This month in The Classroom, we’ll look at thermodynamics, and how to choose and install the correct heatsink for your project.
Many electronic products and projects fail for the simplest of reasons; Overheating due to inability to dissipate so much wasted energy!
Just as a marathon runner is limited by their capacity to perform under the stress of heat, so are electronic components. The amplifier and LCD TV in your home will need heatsinks to dissipate heat, for example. The next time you walk past your LCD TV, put your hand over the top to feel how much heat is coming from it.
HEAT WARMS THE HEART
The heat generated by electronic circuits is almost certainly wasted heat, that does nothing but shorten the life span of components. Waste heat is the other measure of efficiency. When a claim is made that a device is 60% efficient, that also says that it is 40% inefficient; or if efficiency (η%) = 60%, inefficiency = (1 – η) x 100% = 40%.
It is the inefficiency that designers must deal with, and manage. An Analogue Class A Amplifier rated at 100 Watts, is only about 30% efficient. (For the purists, 25% to 50% depending on configuration, allowable distortion and potential use of a transformer).
So an amplifier rated at 100W power out will need to get rid of ~200W of wasted energy.
What is more likely, is that the 100W amplifier is actually taking 100W of DC Input, the energy taken from the supply, giving you 33W audio out and wasting 67W in heat generation.
Before you choke on the wasted energy, and how your 100W amp may only give you 1/3 of the power you thought it would, we must tell you that most analogue Hi-Fi amplifiers were Class AB, or perhaps Class B which are more like 50% efficient. Modern amplifiers may be Class D, or 'Digitally Amplified', with an efficiency much higher, resulting in a smaller and lighter amplifier, generating less heat and wasting less power.
When there is waste heat, the heat must be removed from the component to avoid the component failing, by “boiling its guts out”, i.e. having a meltdown!
Heatsinks are an example of temperature control that reduces the temperature of one location, e.g. an electronic component, by removing the excess energy from that device to some place where it will not cause damage. Such heat flow requires a temperature differential, a heat path, and a thermal conductor.
Physics classifies this process under the science of Thermo-Dynamics; aka 'Thermodynamics'.
Therefore, The Classroom this issue will introduce you to some basics of the science of Thermodynamics, as it applies to Heat Transfer in Electronics.
Typically, the plan is for a component that generates heat through poor efficiency, to have a good thermal conductor, e.g. Aluminium or Copper, attached to the component to soak up the energy, just as a sponge soaks up excess water.
The energy is then transferred away from the device, perhaps by spreading it out over a much larger area where it can be transferred to the atmosphere.
In heat systems, it is all about heat energy (EH), quantity of energy (Q), Energy Flow Rate (Watts, W=Q/T), Thermal Potential Difference (T°1 - T°2) between Energy Reservoirs (Masses, M1, M2, etc.), and Resistance to Energy Flow called Thermal Resistance (k).
The 'System' itself consists, as electrical systems do, of a Source (of Energy), a Path (taken by the energy), and a Sink (where the energy goes). The sink in electrical circuits is often called the 'Ground' but in Thermal Circuits, the sink is the 'ambient' meaning the reference temperature of the local environment. We know we could discuss Absolute Zero or -273°C = 0 Kelvin, but let's keep it simple.
Energy is defined as the capacity to do work, and Heat Energy is energy stored or transferred as heat. For our purposes, we will avoid going into how heat energy is transduced from or to another form of energy, but put simply, the potential energy in electricity is converted into heat potential energy.
Heat energy is transferred differently within different materials. The process called 'Conduction' refers to energy transferred within a solid or liquid, or even a gas, where energy is passed at an atomic level, from atom to atom.
The process called 'Convection' refers to energy transferred when energetic atoms or molecules flow around within a non-solid material, i.e. within a liquid or a gas. Energetic molecules reflect off one another causing the material to expand and float above less energetic molecules, and return to lower levels when the energy is passed away.
The third process, called 'Radiation', refers to energy transfer when atoms give off photonic, electromagnetic energy, that pass readily through some solids, liquids, gas, or even a vacuum.
