Cooling devices: heat sinks and fans HEAT SINKS FANS

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Preface

To ensure a proper function of a system, keeping the temperature of all devices within their working values is very important. Since a common working range of temperature is about from -20°C to +85°C, the lower limit is generally negligible. On the contrary, the higher level could be a problem since all the devices, some more and some less, tend to increase their inner temperature. In certain situations, especially in power circuits, it is necessary to use some cooling devices in order to cool down the hot ones and make the environment well ventilated.

TEMPERATURE PERCEPTION

If you keep the temperature low in your system, it will work better and longer. But, what does “low temperature” mean? Probably if you touch something at 50°C you would say: it’s boiling! In fact, in human perception, everything over 45°C feels hot. The important thing therefore, is to make sure that every component always remains within its working temperature, even if seems that its gone over your perception of cold . So now, don’t worry if you are not able to touch the heat sink of your computer’s µP because it is too hot, read its temperature on the BIOS monitoring section and sigh with relief. :-)

 

Heat sinks

Fig.1 - Example of heat sink

Fig.1 – Example of heat sink

A heat sink is a metal plate, usually a good thermal conductor like aluminum or copper, shaped to dissipate as much heat as possible. You can find lots of different types of heat sinks on the market like that one seen in figure 1.

The cooling capability depends not only on the size of the heat sink but also on the type of the material of which it’s formed and by it’s shape. Copper is better than aluminum thanks to its better thermal conductivity. Regarding the shape, there doesn’t exist a fixed rule. Companies are continuously studying different solutions to increase the efficiency and that’s why there is such a huge variety of them on the market.

Fig.2 - The air through the ducts

Fig.2 – The air through the ducts

Figure 2 shows a drawn detail of  the heat sink in figure 1. The U-shapes form the ducts that allow the air, that should be fresher of the heat sink temperature, to easily pass through it and then cool down the component installed on it. For that reason it is better to place the heat sink in the open air (outside the system box) or in a well ventilated environment.

The holes, clearly visible in the heat sink in figure 1, are necessary to screw the components on it. In particular this heat sink is suitable to install two transistors with TO-3 package like the famous 2N3055.

INSTALL A COMPONENT ON A HEAT SINK

Fig.3 - Non insulated component

Fig.3 – Non insulated component

Fig.4 - Insulated component

Fig.4 – Insulated component

The major part of the power components like transistors, ICs, diodes, etc., are made to be installed on a heat sink by one or more screws. If you are installing a single component on a heat sink, the task is elementary: use screws, washers and bolts as shown in figure 3.

The situation gets more difficult if you need to install two or more components in a single heat sink. The metallic part of the package is generally electrically connected to one pin of the component. If you install all the components directly to the heat sink, you will connect all these metallic parts together creating a short circuit. To avoid that, it is necessary to interpose a mica insulator and a special washer (available in kits) between the component and the heat sink. In this way it is possible to isolate every component electrically, but not thermally (figure 4).

INCREASE THE PERFORMANCE

The first trick consists in putting some thermal grease between the component and the heat sink. It is enough to spread a film of it to make the component perfectly adherent to the heat sink and improve the thermal conductibility.

Another solution is to transform the passive heat sink in an active one. Since it’s the air that cools the heat sink down, if you install a fan on it it will increase the airflow through the ducts. In that way you have the advantage to reduce the size of the heat sink, but a fan involves some disadvantages:

  • the noise. It could be very annoying, it depends on the quality of the fan. This problem could be partially overcome with a speed control circuit that self-regulates the speed, and hence the noise, according to the temperature of the heat sink. The noise is not only audible, electrical noise is also produced;
  • durability. All the mechanical devices have a limited lifetime and in case of failure it might lead to the damage of the component. A proper circuit to monitor the fan-speed or the temperature of the heat sink could save the situation.

The two solutions explained before can be combined to obtain the best performance of a heat sink.

 

Fans

Fig.5 - Example of fan, 80x80x25 mm size

Fig.5 – Example of fan, 80x80x25 mm size

I don’t think it is necessary to explain what a fan is. In addition of the use in a active heat sink as explained just before, fans are useful to make a well ventilated environment. If you have enclosed some PCBs in a box, some air inlets might not be enough to ventilate them properly. Indeed, the sum of the temperature from every single component, could trigger a chain reaction that increases the temperature of the environment until it damages some components and then the device.

Externally fans look quite similar to each other but the technology inside is very different, for this reason it is important to read the characteristics carefully.

PARAMETERS

Choosing the right fan is very important, there are several parameters to consider when you buy a fan. I list them below:

  • Size. Common diameters are: 40mm, 50mm, 60mm, 80mm, 90mm, 120mm, 140mm, and bigger. The height usually changes according to the diameter: 10mm, 25mm or 40mm are the most used. The bigger the fan, more is the airflow.
  • Voltage. Normally they require a DC power supply, standard voltage are 12V or 24V. These voltages might be considered as maximums, the producers sometimes write the start-up value, that is the minimum voltage that is required for the blades to start to turn.
  • Speed. It is written in rpm (revolutions per minute). The faster the fan, more is the airflow but usually the bigger the size, less is the speed.
  • Noise. This is the parameter that probably most affects the price. Its unit is the dB: 10dB (decibel) is a very low value, probably inaudible, values over 20dB could be quite annoying.
  • Air flow. In m³/h corresponds to how much air is moved in one hour. It is the most important parameter.
  • Power consumption. Measured in W (watt), it is important to calibrate the power supply for the fan.
  • MTBF. Mean time between failures, is about reliability expressed in hours.

INSTALL THE FANS

Fig.6 - Rubber mounts

Fig.6 – Rubber mounts

It is very simple to install a fan. You can screw them, even though I suggest you add some rubber washers to dampen the vibrations a bit. Indeed, the box where a fan is installed on, tends to amplify the noise generated by the vibration, which is produced by the rotation of the fan blades. Instead of screws, you can buy rubber mounts that solve the vibration issue quite well (picture 6).

The rotation and the airflow directions are indicated by two arrows on one side of the fan.

Finding the right collocation of the fans it is not always easy. The goal should be the situation displayed in picture 7. It is necessary that all the airflows from the fans are in accordance with each other. From one side of the box the fresh air is sucked, it becomes warmer passing through all the components and escapes from the opposite side.

Pic.7 - Best placement for optimized airflow through a device

Pic.7 – Best placement for optimized airflow through a device

 

If you need some more explanation or if you want to discuss this post, please do not hesitate to contact me or leave your comments below.

 

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Grazie di cuore, Andrea Dal Maso

 


 

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