Do Diodes Have Continuity in Both Directions

Welcome to the world of diodes. In this guide, I'm going to take you through all of the highlights from what a diode is to how to use them in electronics.

This is part of our Basics series on diodes and transistors.​

What Is A Diode?​

The easiest way to define a diode is by this:

diode definition = electrical component that conducts current in mostly one direction ​

This unique capability makes diodes very useful in electronics. They are like one way roads in a city. In that analogy, they let you guide current the way you want it to go.

#1 Lesson for diodes is that they are like one way gates that allow you to control the direction for current to flow through your circuit.

How Does A Diode Work?

In order to see how a diode works, let's look at the behavior of a diode.

Ideal Diode

A diode does two things:

1.  allows current to flow in one direction, called the forward direction
2.  blocks current in the other direction, called the reverse direction​

It is ideal in that it does ​both of these perfectly. If we plotted the current through a diode versus the voltage, it would look like the following ideal diode IV curve.

Due to the process of making things, real world diodes are not this perfect. We'll discuss why as we continue.

Diode Symbol​

It turns out there's an easy way to represent diodes through symbols in a schematic. Here they are:

​Notice that there are many different diode symbols for types of diodes. They are slight deviations for the regular diode diagram.

For example, the zener diode symbol simply has two extra lines facing opposite directions.

​Diode Polarity

Now that we have a good foundation, we should discuss diode polarity. It turns out that diode direction plays a key role in its behavior.

Why?

Well it has to do with the physics in the diode. Let's break down the two ends of the diode as follows:

Diode Anode: positive end of the diode, when​ the voltage here is higher than the cathode and high enough to turn on the diode, current will flow through it

Diode Cathode: negative end of the diode, it will not let current flow through this end unless the voltage gets high enough that the diode can't handle it​, which is known as breakdown.

​PN Junction Diode

The solid state physics behind the workings of a PN diode is related to the manipulation of electrons.

It turns out that we can make types of material that have excess of electrons, N-type, and also an absence of electrons, or P-type.

When we put an N-type material next to a P-type material, we get neat behaviors.

​The P-type section, having an absence of electrons, acts like "holes", which creates positive charge carriers.

The N-type section has an excess of electrons.

So why don't the electrons go join the holes and balance everything out in the material?​

What is neat is that the materials are made in such a way that the excess electrons don't easily flow into the absence of electrons because the two are shifted in relation to each other.

When a diode forward voltage is applied, which means a more positive voltage is put on the anode, then the shift between the electrons and the holes move much closer together, allowing a good movement of electrons (current) through the device.​

That is how your one way street of current is created.​

When a reverse bias voltage is applied, the shift between the electrons and the holes that is already there gets moved even more, making it hard for electrons to flow through the diode.

A road block for current is created.

What Does A Diode Do?​

As we have discussed, diodes are kind of like one way streets. We can use them to help guide current to go certain ways, and prevent it from coming back through certain ways.

We will go into much more detail on different ways to use diodes later in the using diodes section.

First, let's discuss a few more key concepts about diodes.

Forward Bias Diode

What does it mean to have a forward biased diode?​ The answer is pretty simple when you look at it right.

You see, a diode by itself has an N-type and P-type material sandwiched together as we previously discussed.

With these two materials together, and how the materials behave, we get something called a depletion region that prevents current from flowing easily through the device.​

However, if we stick a forward voltage, which is typically 0.7 Volts for the common diode, across ​the anode to the cathode, then we can make the depletion region disappear which allows current to flow easily through the diode.

​We can see this effect in the image above. Notice how 0.7 Volts is put on the anode versus the cathode on the bottom example and now current flows freely through the device because the depletion region is now gone.

Reverse Bias Diode

In the same way that a forward voltage can remove the depletion region, a reverse bias voltage can make the depletion region even bigger.

This has the effect of making the blocking power of the diode even better by not allowing current to flow from the cathode to the anode through the device.​

The typical diode has a reverse bias voltage range of up to 50 Volts. Of course, you can get diodes that go much higher than that. Sometimes in the datasheet, this variable is called the DC blocking voltage.​

As you can see in the image above, by applying a voltage on the cathode end of the diode in the bottom example, you can increase the depletion region even more, blocking any current from flowing through the device.

Reverse Bias Leakage​

Real diodes are not perfect, in that some leakage current will make its way from the cathode to the anode. The amount is typically small though, but if this is an issue for your design, proper diode selection is important.​

Breakdown Voltage

What happens if we continue to increase the voltage on the cathode, and exceed the reverse bias rating on the diode?

Breakdown is what happens. This is when the diode has been pushed beyond its designed expected behavior and now the diode starts allowing current to flow through it from the cathode to the anode.​

Most diodes are typically damaged when this happens.​

Diode Characteristics

To visualize what we just learned, let's look at a graph that shows the different operating modes of a diode. This is the IV curve for a real diode.