All three methods of transfer are commonly used, although Conduction and Convection are the methods we are more concerned with when dealing with heatsinks.
2ND LAW OF THERMODYNAMICS
Wow! Doesn't that sound impressive, but thankfully we only need to understand one aspect of the Second Law of Thermodynamics. "Energy is neither created nor destroyed, but can be converted from one form to another." also meaning that the energy within a system remains constant. In our use, concerned as we are with keeping components cool, whatever heat energy we create from our electronic circuits needs to be removed to the extensive 'ambient' reservoir provided by the Earth, or the whole universe if you like.
SIMPLE HEATSINK EXPERIMENT
Here’s an easy experiment to conduct to allow your kids or students to see and hopefully grasp what was actually happening, and therefore allow them to apply their theories to practice. The fairly simple experiment, shown below, is a simple 10W power resistor with a heatsink attached.
The simple aim of the experiment is to show the cooling effect of a heatsink, but the extended aim is to recognise and identify the thermal path, as shown in the figure above, and to measure temperatures at the various nominated locations.
The easiest method is to take cold measurements, below 25°C and later to take hot measurements when equilibrium is reached, however, as we now have the advantage of using our Arduino, or Raspberry Pi's as data loggers, and laptops/tablets etc as a datalogger/plotter, we can plot a graph of temperature rise versus time, and compare the curves with and without the heatsink attached.
With a bit of maths, we can even calculate the heat flow in the aluminium strip, if there are more curious students amongst the class.
The typical ceramic bodied 10 Watt resistor is 50mm long x 10mm square (roughly). The circuit shown uses a 12VDC source and a 100 or 120Ω 10W resistor, but the actual values can be chosen by the teacher/experimenter, according to what is on hand or available. For example, a ceramic extruder heater element could be used to reference the resistor's use in a 3D printer.
Note: An on/off switch is optional, but recommended for young experimenters.
The simple heatsink is a length of readily available aluminium strip 10mm by 3mm, cut about 100mm longer than the 50mm x 10mm body of the ceramic bodied resistor. i.e. a 150mm length of aluminium strip is fine. The strip should allow for two temperature sensing locations 100mm apart, where thermometers, thermistors or thermocouples can be placed.
The experiment can be extended in many directions, ending up with a working power supply or amplifier if you are keen. However, consider simple steps such as using a different heatsink, heatsink paste, mica insulator, fan cooled, etc. As used in an actual application.
COMPONENT TO HEATSINK TRANSFER
Components that are suitable for Heatsink attachment should have a flat attachment surface, however, round (cylindrical) components are also suitable for heatsinking, as long as the area is in intimate contact with the body of the heatsink.
Surfaces that are rough or have insulating coatings may be unsuitable for heatsinks, as a thermal connection may not be made.
Some surfaces can be cleaned with steel wool, brass or steel wire brush, etc. as long as the hobbyist sets out to make the best possible meeting of the two surfaces without destroying protective coatings.
The following diagram represents a close-up view between a component and heatsink. Using thermal compound or a gasket can improve the heat transfer.
Some components need to be isolated electrically from the heatsink, or more specifically from ground. Therefore, one method is to insulate the whole heatsink, which requires it to be at a safe voltage or 'untouchable'. The more common method is to use an insulating washer or gasket between a component and the heatsink.
In recent history, a mica insulator was used, being a good electrical insulator and usable for high temperatures. Mica is one of those materials we no longer use through the dangers of ingesting the mica dust, but be aware that the material is still out there on old equipment so handle it carefully, and take care in disposing of the material.
Thermal film is one common replacement, being very thin and flexible, and having good electrical insulating qualities as well.
There are also Silicon Rubber Insulators, usually grey in colour, that allow the heatsink as well as the component to mould their shape more intimately to the insulator and pass heat more efficiently.
Also known as Thermal Compound, this is used to provide a thermally conductive compound to fill any space between component and heatsink, or the insulator of course.
Some greases/compounds/pastes contain powdered metal, even Silver, which seem to be safe depending upon what holds it as a paste! Some older white thermal grease contains ceramic powders, and some use Beryllium Oxide, some use Silicon Dioxide, etc.
