Fan pulse alarm using two stage comparator for speed detection

The alarm circuit uses speed detection so that at a fan slowdown speed below a critical low level, a buzzer alarm starts sounding at a low warning volume and then as the fan slows down further the volume increases; that is, the buzzer volume or the buzzing frequency is inversely proportional to the fan speed. The circuit advantageously can be used for a wide range of fans. A pulse detector of a first stage comparator of this circuit detects the pulses or speed of the fan. More particularly, a power supply isolation resistor with an AC coupling capacitor block the DC voltage levels at the fan so that only the AC or pulse component of the fan is picked up. The isolation resistor prevents the power source from attenuating the pulses. The second stage comparator of the circuit sets the fan speed at which the alarm is to start buzzing and powers the buzzer accordingly. Particularly, the pulses are amplified by the first stage comparator and they periodically discharge a charging capacitor. If the pulses are too infrequent, the capacitor will charge up to the level of an error comparator which will trip and sound a buzzer. Since the circuit uses only discrete components and one (multi-vendor) QUAD comparator for all signal processing including alarm activation, it is inexpensive. It is also very small allowing for greater mounting flexibility.

BACKGROUND OF THE INVENTION 
The present invention relates to devices or circuits for monitoring and/or 
detecting the speed and/or failure of a fan and particularly when the fan 
is used to cool critical electronic components of a computer. 
Many of the prior fan alarm circuit designs attempted to differentiate 
between running current of the fan and "not running" current. However, 
this difference is very small when compared with the difference in either 
current for different sized fans. Thus, a different alarm circuit was 
required for each size of fan and for fans with different bearings such as 
sleeve or ball bearing. Additionally, the inherent motor noise pulses 
required special filtering, because these motor pulses invariably were 
greater than the difference in running versus none running current. 
However, using different circuitry is very expensive and requires 
complicated tracking and logistics to ensure that the proper alarm circuit 
is installed. 
Examples of prior art fan and/or temperature sensors or monitors are shown 
in U.S. Pat. Nos. 4,479,115 (Holzhauer), 4,843,378 (Kimura), 4,977,375 
(Toth), 5,115,225 (Dao et al.), 5,436,827 (Gunn et al.), 5,517,175 (Brown 
et al.), 5,534,854 (Bradbury et al.), and 5,574,667 (Dinh et al.). (The 
entire disclosures of each of these patents are hereby incorporated by 
references.) Many of the prior art alerting systems disclosed in these 
patents have numerous components resulting in high costs and large units. 
Since the units are large, the locations where they can be mounted are 
limited. 
One common use for the fans is to cool hard disk drives in computer 
systems. To efficiently handle larger amounts of data storage, larger hard 
drives have been and are being developed. These drives turn faster, 
generating larger amounts of heat. If the cooling fan for that drive slows 
down too much or otherwise fails, the drives can be damaged if they are 
not quickly shut-off or the fans quickly replaced. The drives may not 
spin, errors in data may result and/or they may not acknowledge requests 
for information. Thus, it is even more critical in today's environment 
that there be a reliable indication of fan slowdown or failure so that 
corrective action can be quickly taken to prevent the problems discussed 
above. Early detection also allows corrective action to be taken before 
the drives are shut down to prevent or minimize loss of data in progress 
or other problems caused to open files. 
SUMMARY OF THE INVENTION 
Directed to remedying the problems in the prior art, an improved fan alarm 
system and circuit are herein disclosed. A first stage of the circuit 
accepts fan pulses which exceed a preset limit so as to be insensitive to 
any noise from the power supply. The pulses are used to reset a voltage 
charging capacitor. If no fan pulses are received, a charging capacitor 
charges to a level which exceeds a preset level at a second comparator. 
This second comparator sets the alarm. When the fan is operating at some 
slow speed, the charging capacitor can exceed the "alarm level" until the 
next fan pulse is received. The alarm is then shut off. A low sounding 
alarm is generated, increasing in volume when the fan slows down more. By 
setting the "alarm level" to be greater than the capacitor charge voltage 
generated by the slowest fan operating at one third to one quarter speed, 
the present circuit allows operation over a wide range of fan speeds. 
