The zener diode connected to this speed-sensor output ensures TTL compatibility.įigure 3c. This speed-sensor output is an open collector with a weak pullup resistor, and not necessarily TTL-compatible.įigure 3b. Any changes in supply voltage to control the speed of the fan will also affect the commutation electronics and the speed/alarm sensors.įigure 3a. It is important to note that the alarm and speed sensors share the same supply voltage as the motor and its commutation electronics. Figure 3 shows the output stages available from ebm-papst Inc.
#12 VOLT DC FANCONTROL FULL#
Internally pulled-up outputs can be TTL-compatible or can swing the full supply voltage of the fan. Depending on the manufacturer and the options offered, both speed and alarm sensors can be ordered with either open-collector or internally pulled-up outputs. The most common speed sensor gives two pulses per revolution. Manufacturers also offer fans with speed sensors that give a digital output whose frequency is proportional to the fan speed. NMB Technologies offers a slightly different option that it calls a "Locked Rotor Alarm Signal." This signal goes high whenever the fan stops spinning completely.
#12 VOLT DC FANCONTROL SERIES#
offers an option that generates a series of low-going digital pulses whenever the speed of the fan drops to 75% to 85% of its nominal speed. Alarm sensors typically give a digital signal indicating that the fan has fallen below some threshold of speed or has stopped altogether. These options fall into roughly two categories: alarm sensors and speed sensors. Most fan manufactures offer a variety of ways to do this. This is why it can be important to continuously monitor the condition of the fan. Over time, fan speed and therefore cooling efficiency can slowly degrade or fail completely. Fan speed versus fan voltage (12V-rated fans).Īlthough brushless commutation has gone a long way in increasing the lifetime and the reliability of fans, they are still mechanical devices and prone to mechanical wear and failure. Fan current versus fan voltage (12V-rated fans).įigure 2. To a first order, speed and current are directly proportional to the DC voltage applied. As the DC voltage applied to the fan is varied, its speed and current draw also vary. To the end user, DC brushless fans are fairly simple to characterize electrically. This has greatly increased the lifetime and the reliability of these fans. The end result is a simple-to-use, reliable, 2-wire device. This commutation circuitry is mounted within the fan itself and is totally transparent to the user. Brushless fans have replaced these mechanical brushes with electronic sensors and switches that now perform the necessary commutation. Over time, the fan would wear and eventually fail. Older DC fans used mechanical brushes, which can cause increased electromagnetic interference (EMI) along with dust particles due to mechanical wear throughout the system. In telecom applications, 48V fans are especially popular.īrushless DC fans are called "brushless" because the electric motor within the fan is commutated electronically. As more systems are designed without a 12V power supply, 5V fans will likely become more prevalent. Currently, 12V fans seem to be the most widely used.
A quick glance at fan catalogs reveals that fans operating at a nominal 5V, 12V, 24V, or 48V are available. The simplest approach to system cooling is to connect a fan to a DC power supply and let it run. The basic DC brushless fan is a 2-wire device over which a DC voltage is applied. These fans couple high reliability with ease of use. Brushless DC fans tend to be the solution of choice for most electronic enclosures. Fan speed control and monitoring can ease some of these headaches, resulting in quieter, more reliable fans that use less power.īefore we get into the subject of regulating and monitoring fans, we first need to understand the fans themselves. Unfortunately with fan use comes the usual fan headaches of mechanical failures, increased power consumption, and more noise.
For many applications, this means using fans. A similar version of this article appeared in the Septemissue of EDN.Īs IC designers strive to put more transistors running at higher speeds into smaller packages, there can be only one outcome: heat! Couple this with the fact that these high-power ICs are being designed into ever-shrinking boxes, and you end up with a real thermal management problem.
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