Lab Experiment No. 7
Cooling Fan Control Circuit
I. Introduction
This lab experience involves a project rather than an experiment. The project is to build and test circuits that use a
thermistor to control temperature by activating a cooling fan.
II. The Thermistor: Theory of Operation
A thermistor is a resistor constructed from special material having a resistivity significantly sensitive to temperature
[1,2]. This material allows the resistance of the thermistor to exhibit a predictable variation over a wide range of
temperature. These devices are used as temperature sensors, current limiters, bias current compensators, and circuit
protectors. The resistance-temperature (RT) characteristics of thermistors are very non-linear. For example, the RT
characteristics of one class of thermistors is modeled with an exponential equation derived from the Steinhart-Hart
equation for [2]
Rt T   Rt To  e
1 1
B  
 T To



(1)
In this equation, T is the ambient temperature in °K, To is the reference or nominal temperature (usually 300.15°K or
27°C), Rt(To) is thermistor resistance at the nominal temperature, and B is a model parameter in °K. The plot of a RT
curve for a typical thermistor with a nominal temperature resistance of 6KΩ for two values of B is shown in Figure 1.
Over small ranges of temperature, the RT curve exhibits a near straight-line behavior. In these ranges, the resistance of a
thermistor can be approximated with a first-order relationship to temperature modeled by
 TCR
Rt T   Rt To  1 
T  To 
 100

(2)
where TCR is the temperature coefficient in %/°C. If the TCR is positive, the thermistor is referred to as a positive
temperature coefficient (PTC) device and Rt increases as temperature increases. Conversely, if TCR is negative, then Rt
decreases with an increase in temperature and the thermistor is a negative temperature coefficient (NTC) device. The
TCR for a typical PTC thermistor is on the order of 0.2%/°C to 0.5%/°C while that for a NTC device is between –5%/°C
to –3%/°C.
III. Fan Control Circuits
There are two versions for the fan control circuit used in this project. The schematic of the first version (Ckt. 1) is
shown in Figure 2 where a 5KΩ NTC thermistor RT is connected to a 10KΩ trim pot (R1) to form a voltage divider.
The thermistor is placed next to an object (target) whose ambient temperature is to be regulated by the cooling fan.
The voltage from the divider provides the excitation (Vin) to the 555 timer which is configured as a Schmitt trigger
[3]. Assuming the currents into pins 2 and 6 of the 555 are very small, V in is expressed as
Vin 
Rt T 
Rt T   R1
VCC
(3)
where Rt(T) is the temperature dependent resistance of RT. The output of the 555 (Vo) drives a pair of 2N3904 NPN
bipolar junction transistors (BJT) Q1 and Q2 to control a cooling fan and the light-emitting diode (LED) D1. The
LED provides visual indication of the fan’s condition. The voltage transfer curve (VTC) of the control circuit is
shown in Figure 3. At the target’s nominal operating temperature (T o), Rt(To) is about 5KΩ such that Vin is slightly
larger than 8V which is the ‘high’ threshold voltage (VTH) of the Schmitt trigger. The value for Vin at To can be
adjusted by trimming R1. At this point, Vo is approximately equal to zero volts causing Q1 and Q2 to be turned off
such that the fan and LED are also turned off. As the target’s temperature begins to increase, R t(T) begins to
decrease causing Vin to decrease as well. When Vin reaches the ‘low’ threshold voltage (VTL), the Schmitt trigger
changes state causing Vo to immediately increase to the supply voltage 12V. This voltage is large enough to cause
Q1 and Q2 to conduct, and to turn on the fan and the LED. By positioning the fan to direct a flow of cool air toward
the target, its temperature will begin to decrease such that Rt(T) and, consequently, Vin will start to increase. When
Vin reaches VTH, Vo immediately drops to zero volts which turns Q 1 and Q2 off. As a result, the fan and LED are
also turned off to complete the operating cycle.