Other objects and advantages of the present invention will become more 
apparent to those persons having ordinary skill in the art to which the 
present invention pertains from the foregoing description taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
Referring to FIG. 1, a system of the present invention is shown in block 
diagram form generally at 100. System 100 includes a fan 104 positioned so 
as to create airflow 112 for cooling a device 108. The airflow 112 can 
either be blown towards or exhausted from the device 108. The device 108 
pursuant to one preferred embodiment is an electronic component such as a 
hard disk drive. The fan 104 can be generally any size or type of fan 
which is presently used to cool such components. In other words, the fan 
alarm circuit 116 of this invention has the unique advantage that it can 
accommodate and monitor generally any size fan 104, such as 0.07 to 0.50 
amp or forty or sixty to ninety or one hundred and twenty millimeter fans. 
The construction of the circuit 116 which allows for this flexibility will 
be described in detail later in this disclosure. 
The alarm indicator 120 is a proportional indicator; that is, its signal 
"strength" is inversely proportional to the speed of the fan 104. The 
indicator 120 can be an LED indicator, an input into a computer, a display 
on a computer screen, or preferably an audible buzzer, as will be 
described in further detail. The indicator 120 will typically be remote 
from the fan 104 itself, and may be physically located in another room or 
visible, audible or otherwise detectable in another room. Although the 
indicator 120 is pictured in FIG. 1 as being physically spaced from the 
circuit 116, a preferred embodiment, with the indicator being a buzzer, 
positions the buzzer on the same small board 124 as the circuit 116. This 
board construction is illustrated in FIG. 6. Additionally, the same power 
supply 128 that powers the fan 104 also powers the indicator 120 through 
the circuit 116, as can be understood from FIG. 2. In other words, the fan 
voltage is used to power the entire circuit 116 so as to minimize the 
connections to the fan or fans in an enclosure. In essence, this means 
that the active circuits must operate from ten to fifteen volts DC. 
Referring to FIG. 2, the circuit 116, in addition to having a speed 
detection, has a two stage comparator construction. The first stage 
comparator is shown generally at 132, and the second stage comparator is 
shown generally at 136. The first stage comparator 132 includes resistors 
137 and 138 and comparator 166. The second stage comparator 136 includes 
resistors 202 and 206 and output comparator 170. While the first stage 
comparator 132 detects the pulses coming from the fan 104, the second 
stage comparator 136 sets the speed capacitor 190 at a level at which the 
indicator 120 begins "indicating" and also drives the indicator. The 
pulses are directly related to motor speed of the fan 104. Since the 
variation in speed for different sizes of fans is small in comparison to 
the actual speed, a point range at which the fan alert circuit 116 
indicates a "faulty" fan can be established. The fault conditions include 
the fan 104 not being connected, not turning and turning at a speed which 
is about one-third of its full rated speed. 
Power from the (twelve-volt) power supply 128 is brought into a connector 
140 of the circuit 116. The connector 140 also ties the power supply 128 
into the fan 104 via isolation resistor 148 and the fan operation into the 
circuit 116. Pin 144 of the connector 140 is connected to the fan 104. The 
twelve volts of the power supply 128 pass through the isolation resistor 
148 to the fan 104. Thus, as the fan 104 turns it creates a ripple or 
pulses. If the isolation resistor 148 were not present (or if it had a 
zero resistance), the circuit 116 would see only the power supply 128 and 
thus would not see any pulses. In other words, if the power supply 128 
were tied directly to the fan 104, then the power supply would filter out 
the pulses and they would not be seen by the rest of the circuit 116. 
The pulses are seen at pin 144 and pass to the input AC coupling capacitor 
156. The coupling capacitor 156 acts to block any DC level. This means 
that it does not matter whether the fan 104 is a five, eight, ten, twelve 
or twenty-four volt fan. All that it is being picked up is the AC or pulse 
component of the fan 104. The pulses will be more or less frequent 
depending on how fast the fan 104 is turning. A return resistor 160 after 
the coupling capacitor 156 is also provided for the first stage comparator 
132. 
If the fan 104 stops, nothing passes through the coupling capacitor 156 and 
the output 164 of the pulse detector comparator 166 would then be and stay 
high. This causes the output comparators 170, 174, to generate more power 
for the buzzer 120. For a small buzzer only one comparator is needed, but 
a preferred buzzer of this invention requires two comparators to supply 
sufficient power to it, as shown in FIG. 2. (An alternate circuit would be 
a transistor driver.) The buzzer 120 can be a six to twelve volt buzzer, 
for example. If a six volt buzzer 120 is used and the power supply 128 is 
a twelve volt supply then a limiting resistor 178 is placed in the path to 
limit the current through the buzzer. A noise filter 182 is also provided 
for the buzzer 120. 
