Zener diodes are a type of diode with the ability to allow current to flow in one direction, as well as allowing a large reverse current to flow at a critical reverse voltage. 

Electronic components all heat up at a certain point. 

You might have encountered this with your electronic devices like a laptop or smartphone when you have been using it for extended periods of time. 

So, do zener diodes get hot? Yes, zener diodes do get hot. Zener diodes, like other electronic components, have a maximum power rating. Exceeding this power rating will cause the zener diode to expel excess energy in the form of heat and get hot. 

This article shall delve deeper into why zener diodes get hot.

Keep reading to learn more.

Why do zener diodes get hot?

So, why exactly do zener diodes get hot?

To answer this question, we first have to take a couple steps back and look at some fundamental theories which include energy, power ratings, and current. 

Don’t worry, it will be brief.

Energy

Let’s start with energy.

The First Law Of Thermodynamics states; “Energy cannot be created or destroyed, only changed from one form to another”.

The total amount of energy in the world remains constant. 

So, for example, if you are seated at this moment reading this article, you have Potential Energy. When you get up to maybe go get a new cup of coffee, that potential energy is converted to Kinetic Energy.

Forms of potential energy include;

• Chemical
• Nuclear
• Elastic 
• Gravitational

Forms of kinetic energy include;

• Mechanical
• Electrical
• Thermal
• Sound
• Light

Later we shall see how this applies to zener diodes. 

Power ratings

All electrical/electronic devices and components have Power Ratings.

This defines the maximum allowable power that the device can be subjected to. If you exceed this limit you pose the threat of permanent damage to the device or component.

Electrical appliances will have a label indicating their power rating, while electrical and electronic components will have this information in their datasheets. 

Power rating is a product of a device/components voltage and current rating. 

The equation above shows us that power shares a directly proportional relationship with voltage and current. 

This means that increasing either voltage or current will increase power.

Power ratings have the standard international unit of Watts (W).

What happens inside the zener diode when it gets hot?

But why is power rating important? 

To get a clear picture of this, let’s take a look inside a cable and see what happens. This same concept applies to all other devices and components as well, but the cable is an easier example.

Also, just like components, cables have power ratings. 

Current is the lifeblood that runs through all electrical/electronic circuits and components. Without current the electrical and electronic world would stop still.

It is defined as the flow of electrons past a given point in a circuit. 

Ampere (Amp), is the unit of measurement that tells us the number of electrons passing this given point. The higher the current, the higher the number of electrons, and vice versa.  

Ok, let’s take a look at a cable at the flow of electrons through it.

The image above shows us a sliced up view of a cable. Let’s imagine that this cable is rated at 5 watts.

In this scenario, the cable is operating within its power ratings, so the amount of current (electrons) flowing through the wire is an amount that is within the cable’s capability.

Now, if we increase the amount of current (electrons) drastically, we increase the power through the cable ( as current is directly proportional to power).

This causes a bottleneck effect within the cable as we can see in the image above. 

Here electrons cannot freely move, and bump into each other. As we learnt earlier, energy cannot be created or destroyed.

As electrons move and bump into each other (kinetic energy), they cause friction and release energy in the form of heat.

So why do zener diodes get hot? Because an increase in current through the zener diode causes an increase in power. When this power exceeds the zener diode’s power rating, energy is released in the form of heat as electrons flowing through the zener diode bump into each other. 

Can a zener diode get hot under normal conditions?

But, will zener diodes get hot if you stay within the power ratings?

Zener diodes, like other electronic components, will warm up to a certain extent even when they are operated under their normal power ratings.

This is because heat will still be released as electrons bump into each other. But, the frequency of collisions will be far less compared to when excess current is flowing through the zener diode. 

So, under these normal conditions (and assuming they are wired correctly as well), zener diodes will not get hot. 

And here hot is defined as unusually high temperatures (outside the maximum temperature that the zener diode should be subjected to).

Is there a certain power rating at which a zener diode gets hot?  

Zener Diodes come in a range of sizes, packaging (through hole, SMD), voltage, current and power ratings. This is so that you have the option of selecting the right one for a certain application.

