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General Diodes

Intro

As we examined in the last section, a diode is basically just a P-N junction with leads attached to either side. The anode (A) is the lead attached to the P-type side, the cathode (K) is attached to the N-type side, and the preferential, or forward bias is when electrons flow from cathode to anode (which means anode is hooked to the positive voltage source):

Diagram of a diode connected to a voltage source. The anode is connected to the positive side of a voltage source and the cathode to the negative side.

The symbol for a diode is a triangle pointing in the direction of current (hole-flow) connected to a perpendicular line which represents the junction:

Diode symbol with a triangle pointing in the direction of current flow butting up against a line.{:standalone }

There are a number of different types of diodes, but all of their circuit symbols are based on the one above.

The Wikipedia diode entry lists almost 20 different types of diodes, but in practice, most circuit design only uses a handful of them. As we explore more circuits, we'll also introduce additional, specialized diodes.

Polarity

To differentiate which end of a diode is the anode and which is the cathode, they usually have a marking on the end denoting the cathode. The following photo shows four different diodes, each with their cathode mark on the right:

Photo of four diodes showing a colored band on each diode indicating the cathode side.

Recall that positive charge carriers (holes) flow preferentially from anode to cathode, so the diodes above are in the same orientation as the circuit symbol below:

Illustration of a diode with the anode indicated by a plus sign and the cathode indicated by a minus sign.

Diode Characteristics

Depending on how they're constructed, a diode can have some interesting behaviors. For general diodes, there are a number of characteristics that are used to describe how they work:

  • Forward Voltage (Vf) - This is the amount of voltage drop, or the amount of voltage needed to enable current flow.
  • Maximum Forward Current (If(max)) - This is the maximum amount of current that the diode can safely conduct when forward-biased without breaking.
  • Peak Inverse Voltage (PIV) or maximum reverse voltage (VR(max)) - This is the maximum amount of voltage that can be applied in reverse bias without an avalanche breakdown.
  • Total Power Dissipation (PD(max)) - A diode has some resistance, so some power is lost in the form of heat. As such, a diode usually has a maximum amount of power that it can safely conduct without overheating. Total power dissipation is based on the voltage of the junction potential and the current: PD = Vf * I.
  • Reverse Recovery Time (Trr) - This is how quickly a diode can go from OFF to ON. It's generally only important in fast-switching circuits.

Common General Diodes

  • Switching Diodes - Also known as small signal diodes, they have an extremely fast Trr, and are used for fast current switching.
  • Rectifier/Power Diodes - High current capacity, usually 1A or more. Used for converting AC to DC and protection of circuits from power spikes.
  • Schottky Barrier Diodes - Have a very low voltage drop, so they can be used in a clever way to get to a digital 0/OFF. Also very fast. Used for basic logic control.
  • Zener Diodes - Have a precise breakdown voltage. Typically used in a reverse bias configuration to provide a voltage reference.

Switching/Small Signal Diodes

Switching diodes are one of the most common diodes available, largely because of their usefulness in high frequency circuits like radios, where their fast switching abilities are required.

They typically have small power ratings, 150mA or less, and a Vf of 0.7V.

Despite their commonality, most of the digital circuits we'll explore will use other diodes.

Power/Rectifier Diodes

Power (aka rectifier) diodes are similar to signal diodes but are designed to handle higher current. The 1N4001 is a common rectifier diode and is capable of handling 1A, but the Vf (forward voltage) drop is 1.1V. These are commonly used to protect certain parts of circuits from voltage spikes, as well as for conversion of AC to DC electricity.

Because diodes control the direction of current, they're often used to protect circuits from unwanted current, especially power spikes.

Flywheel Diodes

Flywheel diodes protect circuits from collapsing magnetic fields created by de-powering electric motors and other coils by feeding excess current back into the coil. We'll use a flywheel diode in a later chapter to safely control a relay.

Voltage Clamping

Voltage clamping refers to clipping a signal to a maximum/minimum value to prevent it from going outside a particular range, or shifting an entire AC signal wave above or below 0V.

Diagram of two graphs that illustrate how a diode restricts output voltage to a reduced range.

Rectifiers

Alternating Current (AC) electrical signals can be converted into Direct Current (DC) through a clever arrangement of diodes known as a rectifier:

Diagram of four diodes arranged in square pattern to form a rectifier.