Pin 186 into comparator 170 is another reference level. Thus, when the 
charging capacitor 190 is charged up and exceeds the value at pin 186 it 
makes the output pin 194 go low, which turns the buzzer 120 on. The 
charging capacitor 190 gets its charge from charging resistor 198, which 
charges the charging capacitor and establishes how long it takes for the 
capacitor to reach the trip point level for the buzzer 120. The diode 200, 
in turn, allows rapid discharge by bypassing resistor 198. 
The charging resistor 198 and the charging capacitor 190 are selected by 
the designer pursuant to this invention to be the correct size. If they 
are too large, the charging capacitor 198 will never get charged up enough 
so that the buzzer 120 comes on. On the other hand, if they are too small, 
the buzzer 120 will always be going off. A workable pulse range is three 
to five or six millisecond pulse intervals. Resistors 202 and 206 
establish the level that the charging capacitor 190 gets charged to before 
the buzzer 120 is energized. 
Thus, the output comparators 170, 174 have two functions. One is as a 
comparator to set the point at which the alarm is tripped, and the other 
is to power or drive the alarm or buzzer. 
The circuit 116 works extremely well for fans 120 of sizes from fifty to 
ninety millimeters. And for the larger one hundred and twenty millimeter 
fans 120, the isolation resistor 148 can be changed from one-half watt to 
one watt. However, this larger resistor does not work well for a small 
fifteen millimeter fan 120, which requires less current and would not 
necessarily generate as high a pulse. If the threshold level of the pulse 
detector 166 is reduced to take this into account, a potential resulting 
problem is that some power supplies are so noisy that the circuit 116 may 
be fooled into thinking that it is really fan noise. Thus, the designer 
must factor this in when designing circuits (116) pursuant to this 
invention for very large and very small fans 104. 
Representative waveforms useful in understanding the present circuit 116 
are shown in FIGS. 3a, 3b and 3c generally at 210, 214, 218, respectively, 
for a forty millimeter fan 104. FIG. 3a is a waveform 210 after the 
coupling capacitor 156. FIG. 3b is a waveform 214 at a "normal" running at 
the output of the pulse detector comparator 166. FIG. 3c is a waveform 218 
at a "slow down" running at the same location, as by purposely placing a 
finger on the fan 104 to slow it down significantly. It shows the 
capacitor discharge pulses, the alarm speed. The pulse intervals stretch 
out and the charging capacitor 190 will, at a certain level, charge up 
sufficiently to exceed the trip level established by the output 
comparator(s) 170 (and 174) and the buzzer 120 will start buzzing. In 
comparison to a typical prior art system, there are no pulses, just DC 
levels. So at some point as the fan slows down, the output to the buzzer 
would start changing in amplitude and no noise (or other indicating 
signals) would be emitted until the fan was almost stopped. 
FIGS. 4, 4b and 4c show waveforms similar to FIGS. 3a, 3b and 3c, but for a 
fifty millimeter fan. Likewise, FIGS. 5a, 5b and 5c show waveforms for a 
sixty millimeter fan. 
A board layout of the present circuit 116 and buzzer or indicator 120 is 
shown in FIG. 6 generally at 226. As can be seen, the size of the board 
124 is extremely small--for example, a 0.8 by 1.25 inch board as compared 
with prior art boards which are typically more than twice as large. It is 
so small and light that it can be positioned or mounted almost anywhere 
using self-adhesive clips. Field retrofit is easy, and no modifications to 
the sheet metal enclosures need to be made. No holes need to be drilled 
into the chassis. The board 124 can be clipped into any small place. It 
can easily mount to the side of small (sixty millimeter by twenty 
millimeter) fans. The board 124 can even be packaged within the confines 
of various front panel plastic bezels. This small board size provides 
greater mounting flexibility and use in applications where space is at a 
premium than was previously possible. 
Also, as can be appreciated from FIG. 6, the circuit 116 uses only discrete 
components and one standard simple integrated circuit, e.g., resistors, 
capacitors and diodes, and one multi-vendor QUAD (even though only three 
amplifiers are shown in FIG. 2, so there is a spare amplifier) comparator 
230 for all signals processing including alarm activation. That is, the 
circuit 116 uses an inexpensive common chip along with several 
non-precision discrete parts, such as ceramic capacitors and five-percent 
resistors. In addition to providing for the use of a very small board 124, 
the use of only these components also means that the construction of the 
circuit 116 is very inexpensive. 
From the foregoing detailed description, it will be evident that there are 
a number of changes, adaptations and modifications of the present 
invention which come within the province of those skilled in the art. 
However, it is intended that all such variations not departing from the 
spirit of the invention be considered as within the scope thereof as 
limited solely by the claims appended hereto.