So there won’t be a specific number of voltage, current, or power rating, at which all zener diodes get hot. Every zener diode will have been tested accordingly and have its own unique maximum power rating. 

And as we learnt earlier, getting near to, or exceeding the power rating will cause the zener diode to get hot.

Also, the size of the zener diode will play a big part in how much current it can handle.The bigger its size, the more current it can handle and therefore it will have a higher power rating. 

To find the power rating information of zener diodes you will have to look at its datasheet which is provided by the manufacturer (this can be found online). 

What happens when zener diodes get hot?

Maximum power ratings are there for a reason.

They help us stay within the limits of the components to protect them. Exceeding this power rating will cause excess current which leads to it getting hot.

But, what happens when it gets hot?

There are a couple of things that can happen.

The first occurrence is a phenomenon known as Thermal Runaway, which describes a scenario where an increase in temperature dissipates energy (in the form of heat), further causing an increase in temperature.

This will cause the zener to get hotter. 

Next, once the temperature has reached a level which the casing (structure) of the zener diode cannot handle, it is going to rupture which could lead to the diode smoking.

Or, in a worst case scenario, a small explosion. 

The zener diode is now permanently damaged and will need to be replaced. 

How to avoid zener diodes getting hot

Whether it be with zener diodes, or any other aspect of life, accidents are going to occur. But, being prepared is the best way to minimize and reduce the chances of accidents occurring. 

So what is a good way of avoiding zener diodes overheating?

The first step is to know the maximum power of the circuit/application where the zener diode will be used. 

Next is to choose a zener diode that has power ratings which matches, or is slightly higher than the circuit’s power rating. 

The rule of thumb is to choose a rating slightly higher to give it some allowance.

Always check the datasheet of the zener diode you are purchasing and make sure to check its power, voltage and current ratings. 

Bigger zener diodes will be dealing with higher currents and therefore are going to get hot (which is normal). 
In this scenario you will need to use a Heat-sink, which is a component used to dissipate excessive heat.

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They have many uses within these circuits, but the main one being to limit the flow of current.

Sometimes resistors can heat up. 

Why do resistors get hot?

Resistors get hot because excess energy is dissipated in the form of heat when current flows through the resistors. Exceeding a resistor’s power rating causes more current to flow through the resistor. This increase in current sees an increase in electrons which start to bump and rub into each other causing friction resulting in energy released as heat. This is the main reason resistors get hot.

A closer look at current through a resistor

Let’s start with Current.

Without current, the electrical and electronic world would stop still. Current is the blood that flows through every circuit giving it life.

Current is defined as the flow of electrons (specifically past a given point) in a circuit. 

The unit of measurement for current is known as Ampere or Amp (A) which tells us the exact number of electrons that are passing a given point.

The higher the current, the higher number of electrons and vice versa. 

However, for electrons to flow freely, it requires a Conductor. Conductors are materials that have higher electrical conductivity which means that electrons can move through them more easily. 

On the other hand are materials known as Insulators, which inhibit (prevent) the flow of electrons. 

Copper is the go to conductor for making wires and components due to its high conductivity.If we were to slice up the conductor (such as copper) when electrons are flowing, we would see something similar to what is in the image below. 

Note, for electrons to move in a uniform manner in a given direction, it requires an energy source such as a battery, otherwise electrons move about randomly. 

Deeper look at a resistor

Let’s take a closer look at the Resistor. 

There are many different types of components and devices in the electrical and electronic world, each with their own unique abilities which help perform a certain function within a circuit. 

The Resistor is a fundamental electronic component that can be found in almost all circuits.

So what is the definition of a resistor? 

A resistor is a passive electrical and electronic component, whose main purpose is to ‘resist’ the flow of current in a circuit. Also, rather than having an ambiguous resistance value, resistors are created with a set resistance value. 

The bigger the resistance, the less current can flow, and the lower the resistance, the more current can flow.

It is known as a passive component due to the fact that it has no means of generating its own power, but rather dissipates power in the form of heat. 