Notice how current in the y-axis flows through the diode well at the forward voltage of 0.7 Volts for a typical diode. The breakdown voltage is where the current will start flowing in the opposite direction, which is -50 Volts for the typical diode.

All real diodes will have leakage current as well, in which the current will flow from cathode to anode when not forward biased.

Sometimes, there are other characteristics that you may need to understand such as diode resistance. For many circuits, it's not really a factor that comes up.

However, for more sensitive circuits, one way to figure out the diode resistance in forward bias mode is that you can use the classic resistance = voltage / current equation.

In this case, you can measure the diode voltage drop across the diode for the different circuit modes that you are curious about in relation to the current through the diode.

Diode Equation​

A useful exercise in understanding diode behavior is to learn about the diode current equation.

Let's first look at the ideal diode equation, and then see how real world effects change its behavior. It is as follows:

​where:

• Is = dark saturation current
• q = value of electron charge
• Vd = voltage across the diode
• n = ideal factor, n = 1 for ideal diodes, and n = 1 to 2 for real diodes
• k = Boltzmann's constant, 1.38064852E-23 Joules/Kelvin
• T =​ temperature (Kelvin)

​In order to reduce the equation, we know that kT/q is something called the thermal voltage, or Vt. We can rearrange the equation as follows:

Here, Vt = 0.026 Volts at normal temperatures.

As you can see, the equation is non linear, which makes diode behavior a little complicated to simulate. This just means real diodes mostly ​do what ideal diodes do, just not perfectly.

If you are interested in the modeling of diodes, there is a great write up on it here.​

Diode Types

Avalanche Diode

​Avalanche diodes are ones that are designed to handle breakdown voltage mode on purpose. Therefore, they are not damaged when put in breakdown mode because their design spreads out the current density more evenly.

These diodes are typically used as a form of protection from unwanted or unexpected voltages. They have the ability of being put into breakdown mode and conducting excess energy into ground, saving a circuit that is not designed to handle those voltages.​

Germanium​ Diode

Common diodes are made out of Silicon, which has specific properties that give rise to its forward voltage of 0.7 Volts. But what if you need a diode that has a lower voltage?

That is where a Germanium diode can be handy. Given its material properties, these diodes have a typical forward voltage of 0.3 Volts.

The lower voltage makes this type of diode handy in audio and FM circuits. It used to be the popular diode of choice back in the day before Silicon diodes became mainstream.​

Gunn Diode

The gunn diode is also known as a transferred electron device​ (TED). It is different from other diodes in that it only has N-type material (there is no P-type material in it).

​It has two sections of N-type material connected by a thin section of N-type material. What happens is that as the voltage across the device increases, the current increases up until a certain point in which the current starts to decrease.

This makes the device act as if it has negative resistance. It also can conduct current both ways because of the missing P-type material.

They are commonly used in electron oscillator circuits to create microwaves, including radar speed guns and automatic door openers.​

LED Diode

An LED diode stands for a light emitting diode. ​A diode LED is a device that emits photons when current passes through it.

LEDs are extremely common these days and can be found everywhere in electronics. The price has been driven down so cheap that they are even used in circuits to indicate board level functions.​

Newer technology pushes are working to improve the cost of organic light emitting diodes that offer even more benefits, including flexible displays.​

Photo Diode

A p​hoto diode is a device that generates current when it absorbs photons. Therefore, these devices are handy in detecting photons across many different wavelengths.

In fact, all digital camera technology works by using an array of photo diodes, where each diode is considered a pixel.​

There are even things called diode array detectors that have an array of photodiodes that work at detecting different wavelengths​ of light, so that a wide spectral range of information can be gathered.

PIN Diode

PIN diodes, as the name implies, is where an undoped material is put in between the P-type and N-type materials.  The undoped material creates something called an intrinsic region.

These diodes are handy in high frequency circuits. They make great RF and microwave attenuators and switches.​

Schottky Diode

A Schottky​​ diode is one where the P-type material is removed and instead a metal is used with the N-type material to create the diode.

The advantage is a lower forward voltage, which helps increase switching frequencies in certain applications. This in combination with faster recovery times makes them useful in circuits like switching power supplies.​

Shockley Diode

The Shockley diode is one of the first ever invented by William Shockley. It had four layers of PNPN material.

These diodes are not manufactured anymore, but their behavior can be mimicked by a dynistor.​

Silicon Diode

Silicon diodes are the every day run of the mill diodes that you will find in circuits. They are the most common and typically have a forward voltage of around 0.7 Volts.​

An image of the 1N914 can be seen below.​

Tunnel Diode

Tunnel diodes take advantage of the effect called quantum tunneling.

What's cool about these devices is that at first, it is very easy for current to travel through from anode to cathode. Then as the forward voltage increases, ​the current flowing through the device decreases, creating negative resistance.

