Diodes

A diode is a component which only allows current flow in one direction. This property is commonly used in many different ways, from rectifying and regulating voltage, to combining and otherwise dealing with analog and digital signals, to more esoteric applications such as in oscillators and RF circuits of various kinds.

Diodes have two poles, called the anode (+) and cathode (-). As a convenient mnemonic, you can think of the arrow in the symbol as showing the (conventional) direction the current can flow, and the line as “stopping” any current. On many diodes, the line in the symbol corresponds to a color marking at one end of the diode’s capsule.

Important parameters for diodes depend on the application, but include:

Forward voltage drop (V_F): The voltage needed for the diode to begin conducting. 0.6V is a typiccal value.

Voltage (V_R(max) – peak inverse voltage/maximum reverse voltage): How high a voltage the diode can block. If exceeded, the diode will be destroyed! Smaller signal diodes can normally withstand less than 100V, while rectifier diodes often will often withstand 1000V or more. A rectifier diode that can withstand 1000V can replace one with a lower resistance, but never the other way around!

Current (I_F(max) – maximum forward current): The maximum current that can pass through the diode. When current passes through the diode, it gets warm. Diodes that can withstand higher currents do this by warming up less and/or withstanding higher temperatures. For large diodes, mounting on a heatsink may be required for them to be capable of the rated current. Just as with voltage, it is fine or good to use a diod with higher current handling than strictly necessary. Lower heat generation is always a good thing, both for the diode itself, the PCB and nearby components.

There are many different types of diodes, with different areas of use:

Silicon diode. The most common type of diode. Available both for lower voltage and current – often called signal diodes – and more powerful types, which are often called rectifier diodes or just rectifiers. Silicon diodes are used, among other things, for protection against incorrect polarity, protection against current spikes and for rectifying alternating current.

Diode bridge. Consist of four internal diodes, commonly used to rectify alternating voltage. Diode bridges are available in many different packages, from small IC-style chips to bigger devices for chassis mounting with a bolt. Often called rectifier bridges or just rectifiers.

Schottky diode. Similar to silicon diodes in many respects, and share many common uses. However, schottky diodes are characterized by a lower voltage drop, typically 0.45V. For this reason, they are found in particularly efficient circuits such as switched power supplies, often a single diode – less often in diode bridges in linear power supplies. They generally withstand lower voltages than silicon diodes.

Zener diode. Zener diodes have a certain voltage at which they start conducting current “backwards”, known as the Zener voltage. They are normally used in this way, which means they are wired "backwards". In the other direction, they function much like ordinary signal diodes. Used to regulate voltages, for protection, or as voltage references.

TVS diode. Transient Voltage Suppression diodes are specially designed to protect against overvoltages and spikes from e.g. static electricity. Similar to zener diodes, but are optimized to work in other ways. Available in bidirectional and unidirectional versions, and in combined packages with a combination of silicon diodes and transient protection, for one or more channels.

Avalanche diode. Similar to zener diodes and are also connected in the opposite direction to reach their breakdown voltage and start conducting. They are based on a different technology than zeners, and the voltage is more constant with varying current compared to zener diodes.

Germanium diode. Similar to silicon and schottky diodes but with an even lower forward voltage, down to 0.2V. The germanium diode was the first diode made of semiconductor material, and is mainly found in older electronics, but also in modern effects pedals.

Varicap diode. Short for variable capacitance diode, this diode changes capacitance according to the voltage it sees. Commonly used in oscillators in RF applications, and functions as a voltage-controlled capacitor rather than as a diode. The symbol is a combination of capacitor and diode. Other common names are varactor or tuning diode.

PIN diode. A special type of diode which has a lightly doped middle region between the P and N regions of the diode (hence the I in the name). Used in RF applications . Often has very low and stable capacitance, and can function as transient protection for sensitive electronics for high frequency signals.

Light emitting diode. Diodes that light up when they conduct current – LEDs. Have higher voltage drops than regular silicon diodes, but in principle function just like them.

Photodiode. A special kind of PIN diode used to detect light.

Key applications

Some common applications for diodes are described below. In many cases, many types of diodes may be used for the same function, whilst certain functions require a specific type of diode.

Freewheeling diode

A flyback diode (also known as a freewheeling diode or snubber diode) is a protective diode used in conjunction with inductive loads, such as DC motors, relays, solenoids or electromagnets. Their goal is to protect other electronic components, such as drive circuits, from dangerous voltages. The coil in inductive loads ”want” the current passing through them to remain constant. When the current is interrupted – for example, during PWM control of DC motors, or when a relay is switched off – the coil’s magnetic field collapses. The inductance will then attempt to maintain the current by inducing a brief voltage pulse with reverse polarity.

This voltage spike is referred to as back EMF (EMF stands for Electromotive Force). The voltage can be very high, and without a protective diode, it will find its way through some surrounding electronics, such as transistors or motor drivers. As well as potentially damaging electronics, such brief current spikes will also cause electromagnetic interference (EMI). Such interference can be transmitted to other equipment via PCB traces or cables – which may e.g. cause cause microcontrollers to restart or behave erratically – or affect wireless communication.

In use, flyback diodes are wired in parallel with the coil, in reverse bias (i.e. with the cathode connected to the positive voltage). The adjacent schematic shows how to wire a protective diode for use with a relay.

When the transistor switch is opened, current flows through the coil in the relay (and the lamp is lit). The diode blocks voltage from passing through it, as it is reverse-biased. When the switch closes, the current cannot continue through the transistor. Instead, the circuit is completed via the diode which begins to conduct, and dissipates the rising voltage spike, thus thereby preventing the coil from generating harmful voltages.

To minimise electromagnetic interference, it is recommended that the diode is placed in close proximity to the inductive component.

Choosing a diode for flyback protection

Flyback diodes are not a type of diode, but rather a name for any diode used for this purpose. Many different types of diodes will work as flyback diodes. Silicon diodes are most common for discrete inductive loads (e.g. relays, DC motors etc.), whilst Schottky diodes are commonly found inside DC-DC converters.

The role as flyback diode is not a particularly demanding application for diodes. Often, the voltages they need to block are not very high – a good rule of thumb is that the diode should be able to withstand at least twice the relay’s coil voltage. For relays and motors operating at 24V and below, a standard 1N4148 is therefore perfectly adequate, as it can withstand at least 75V.

The diode must also withstand the relay’s coil current, as this is the current that the inductor will strive to maintain. In this respect too, the 1N4148 is often more than adequate, as its maximum average forward current is 200mA, and many relays have a coil current well below 100mA.

A simple and effective alternative to worrying about forward current or voltage ratings is to simply choose a more robust diode, e.g. one from the classic 1N400x series. For instance, the diode 1N4007 can withstand 1000 V reverse voltage and 1 A continuous forward current. Whichever you choose, the most important thing is that you use a protective flyback diode!