light-sensitive diodes that produce a current in response to incident light. They are used in light detection and optical communication systems.

Measuring a Diode (Figure 4)

Testing a Diode

Make sure the diode does not have any physical damage such as liquid damage/corrosion first! That's the quickest way of identifying a faulty diode.

Testing a diode can help determine if it is functioning correctly. Most diodes can be tested using a digital multimeter in diode test mode. It's best to test the diode outside the circuit.

Regular / Schottky / Zener / LED

1. Identify the anode and cathode terminals of the diode. The cathode is typically marked with a band or line on the diode's body.
2. Set your multimeter to the diode test mode (usually indicated by a diode symbol).
3. Place the red probe on the anode and the black probe on the cathode.
4. The multimeter should display a voltage drop reading (around 0.2 to 0.7 V for silicon diodes). If it shows "OL" or less than 0.2, the diode is likely faulty.
5. Polarity is a concern. If the diode has a reading when the probes are flipped that means the diode is faulty

You can test the diode in-circuit, but keep in mind that other components in the circuit may affect the measurement but only lowering it.

Measuring a full bridge rectifier diode package is the same as measuring 4 regular diodes!

Diodes usually either fail in a fully open or fully shorted state. Rarely, the diode is damaged in such way that the voltage drop across it is outside the rage.

Identifying a Zener diode's zener voltage

1. Remove the diode from the circuit.
2. Solder a 1 Kilo Ohm resistor in series with the Zener diode, connecting it to the anode terminal, for example.
3. Using a Lab-Bench Power Supply, connect the positive terminal to the cathode of the diode and the negative terminal to the other end of the resistor.
4. Place your multimeter in DC voltage mode and connect its probes across the diode.
5. Gradually increase the power supply voltage until the reading on your multimeter no longer rises.
6. The voltage at which the multimeter reading plateaus represents the Zener voltage for this diode.

Replacement Considerations

When replacing a diode, always read the datasheets and pay attention to the following parameters:

1. Type: Ensure the replacement diode is of the same type and has similar electrical characteristics as the faulty diode.
2. Forward Voltage: Match the forward voltage rating closely to the original diode. Using a diode with a significantly different forward voltage may affect circuit performance.
3. Reverse Voltage (Zener): Pay attention to the reverse voltage rating. That's the most important parameter for Zener diodes.
4. Power Rating: Match or exceed the power rating of the replacement diode with the original diode.
5. Package Type: Choose a replacement diode with a compatible package type and size. Different packages have different mounting and pin configurations. If space is not a concern, you can use any size as it won't affect the circuit behavior.
6. Reverse Recovery Time: In high-frequency applications, the reverse recovery time of the diode is crucial. Ensure the replacement diode has a similar or faster reverse recovery time.

Replacing a diode with the wrong specifications can lead to circuit malfunctions or failure, so it's essential to choose a replacement carefully.