A simple precautionary procedure is to try to only use chemicals you are sure are safe, know the constituent chemistry, avoid touching with your hands, and make sure you wash thoroughly before and after using them.
THE ACTUAL HEATSINK
To be useful, any heatsink must be better than ambient airflow at removing heat from the component, yet ultimately the heatsink is limited by the amount of heat energy it can, in turn, dissipate to the ambient air.
If you remember how an electrical resistance works, the resistance is determined by the 'resistivity' of the material, the length of the path the current must take, and the cross-sectional area of the conductor.Thermal Resistance is determined by essentially the same parameters which we now look at in a little more detail.
Metallic materials have the best thermal 'conductivity' just as they have the best electrical conductivity, remembering that conductivity = 1/resistivity.
Using the following table, Thermal Conductivity of common metals is expressed in the power transferred along one metre of the material for a temperature differential of one degree Centigrade or Kelvin (Remember 1°C = 1 Kelvin).
Note: The Quantity of Heat Energy is not given and neither is the area of application.
While ultimately, the best heat transfer is available using a Diamond or Silver heatsink, most heatsinks are made of Aluminium or Copper. I guess we can imagine why!
|MATERIAL||Thermal Conductivity (W/m K)|
Compared to Air, at 0.024W/m K, Aluminium is an 8500 times better conductor of thermal energy, so the length of the path is almost negligible in the calculation, because the heatsink must in almost every case, eventually connect to the Air.
AREA OF THE CONNECTION
The transition between the component and the heatsink is typically metal to metal, probably Copper to Aluminium, possibly with a thin gasket between the surfaces. On the other hand, especially at the heatsink/Air transition, the area is very important.
Spreading the resistance over a very large area is the reason heatsinks are typically made up of a gang of fins all stemming from a thick base block to which the component/s is/are connected.
HEATSINK AND FIN DESIGN
While you will usually only have the opportunity to select between a number of available heatsinks, engineers may be required to design a heatsink to fit within pre-defined dimensions. They may have a set requirement of mounting positions for components, and fixed access to the environment, i.e. the ambient air.
At the transition between the two, the maximum area will only work if the heat can get to and from that area. Getting to the surface of the heatsink is made relatively easy because of the 8500 advantage of Aluminium compared to the air, but it will only work if there is airflow, either natural airflow, forced airflow, or airflow created by the heat itself as it creates convection.
Therefore, fin design must be optimised for many parameters.
Fins are most commonly parallel to one another, being spaced at least three fin thicknesses apart, and often much more, being very thin for weight requirements as well.
The space between the fins depends almost entirely upon airflow; natural convection needs plenty of space to allow the air to pass slowly and perhaps turbulent to present any cool air to the hotter fins. Fins may be arranged radially to enhance the passive airflow.
Forced airflow, induced by a fan, or the motion of heatsink itself, tends to be more compact in an attempt to transfer as much heat as possible to the airflow.
Fins don't always fan out from the heatsink, and may in some designs fan inwards (inverted fins), within a cavity or air tube, or air duct. The duct may be constructed for example in a square tube of dimensions suitable to fixing a standard fan, (95mm computer fan, for example), and possibly one at each end of the tube to both push and draw air through the duct, between the enclosed fins.
While plain Aluminium fins will work well with forced airflow for conduction/convection thermal transfer, heatsinks designed for passive use (non-forced airflow) may be coloured matt black. This is done by anodising Aluminium to form a thin oxide layer, and dying the anodised oxides black. The black helps dissipate heat by radiation, as a Thermal Black Box. Most other colours are purely for fashion or trade identity only.
Please remember that an insulating layer will hinder the heatsink from doing its job, and that includes painting the heatsink. Never paint a heatsink.
As we seem to be even more determined to compact as much computing power as possible into the tightest confinements, computer manufacturers have resorted to using Heat Pipes, copper tubes filled with a heavy gas or liquid to transport the heat from its source to the fins for fan cooling.