Construction of a resistor and how it provides resistance

So, we know that conductors allow current to flow more freely compared to an insulator. 

Under the umbrella of resistors, there are many different variations of how they are constructed and the materials used. 

But, to save you time, I shall concentrate on the most common type which is the Wire-wound.

Below is an image of what a wire-wound resistor looks like on the inside;

As you can see, there is a rod that is wrapped by a wire. The rod is typically an insulating ceramic, and the wire is copper (due to its great conductivity)

But, copper has low resistance! So, why use it in a component meant to resist the flow of current?

While resistors need to oppose the flow of current, they still need to let the current pass. But, just sticking a straight piece of copper wire inside a resistor is not going to add much resistance. 

The key to increasing the resistance of a set length of copper wire comes down to coiling the wire. This is why the copper wire is wound (coiled) around the insulating material.

The resistance of a resistor can be accurately set by controlling the number of turns of the coil of copper wire.

Also, we saw the resistance of a material is controlled by a number of factors; length and cross-sectional area.

If the length of copper wire is set, we can further increase its resistance after coiling it by reducing its cross-sectional area (make it thinner). This is thanks to the inversely-proportional relationship between resistance and the cross-sectional area of a wire.

But, what if you want to reduce the resistance of a resistor?

Just do the opposite! Reduce the number of turns, and increase the cross-sectional area of the wire.

Resistor power ratings

One last thing to note before we dive into why resistors get hot, are power ratings.

Every electrical and electronic device, and component has a power rating. This is a value that indicates how much electrical power is needed by the component to work effectively.

Also, the power rating value indicates the maximum allowable electrical power the device or component can handle.

Exceeding this value will cause damage to the component. 

The power rating can be broken further into; Voltage ratings, and Current ratings. 

This is because power is a product of voltage and current ( P = V x I).

Components will have values for maximum voltage and maximum current ratings that should not be exceeded. These two values can be multiplied to get the maximum power rating.

The same holds true for resistor power ratings, which is a value that denotes the maximum power that a resistor can withstand. 

Common resistor power ratings include;

• 0.25 Watt
• 0.5 Watt
• 1 Watt
• 2 Watt
• 25 Watt

The main reason resistors get hot

Ok, so we have learned a bit about current, resistors, and their power ratings. With this knowledge in hand we can take a look at the main reason why resistors get hot.

Earlier we saw how electrons flow through a conductor (which would be the same if we were to take a look at how electrons flow inside a resistor). 

The main reason that resistors get hot is that when current passes through it energy is lost in the form of heat. 

This happens due to friction as electrons bump into each other, as well as the conductor resulting in heat. 

But, the resistor shouldn’t be very hot to the touch under normal conditions (if its power ratings are not exceeded).

The main purpose for power ratings is to tell us the maximum capacity of current (electrons) components like a resistor can handle for a fixed area. 

The higher the current that the resistor is subject to, the higher the number of electrons that will start to flow.

However, the resistor’s physical area remains the same, but now has more electrons flowing through it, and with less space to do so. As you can imagine there is going to be more collisions between electrons resulting in more friction which leads to the resistor getting hotter. 

The diagram below shows an increase in current (which results in an increase in electrons) for the same given area within a resistor. 

Also, there is a phenomenon known as Thermal Runaway, which describes a scenario where an increase in temperature dissipates energy (in the form of heat), further causing an increase in temperature. 

What happens to the resistors when they get too hot?

In any electrical and electronic circuit, components and devices getting too hot is not wanted. The same is true for resistors.

There are a couple of things that can happen to a resistor if it gets too hot.

Resistance of resistors change if it gets to hot

The first thing that can happen if a resistor gets too hot is that its resistance can change. 

When it comes to resistors, there is an important parameter which is known as the Temperature Coefficient of Resistance (or TCR).

TCR shows us the change in resistance for any given material (such as a conductor) per degree of temperature change. 

Every material has its own TCR value. 

The resistance increases as the temperature increases regardless of the material as well as the fixed length and area of the resistor (but, some resistors are more resistant to changes in temperature than others).

