How To Control Computer Fan Speed

[Lu Rounsaville]

Ive been messing with a 12v computer fan recently and need to control it's speed.

I'm currently using PWM from the Arduino with a transistor to gate the 12v line. It's working fine, but when I'm not in 100% duty cycle (sending 255 through the PWM port), the fan start to produce a high pitched noise, that gets higher as I slow down the fan. Why am I hearing that noise and should I be worried about it? Is it because I'm using PWM instead of a regulated voltage?

It would help if we had some idea which fan you were controlling (whether it is meant to be PWM'ed for instance), which transistor you are using (is it capable of switching as fast as the Arduino switches), and how the transistor, fan, and Arduino are wired.

Many fans work by generating there own PWM current pulses. By giving it an external PWM you are in effect chopping of the voltage when it needs it. The result is that the two frequencies beat with one another to produce the noise you here.

Should you be worried about it? It depends on your application. The fan will not last as long in this mode but it is hard to say by how much.

I've used PWM because I couldn't think of a cheap & easy way to output a controlled voltage varying from 0 to 12V using the Arduino, so I'm running a 12V line where the fan and the transistor are wired and I'm connecting pin 11 of the Arduino to the gate of the transistor, so I can have a 12V PWM signal. Here's the basic circuit:

That it should do, the most common computer fan controllers out there are simple resistors to act as a voltage drop, the vast majority of fans will run till at least 7v, most will run till 5v if already turning. Starting takes more voltage though.

Interest has been growing in integrated circuits for controlling the speed of cooling fans in personal computers and other electronic equipment. Compact electrical fans are cheap and have been used for cooling electronic equipment for more than half a century. However, in recent years, the technology of using these fans has evolved significantly. This article will describe how and why this evolution has taken place and will suggest some useful approaches for the designer.

There are many different types of fans and ways of controlling them. We will discuss here various fan types and the advantages and disadvantages of control methods in use today. One way to classify fans is as:

In order to be sure of a correct fan speed reading under PWM control, it is necessary to periodically switch the fan on long enough to get a complete tach cycle. This feature is implemented in a number of Analog Devices fan controllers, such as the ADM1031 and the ADT7460.

Linear control: At the next level of fan control, linear control, the voltage applied to the fan is variable. For lower speed (less cooling and quieter operation) the voltage is decreased, and for higher speed it is increased. The relationship has limitations. Consider, for example, a 12-V fan (rated maximum voltage). Such a fan may require at least 7 V to start spinning. When it does start spinning, it will probably spin at about half its full speed with 7 V applied. Because of the need to overcome inertia, the voltage required to start a fan is higher than the voltage required to keep it spinning. So as the voltage applied to the fan is reduced, it may spin at slower speeds until, say, 4 V, at which point it will stall. These values will differ, from manufacturer to manufacturer, from model to model, and even from fan to fan.

The Analog Devices ADM1028 linear fan-control IC has a programmable output and just about every feature that might be needed in fan control, including the ability to interface accurately to the temperature-sensing diode provided on chips, such as microprocessors, that account for most of the dissipation in a system. (The purpose of the diode is to provide a rapid indication of critical junction temperatures, avoiding all the thermal lags inherent in a system. It permits immediate initiation of cooling, based on a rise in chip temperature.) In order to keep the power used by the ADM1028 at a minimum, it operates on supply voltages from 3.0 V to 5.5 V, with +2.5-V full scale output.

5-V fans allow only a limited range of speed control, since their start-up voltage is close to their 5-V full speed level. But the ADM1028 can be used with 12-V fans by employing a simple step-up booster amplifier with a circuit such as that shown in Figure 4.

The principal advantage of linear control is that it is quiet. However, as we have noted, the speed-control range is limited. For example, a 12-V fan with a control voltage range from 7 V to 12 V could be running at half speed at 7 V. The situation is even worse with a 5-V fan. Typically, 5-V fans will require that 3.5 V or 4 V be applied to get them started, but at that voltage they will be running at close to full speed, with a very limited range of speed control. But running at 12 V, using circuits such as that shown in Figure 4, is far from optimum from an efficiency perspective. That is because the boost transistor dissipates a relatively large amount of power (when the fan is operating at 8 V, the 4-V drop across the transistor is not very efficient). The external circuit required is also relatively expensive.