Yet other schemes use water cooling in the same process, as water is about 25 times better than air as a conductor of heat.
While Airflow has been covered already; relating to fins and forced versus passive, etc., there are some other considerations that are often forgotten, in the design stage and the selection stages.
A heatsink for a box that sits on a bench must have airflow that draws in fresh air and flows away as heated air. The options can be simple, but mistakes can be made. For example, a box that sits flat on a surface has airflow blocked from below by the bench surface, unless the box is intended to overhang the back of the bench, but then what if the user isn't aware of that need.
A box may have been designed to sit flat, as desktop computers once did, but what about somebody suddenly deciding to sit it on one side? The issue may arise that the fins that were designed for vertical airflow are now horizontal.
The box designed with an internal heatsink cooled by rising airflow from beneath the box may find itself sitting on someone's bed, or rug thus blocking off airflow. All of these issues can be forgotten during the design stage, if the designer only thinks of how they intend the box to be used, rather than how it might get used.
An electronic appliance designed in one place, and tested in that place, is not necessarily suited to another place or application. For example, a device designed for use in one climate or environment may not work in another.
Temperature can vary greatly depending on where you live. For example. our writer Bob lives in a hotter part of Australia and had an issue with motor start capacitors on his home water pump. The capacitors were rated for 42°C intermittent operation, and it gets to that temperature in Australia in the open air at times. Therefore, when mounted inside an electric motor, inside a weather resistant box, the capacitors regularly melted, oozing dark grey matter and no longer operating. They needed to be replaced with better quality, high-temperature capacitors.
Quite a few switchmode power supplies are also designed for cooler temperatures, using lower temperature rated capacitors, e.g. 85°C, when 105°C capacitors would be a better choice for Australia.
The people who designed the European made pumps did not imagine they would be used at an ambient temperature of >40°C. Nor do many electronics designers when choosing a heatsink, and therefore, the heatsink in your favourite piece of equipment may be undersized for our Australian heat!
Air gets thinner with altitude, and although we are not blessed with high altitudes here in Australia, heatsinks, like humans, have difficulty as the air gets thinner. Some equipment designed for use on boats, or in cars, may find itself being used in aircraft, and in a confined space to boot!
Heatsinks are not only attached to components, but to the box containing the components, and in some cases are quite heavy. Therefore, there is a need to think through the design, perhaps using the heatsink as a part of the case.
Mounting of components, as well as the heatsink itself, can require drilling, threading, and other machining operations for mounting hardware such as antenna sockets or mounts, power entry connectors, etc., all of which must have a place to mount, without affecting the thermal design of the heatsink.
The mounting screws may need to be insulated by plastic bushes and washers, which require an oversized hole to insulate the bore from the screw.
Let's look at using a heatsink from a catalogue, in this case for a TO-220 Transistor or Regulator mounted on a heatsink, part number HH8516, from the Jaycar catalogue.
|Heatsink Mounting Method:||PCB|
|Heatsink Fins:||2pc (Mounted by two pins)|
|Heatsink Thermal Resistance:||19°C/W|
|Component Mount Area Height:||16mm|
|Cooling Fan Mount Size:||21.8mm|
The heatsink is intended to be mounted to a TO-220 cased component, though other uses can be made of the heatsink. While not seen in the image, there is likely to be a mounting hole so the TO-220 device can be fastened to the heatsink with a suitable screw, possibly a self-tapping screw.
This heatsink is Black Anodised Aluminium to transfer heat as efficiently as it can to the ambient environment, which we will assume to mean 25°C in still air. i.e. Airflow is only generated by the convection currents caused by the heatsink heating the surrounding air.
Orientation, therefore, should place the fins vertical, such that airflow rises between the fins. The mounting pins, however, mount the heatsink flat on the PCB, by design. Efficiency would, therefore, be improved if the fins overhung the side of the PCB, or a hole was designed to be cut through the PCB under the fins of the heatsink. The PCB might also supply added heat-sinking capacity if the pins were soldered to a copper plane, such as the earth plane.