This happens because the atoms within a material get excited as the temperature increases. This causes the atoms to move about more hastily, making it harder for electrons to get past.

The TCR of a resistor is usually affected by ambient temperature. However, if a resistor is housed within an enclosed area, the heat given off by the resistor as it gets too hot can increase the ambient temperature, thereby increasing its resistance. 

Resistor can get  physically damaged if it gets too hot

The next major thing that can happen is that the resistor can get damaged. 

A resistor is constructed using specific materials that give the resistor its abilities. However, these materials are capable of handling a certain range of temperatures. 

Exceeding these temperatures will damage materials of the resistor on the inside as well as outside (the body)

Do all resistors dissipate the same amount of heat

The amount of heat that a resistor will dissipate depends on the physical size of the resistor, as well as the power rating (which is largely dependent on its size too).

A smaller resistor will get hotter faster due to its lower power rating. However, the temperature might be lower also due to its power rating and physical size.

Whereas a bigger resistor (with a higher power rating) can withstand much more power and not get hot till subject to higher currents compared to smaller resistor. 

How to stop resistors from getting hot

We know that resistors get too hot if too much power is supplied to them (the power is higher than their maximum rating). 

One of the easiest ways of stopping a resistor heating up is by limiting the amount of power it is subject to.

There are many equations for power, but below are some of the most common.

To limit the power to a resistor, you can either reduce the voltage or current. 

Other ways to stop a resistor heating up are to use Heat Sinks or a Fan.

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The main reason is to create an electrical/mechanical bond between these two metals (or more) to allow electricity to flow.

However, soldering is not limited to just electrical and electronic applications. It can be used for sheet metal, as well as Jewelry and stained glass work.

Solder is the metal alloy material that is used as the ‘glue’ that creates the bond between the two pieces of metal. 

But, why do soldering irons get hot? Soldering irons get hot in order to melt the solder which is used to create an electrical and mechanical bond between two pieces of metal. The solder when not heated is a hard substance which will not be able to adhere to the metals being connected together. So, it needs to be heated until it melts with the help of a soldering iron. 

Main reason why a soldering irons get hot

I briefly mentioned above why a soldering iron gets hot, but let’s take a deeper look at what is involved with the process of soldering, soldering irons and solder.

Soldering is synonymous with Electrical and Electronic applications. 

It can be used to;

• Solder components onto a Printed Circuit Board (PCB)
• Connect two pieces of wire together
• Connect components together
• Fix a broken PCB tracing 

These are just some of the many uses of a soldering iron with electrical and electronic applications. 

The main objective as we mentioned earlier is to create semi permanent electrical and mechanical bonds since electricity requires a conducting material to move through. 

I say semi permanent because it can be reversed using a process known as Desoldering.

Desoldering involves using a soldering iron and either Solder Wick or a Desoldering Gun to remove solder and thereby removing the bond. 

This can be helpful if you make a mistake (and many will be made), as well as when components that have failed and need replacing.

What is solder

Solder is a vital component and is the main reason soldering irons get hot. 

There are many different types of solder available that come in a variety of metal alloy compositions. 

Below are some of the most common solder compositions;

• Tin-Lead (Lead based)
• Tin-Silver-Copper (Lead free)
• Tin-Antimony
• Tin-Copper
• Tin-Silver 

However, each of these different compositions of solder does not have the same melting point.

 Due to their chemical makeup, and metals used, they vary in temperatures at which they will melt. 

Different types of soldering

There are typically three types of soldering processes used; Soft soldering, Hard soldering, and Brazing.

Soft soldering – this type of soldering involves using solder with the lowest melting point and which are typically alloys. Melting temperatures can range from 90°C (194°F) – 450°C (842°F). 

Hard soldering – in this process, Brass or Silver are the metals that are used as the solder to create bonds. The melting temperatures involved in hard soldering range from 450°C (842°F) – 600°C (1112°F). Blowtorches are sometimes used to reach these temperatures if a soldering iron is unable to do so. 