Then as voltage creases even more, it starts operating like a regular diode. However, the diode is desired for its negative resistance region. They are useful in frequency converter and detector circuits.

Varactor Diode

The purpose of a varactor diode is to exploit the capacitance, which is voltage dependent, of the diode in reverse bias mode.​

In effect, they can be used as voltage controlled capacitors and are handy in oscillator and frequency multiplier circuits.​

Zener Diode

Zeners have a much sharper current curve than other diodes in the breakdown region.

This means that while they operate like other regular diodes (anode to cathode), they can also pass current in reverse (from cathode to anode) when the reverse bias voltage is reached.​

Other diodes are not designed to operate in breakdown voltage mode, whereas Zeners are designed to work there.​

Common Diodes

1N400X Series

A great general purpose diode for many different applications is the 1N400X series. They are often found in DC power circuits for protection. A picture of the 1N4001 diode can be seen below.

Here is why they are great:

• low cost
• low reverse leakage
• high forward surge current
• forward current max =  1 Amp
• max forward voltage at max current = 1.1 Volts
• max reverse bias voltage varies on X part selected, from 50 Volts up to 1000 Volts ​

Some specific examples are:

1. ​1N4001 diode - reverse bias = 50 Volts, link
2. 1N4004 diode - reverse bias = 400 Volts, link
3. 1N4007 diode - reverse bias = 1000 Volts, link

1N540X Series​

​If you need more forward current, then the 1N540X series is a great option. They are very similar to the 1N400X series, except:

• forward current max = 3 Amps​
• surge current is much higher

Example:

1N5408 diode - reverse bias = 1000 Volts, link​

Small Signal

For other types of circuit, including small signal applications, there are better suited diodes available.

When you are dealing with lower current and lower voltage situations, these diodes come in handy.

Some great examples include:

1. ​1N914 diode - reverse bias = 100 Volts, forward current = 0.2 Amps, link
2. 1N4148 diode - reverse bias = 100 Volts, forward current = 0.2 Amps, link

Diode Packaging​

Diodes come in many different package options, including through hole, surface mount, and bigger packages like that used in RF and high power applications.​

Depending on the specifications of the diode, the size will vary. For example, high voltage diodes will tend to be much bigger than low voltage options.​

Diode Markings​

Diodes will have certain markings to signify the part number, as well as the polarity of the device.

For example, through hole diodes will have numbers printed on the part and will also have a thin band at one end of the diode which signifies the cathode.

The datasheet of the part will show you what the markings consist of and what they mean.​

An example of a Germanium through hole diode can be seen below.​

Using Diodes​

Let's look at some of the most popular diode circuits out there to get a better idea of how to use diodes.

Rectifier Diode

​A diode rectifier is one of the most common ways of using a diode. Let's take a look at some specific examples next.

Diode Bridge

There are two varieties worth mentioning here: half wave and full wave rectifiers.​

Half Wave Rectifier​

Say for example you have an alternating current (AC) signal, and you only want the part of the signal that is above 0 Volts. You can use a diode to do this.

A common place where this type of circuit is used is for a 120 Volt AC rectifier, as shown below.​ This is called a half wave rectifier.

Notice how only the positive components of the input signal are passed, while the negative components are not.

The issue here is you only get a half of the signal in this example, the positive half. In many situations that might be all that you want.

In situations where you want both components, then you'll need a full wave, which we will cover next.

Full Wave Rectifier

A full wave rectifier is a combination of 4 diodes together to get both the positive and negative components of a signal into an output that is positive.

The diodes are arranged in such a way such that the input signal always has a path through the diodes, whether its positive or negative voltage. This diode arrangement can be seen below.

The input signal is transformed to all positive as seen below (input and output are color coordinated with the above diagram.

Full wave rectifiers come in premade packaging with higher current limits. An example can be seen below.

You can also arrange your own diodes individually to create your own full wave bridge. You will want to choose good power diodes, or diodes that have higher forward current and higher breakdown voltages for your application.

The 1N4007 and 1N5408 parts are great choices for rectifying 120V AC directly, depending on your max current requirements. Note that the max reverse bias is critical here and the 1000 Volt rating on these parts give you plenty of safety margin.

If you are using a step down transformer between the 120 Volt AC and the full wave bridge, then determine the max voltage and make sure the diodes you select have plenty of margin (2-3 times above) for the reverse bias.​

If you are interested in smoothing out the ripple, you can use a capacitor on the output that is appropriate for your current pull in your circuit and get a nice DC voltage on the output.​

Flyback Diode

There are many names for this same type of diode, including snubber diode, freewheeling diode, and suppressor diode.​

A flyback diode is a handy way of using a diode to reduce sudden voltage spikes that occur when the current through an inductive load suddenly changes.