In clear still air, the heatsink is rated at 19°C/W, so for every watt of heat generated by the component, the temperature of the heatsink is expected to rise by 19°C.
For an LM7805 regulator running on 9V DC, 4V DC must be wasted as heat. Therefore; for the maximum 1 Amp of load current, the heat generated will be 4 Volts times 1 Amp, or 4 Watts.
The temperature would rise from 25°C to 25 + (4 x 19) = (25 + 76)°C = 101°C, enough to boil water. The LM7805 has a maximum recommended operating temperature of 70°C.
That leaves us with a number of options; reduce the load current, reduce the overhead voltage, use a larger heatsink, add more/better airflow, or change to a more efficient switch mode regulator!
Alternatively, working backward; for a maximum temperature of 70°C the maximum allowable temperature rise is 70 - 25 = 45°C. Therefore, the maximum continuous load is limited to 45/19 = 2.37W, so for a 4V drop, the load current should be kept below 2.37/4 = ~590mA.
Larger heatsinks are obviously available, but space to mount it may not be.
MEASURING HEATSINK THERMAL RESISTANCE
If you find yourself with an existing case or heatsink, but without any other information to determine the suitability of what you have for what you intend to do with it, you might consider a small experiment similar to the one we mentioned earlier.
Essentially, a transistor or resistor can be mounted on the chosen heatsink, or case, and a known, but safe current passed through the device while both current and voltage are measured and kept constant. The temperature of the device at the case should also be measured over time.
The setup should typify the intended final setup as closely as possible, including ambient conditions and any additional cooling.
Then the setup should be run from a cold start (<25°C) through to the maximum safe component temperature, e.g. 70°C for an LM7805 regulator, and many common transistors.
The effects of any ventilation or ventilation restrictions can be assessed, and in compliance testing, the ambient temperature can be altered to test performance at 40°C room temperature, for example.
In effect, torture the poor electronic device but make sure it never goes outside of it's specified operating range.
Immersion In Liquid
Heat has long been a huge recurring problem for electronics, as we have discussed here. What is interesting, however, is when we start immersing electronics in liquids.
Now I know what you're thinking... electronics in liquids??? Have you lost your mind???
Well, that's up for debate - however it's entirely feasible to immerse electronics in liquid. As long as that liquid doesn't conduct electricity!
Unfortunately, as you may expect, this rules out water. However oil, in various forms, is actually quite feasible to do. In fact oil cooling has been used for electronics for a very long time in certain applications. High voltage transformers, certain types of electric motors, and many other items use oil immersion to handle cooling.
When we immerse electronics (or at least the parts that get hot) in liquid, we see a huge increase in heat extraction, and depending on the device. This is largely because we're making contact on every exposed surface with our liquid, which is extremely efficient at extracting heat. It's virtually impossible to achieve this with traditional heatsink approaches.
Naturally, oil cooling isn't entirely without drawbacks, which is likely why it isn't common in regular consumer electronics. A few of these are:
LEAKAGE: Just like keeping water in a water tank, the oil must be retained within the unit, which means it must be watertight. This creates a huge challenge for anything with plugs and sockets, knobs, or just about anything.
WEIGHT: Filling something with liquid instead of air means the weight increases drastically. This has implications for freight, and general logistics.
COST: Oil isn't free, however, air essentially is. Manufacturers aren't paying for the air inside their products they make!
IT'S OVERKILL IN MOST CASES: In most circumstances, it's just not necessary and the additional costs would be difficult to justify.
Ultimately, you can actually build a water-tight PC case and fill it with mineral oil, with a surprisingly small number of adjustments. However, what would be the benefit?
WHERE TO FROM HERE?
Once you have a basic knowledge of how heatsinks are rated, and how they are used, you will be in a better position to choose and use heatsinks to allow your circuit components to both perform better and last longer.
Make sure you stock up on heatsink paste, thermal film and/or silicon rubber insulators in your workspace or toolbox.
Always consider how you are going to house your next project before you start assembly so you can choose the best enclosure. The size, ventilation, build material, etc. are all important factors to consider. Also whether it has space for a fan to be integrated, if necessary.