Brazing – Solder of much higher melting points are used in this process compared to the other two. Temperatures can be higher than 450°C (842°F). 

What is a soldering iron

Without a soldering iron, solder would be useless. So, while solder is a vital component, it is one half of the picture. 

A soldering iron is the other half that is required that helps unleash the superpowers of solder. 

It is an electrical tool which aids in the process of soldering by providing sufficient heat to melt solder in order to join metals together. 

Just like solder, soldering irons come in a variety of prices, shapes, sizes, temperature capabilities etc.

Depending on what type of soldering process you are using, as well as what type of solder you will be using, you will need to choose a soldering iron capable of getting hot enough to melt the solder.

But, the better the quality of the soldering iron, the more expensive it is going to be. 

So, deciding if you need an expensive soldering iron depends on a couple of factors.

If you need help deciding whether you need an expensive soldering iron or not, click here.

However, you want a soldering capable of a range of temperatures as we now know that different solders have different melting points. 

Cheaper soldering irons tend to have one temperature and low wattage which do not get hot enough to melt solder.

Other applications where soldering irons get hot

While its main use is in the electrical and electronic field, a soldering iron is not limited to just them. It has many other applications.

These include;

• Roofing
• Metal Gutters
• Auto Repair
• Jewelry
• Desoldering
• Plumbing
• Stained glass and Mosaics
• Wood burning

With applications like roofing, auto repair, plumbing, jewelry, etc solder is still used so the soldering iron needs to get hot in order to melt the solder.

With wood burning letters are engraved on wood using the soldering iron. Trying to do this with a cold soldering iron is near impossible.

So, again the soldering iron needs to be hot to be able to engrave on wood. 

What is the right temperature to set the soldering iron to get hot?

As you saw earlier, there are different types of solder as well as soldering processes which have a range of temperatures at which the solder melts.

So, determining how hot a soldering should be set comes down to the solder you are using. 

The ranges of temperature at which they melt should be included in the solder’s datasheet which can be obtained from the manufacturer’s website. 

Below is a snippet from a solder’s datasheet. As you can see, it specifies the temperature range at which you can set your soldering iron to get hot enough to melt the solder. 

If there is no information available, and the solder you are using is a metal alloy, 360 – 400°C is the general temperature to work with. 

How hot can a soldering iron get ?

This all depends on the complexity of your soldering iron. 

There are typically two types of soldering irons; ones that come as a single soldering iron, and the ones that come as a station with temperature control. 

The first type (just the soldering iron) tends to only have one temperature setting (on occasions you can get up to 3 temperature settings). 

The temperature output is largely determined by the wattage of the soldering iron. 

A soldering iron station will have temperature control which allows you to set a range of temperatures. 

Again, the temperature range will vary from brand to brand. 

How long does it take for a soldering iron to heat up?

Again, without sounding like a broken record, the time it takes your soldering iron to get hot will depend on the soldering iron itself, as not all soldering irons are the same.

The bigger brand soldering stations will get hot in about 20 – 30 seconds. 

Whereas, the soldering irons that are cheaper and have no temperature control can take much longer as they are unregulated. 

If you want to be sure of how long it takes, you can always use a timer to see how long it takes from when you turn on your soldering iron to when it starts melting solder.

How do you tell if the soldering iron is hot enough?

The last thing you want to do is burn yourself because you were trying to see if your soldering iron is hot enough. 

There are other ways to go about testing whether it is hot or not.

• If you have a digital display it should indicate when it has reached the set temperatures (some soldering irons have an LED indicator)
• Wet sponges are used to clean the soldering iron tip when soldering. Tapping the wet sponge should produce a ‘hissing’ sound indicating it is hot. Also, vapour should be visible.
• Touch solder to the soldering iron to see if it melts. However, if it takes time to melt and only melts a little bit, the soldering is not hot enough. It should melt instantly and turn to ‘liquid’.

What if your soldering iron is not getting hot?  

If for some reason your soldering iron is just not getting hot, there are some potential reasons as to why it is not getting hot. 

Check this article for more information 6 reasons your soldering iron is not melting solder.

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