The above circuit will transform a two-phase AC wave form into positive and ground voltages:

Illustration of how a rectifier changes a two-phase AC wave that varies between -110v to +110v into a wave that varies between 0v and +110v.

With some additional components, the DC wave output above can be transformed into a smooth, level DC signal.

Rectifiers are used as the first stage in converting household mains AC current into DC current for use in electronics in nearly all AC power adapters. For example, the USB wall adapter that you use to charge your phone converts AC to DC using a rectifier. In fact, nearly every electronic device around you that plugs into the wall contains a rectifier circuit.

Schottky Barrier Diodes

Schottky diodes use a piece of metal against the N-type side instead of a P-type semiconductor. This results in a much lower voltage drop and fast switching speeds.

The circuit symbol for a Schottky diode looks similar to the diode symbol, except that the perpendicular line looks similar to an S.

Illustration of the Schottky diode symbol.

Schottky diodes typically have a very low voltage drop (Vf), typically around 0.2V (0.15V to 0.45V) which makes them very fast and also makes them ideal for use in simple circuit logic.

Reverse Current Protection

Because diodes act like one-way valves, they can prevent reverse current from happening in situations where a battery is plugged in the wrong way. Schottky diodes are ideal for this because their low Vf means that it doesn't cost much battery voltage to protect the circuit.

Diode Logic Gates

These logic circuits can be arranged to provide operations such as AND and OR, called logic gates. This allows you to determine if multiple inputs are all ON as well as if any of the inputs are ON or OFF.

Consider the following circuits:

Illustration of Shottky diodes used to form logical gates.

OR Gate Truth Table

Truth tables are used to describe the resulting output from inputs based on the operation/function. The following truth table describes the resulting output based on the possible inputs to an OR gate:

Input 1Input 2Output
000
011
101
111

In the truth table, 0 and 1 represent LOW and HIGH, respectively. In the case of our circuit, this means somewhere around 0V for LOW and 3.3V for HIGH.

In the case of an OR gate, as long as any input is 1, the output will also be 1. This makes sense, because applying voltage to either diode will provide voltage to the output.

However, recall from earlier that digital logic levels don't have to be exactly 0V and 3.3V, but rather, LOW can be up to 1.1V, and HIGH can be as low as 2.2V. This is important, because in a practical circuit implementation of the above, some voltage is actually lost to the voltage drop of the diode. If the Vf of the diodes are 0.2V, then only 3.1V is actually read on the output.

AND Gate Truth Table

The AND gate is a slightly more clever than the OR gate. In the case of the AND operation, a HIGH voltage is only seen at the output when both inputs are HIGH, therefore, blocking the ground path for the 3.3V voltage source:

Input 1Input 2Output
000
010
100
111
Logic Gate Uses

In this way, low Vf Schottky diodes can be used to determine if conditions are met within a circuit, such as if multiple inputs are ON. We'll explore this more by building these gates in one of this chapter's labs.

Zener Diodes

Zener diodes have a known, precise breakdown voltage, which makes them very useful for providing a reference voltage.

Voltage Reference

Sometimes, a circuit needs a reference signal at a precise voltage. By utilizing the breakdown voltage of a reverse-biased Zener in a circuit, its Vf back-pressure can provide that voltage reference. Consider the following circuit:

Illustration of a circuit utilizing a Zener diode to provide a 5V voltage from a 9V source.

In the circuit above, a Zener with a 5V breakdown voltage is being reverse biased with 9V, which means that it's breakdown threshold has been reached, and will conduct current, with a 5V voltage drop. Since the voltage drop acts like a dam, no matter how much voltage is applied, 5V of back pressure will always be present:

Illustration of the analogy of a Zener diode acting like a dam for water flow.

As long as the current is limited, in this case with a resistor, it will stay within that precise operating band. Recall the diode behavior chart from before, specifically the breakdown behavior:

Illustration of the diode breakdown curve.

While this circuit looks a lot like a two resistor voltage divider, it's got a huge advantage over a divider; as long as the current is limited, no matter what amount of voltage is applied (within the diode's tolerance), the Vout reference will always be the same.

And while the limited amount of current prevents this from being a useful voltage regulator, it does serve as a reliable voltage reference, which is used in ADC conversions, voltage regulator circuits, and more.

Zener Diodes come in many Reverse Voltage Ratings

Because Zener diodes are often used as voltage references, they come in a wide variety of reverse voltages. In fact, Zeners cover all common voltages you're likely to find in modern circuits.