As we discuss in the article on inductors, whenever an inductor sees a change in the current going through it, it will create an EMF voltage spike to try to stabilize the current change.

In many circuits, this generated EMF is usually unwanted, and can sometimes cause damage to other parts of the circuit.

In order to eliminate damage, a diode can be placed such that current is encouraged to flow through the diode in the event of an EMF voltage spike, instead of through other circuit components that might be damaged.​

A common circuit where this is useful is the control of a small fan or relay inductor. Typically, most digital pins can source less than 20 milli-amps, so it's necessary for a current amplifier. See the example diode schematic below.

An NPN transistor works well here, because the digital pin can source the 10 milli-amps to turn on the NPN transistor, and the transistor can handle the amp or so of current needed for the fan or relay inductor.

Whenever the transistor turns off, the inductor sees a sharp drop in current and produces a back EMF spike.

Without a diode, the spike will flow through the transistor, usually damaging it. By placing a diode in parallel with the inductor, the EMF voltage spike turns the diode on and allows the current to flow through the diode and back into the inductor where it will dissipate.

This flow back of current back into the inductor is where the name for this type of diode comes from.​

For the D1 diode in the above circuit, a common part choice is the 1N4001, which has a 1 amp forward current, high surge current, and 50 Volt reverse bias specifications.​ This works well in 12 Volt circuits. If your voltage is higher, you might need a more capable part.

Zener Diode Voltage Regulator​

As we discussed earlier, Zeners are designed to operate in the breakdown voltage mode.

One way to take advantage of this is with a Zener regulator. We simply need a resistor and a Zener, properly selected, in order to give us a desired voltage output.​

An example Zener diode circuit can be seen below.

The Zener will clip the input voltage down to the breakdown voltage of the diode in this circuit for the output. In order to do this, it must allow current to flow through the diode which will be dissipated as heat, but only when the input voltage is above the breakdown voltage.

The desired output voltage will determine the Zener, as you will select the diode based on its breakdown voltage to match the output voltage. You must get a diode that can handle the power that must be dissipated.

The resistor must be carefully selected based on the current pull of the circuit. A great calculator for choosing these parts is here.

Blocking Diode

This use of a diode is simply a name given to the situation where a diode is used to steer current to flow in one direction only.

A great example is with a solar panel and battery charger circuit. When the sun is out and the solar panels are producing current, they will typically be at a higher voltage than the battery that the circuit is charging, so current will flow from the panels and into the battery.

However, at night time, there is no sunlight hitting the solar panels so they will not be producing current. The battery at this point will be at a higher voltage and without a blocking diode, current will flow from the battery into the panels, wasting energy.

When a diode is placed between the solar panels and the battery, it will allow current to flow from the panels into the battery, but will not allow current to flow from the battery into the panels.

Therefore, it "blocks" current from flowing the undesired way.​

Another place where this is useful is for batteries in a circuit. Any time it is possible that someone can put batteries in backwards, or plug in DC power backwards, a great way to protect the circuit is with a blocking diode.

​The diode ensures that only the correct voltage polarity will allow current to flow into the circuits, protecting them from negative voltages.

The catch is that you must select a diode that can handle the max forward current that the circuits will pull. Also, the circuit voltage will be decreased by the forward voltage of the diode.

Clamping Diode

A clamping diode is simply a way to use a capacitor and a diode in order to manipulate the DC level of a signal.

In the example circuit that follows, the capacitor and diode create a DC offset on the input AC signal.​

If we want to go the other direction for DC offset, then we simply reverse the direction of the diode, as shown below.

You can go even further if you put a voltage source between the diode and ground, such that you can add more DC bias in the desired direction.

Clipping Diode

In contrast to clamping is clipping. ​This is where you can use a series resistor and a diode in order to clip off an unwanted part of the input signal.

For positive clipping, the diode is arranged so that it is on when the signal is above the forward voltage, and therefore the diode conducts current, clipping the upper voltage at around 0.7 Volts.​

An example can be seen below. R2 is simply an example resistor and is not required.

In the example above, notice how the max upper voltage is clipped at 0.7 Volts, which is the forward voltage of the diode.

If negative clipping is desired, you can simply flip the diode around. In this case, whenever the signal input is negative beyond the forward voltage, the diode will be on and conduct current, clipping the negative signal at -0.7 Volts.

An example is below. Again, R2 is not required.

In the example above, notice how the negative portion of the signal is clipped at -0.7 Volts.

To go even further, you can add a voltage between the diode and ground to shift where the clipping occurs on the input signal.

You can also do both positive and negative clipping together by placing two diodes in parallel facing opposite polarities to clip the upper and lower portions of the signal.​

Did you like this article or do you have some interesting experiences with diodes? Let us know about it in the comments below!

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Source: https://www.electronicsdesignhq.com/diodes/

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