Another disadvantage of low-frequency PWM is commutation noise. With the fan coils continuously switched on and off, audible noise may be present. To deal with this noise, the newest Analog Devices fan controllers are designed to drive the fan at a frequency of 22.5 kHz, which is outside the audible range. The external control circuit is simpler with high-frequency PWM, but it can only be used with 4-wire fans. Although these fans are relatively new to the market, they are rapidly becoming more popular. Figure 7 depicts the circuit used for high-frequency PWM.

Fan control is the management of the rotational speed of an electric fan. In computers, various types of computer fans are used to provide adequate cooling, and different fan control mechanisms balance their cooling capacities and noise they generate. This is commonly accomplished by the motherboards having hardware monitoring circuitry, which can be configured by the end-user through BIOS or other software to perform fan control.[1]

As modern PCs grow more powerful so do their requirements for electrical power. Computers emit this electrical power as heat generated by all major components. Heat production varies with system load, where periods of compute-intensive activity generate much more heat than the idle time does.[1]

Processors in most early x86-based computers, up to some of the early 486s, did not need active ventilation. Power supplies needed forced cooling, and power supply fans also circulated cooling air through the rest of the PC with the ATX standard. The byproduct of increased heat generation is that the fan(s) need to move increasing amounts of air and thus need to be more powerful. Since they must move more air through the same area of space, fans will become more noisy.

Fans installed in a PC case can produce noise levels of up to 70 dB. Since fan noise increases with the fifth power of the fan rotation speed,[2] reducing revolutions per minute (RPM) by a small amount potentially means a large reduction in fan noise. This must be done cautiously, as excessive reduction in speed may cause components to overheat and be damaged.[needs update] If done properly, fan noise can be drastically reduced.

The common cooling fans used in computers use standardized connectors with two to four pins. The first two pins are always used to deliver power to the fan motor, while the rest can be optional, depending on fan design and type:

The color of the wires connected to these pins varies depending on the number of connectors, but the role of each pin is standardized and guaranteed to be the same on any system. Cooling fans equipped with either two- or three-pin connectors are usually designed to accept a wide range of input voltages, which directly affects the rotation speed of the blades.

In this style of fan control, the fan is either on or off. Temperature inside the chassis is checked, and if an outside-of-range temperature is detected, fans are set to their maximum speed. When the temperature drops below a threshold again, the fans are turned back off. This control method reduces noise issues and power requirements during periods of low usage, but when the system is operating at capacity, the fan noise can become a problem again.

A standard cooling fan is a DC motor with blades attached. By varying the voltage input across the acceptable range for a fan, the speed of the fan will increase (to added voltage) and decrease (to reduced voltage); a faster fan means more air moved and thus a higher heat exchange rate. There are a few ways to perform this regulation, as described below.

Resistors in series with a fan's power pin are the simplest method of reducing fan noise, but they add to the heat generated inside the computer case. Since the voltage drop is proportional to the current, the fan may not start. They need to be of the appropriate power rating. For variable fan control, potentiometers could be used along with a transistor such as a MOSFET whose output voltage is controlled by the potentiometer. It is possible to use a rheostat instead.

A diode in series with the fan will reduce the voltage being output to the fan. A silicon diode provides a relatively constant voltage drop of about 0.7 V per diode; data sheets for a specific diode specify its voltage drop, for example the 1N4001 silicon diode's voltage drop varies from approximately 0.7 to 0.9 V as the current varies from 0.01 to 1 A.[3] The power rating should be noted and some diodes may require cooling to operate at their rated current. The voltage drop across the diode will fall with temperature, causing the fan to speed up.

The voltage a computer cooling fan receives is defined by the difference between the voltage wire (+12 V) and the ground wire (+0 V). By connecting one or both wires to a different voltage, the voltage the fan receives will be different from the default 12 V the fan was designed for.

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