The schematic for the second version (Ckt. 2) of the control circuit is shown in Figure 4. In this circuit, the pchannel junction field-effect transistor (PJFET) J1 and the 10KΩ trim pot R1 make a current source that forces
current into the thermistor RT. The operation of this circuit to control the cooling fan is basically identical to that of
the first version.
IV. Components and Instruments
The components and instruments required for this lab are listed below.
Components:
Resistors:
100Ω
1KΩ (2)
10KΩ trim pot
NTC thermistor
Capacitors:
0.01µF (2)
10µF
Active devices:
IC: 555 timer
Red LED
NPN BJT: 2N3904 (2)
PJFET: J271
Instruments:
Power supply
Agilent E3620A
Multimeter
Agilent 34401A
Additional:
12V cooling fan
Tool box
Breadboard
Hook-up wire
V. Project Procedure
Both circuit versions described above are to be built and tested in the lab. The following tasks are to be performed.
(a) Download, store, and print data sheets for the components listed below
NTC502-RC thermistor (Xicon)
555 timer (Fairchild Semiconductor Corp. or National Semiconductor Corp.)
NPN BJT 2N3904 (Fairchild Semiconductor Corp.)
PJFET J271 (Fairchild Semiconductor Corp.)
(b) Obtain a fan from the GTA. Confirm the operation of the fan by connecting it to the Agilent E3620A power
supply. Connect the red lead to positive (+) and the black lead to negative (−). Adjust the voltage to 12V and
verify that the fan is operational. If the fan does not operate, obtain another from the GTA. (Note: Be sure to
return all fans to the GTA after the project is completed.)
(c) Build Ckt. 1 shown in Figure 2 on your breadboard. Follow the breadboard layout shown in the photo in Figure
5. Place the fan and thermistor connections at the far end of the breadboard for convenient access.
(d) With the power supply voltage Vps of 12V, adjust R1 such that Vin is slightly larger than 8V. Measure and
record Vin, and indicate that the fan and LED are off.
(e) Use a heat source (hair dryer) to blow hot air onto the thermistor. Measure and record V in when the fan and
LED turn on. This voltage should be slightly less than 4V.
(f) Remove the heat source and direct the air flow from the fan onto the thermistor. Measure and record V in as the
temperature of the thermistor decreases to nominal. As V in exceeds 8V, the fan and LED should turn off.
(g) Repeat (c) through (f) for Ckt. 2 shown in Figure 4.
VI. References
[1]
M. Sapoff and R.M. Oppenheim, “Theory and application of self-heated themistors,” Proc. IEEE, vol. 51, pp.
1292-1305, Oct. 1963.
[2]
E.A. Boucher, “Theory and applications of thermistors,” Chemical Instrumentation, vol. 44, no. 11, pp.
A935-A966, Nov. 1967.
[3]
S. Franco, Design with Operational Amplifiers and Analog Integrated Circuits, 3rd Edition, The McGrawHill Companies, Inc., New York, NY, 2002. (ISBN 0-07-232084-2)
1 10
3
Resistance (Kohms)
100
10
1
0.1
50
25
0
25
50
Temperature (C)
75
100
125
150
B = 3900K
B = 4100K
Figure 1
RT curve for a typical thermistor
Agilent
power supply
+VCC
red
C1
10F
25V
R1
black
10K
trimpot
C2
8
2
Vps
555
timer
Vin
6
12V
RT
Rt(T)
D1
red
LED
fan
5K NTC
thermistor
3
Vo
R2 1K
R4
C3
0.01F
Q1
R3 1K
1
Q1, Q2 - 2N3904
Figure 2
Fan control circuit Ckt. 1 with a
thermistor in a voltage divider
100
0.01F
Q2
Vo
12V
fan on
hysteresis band
fan off
0V
VTL
4V
0V
VTH
8V
12V
Vin
increasing <---- T ----> decreasing
Figure 3
Control circuit VTC
Agilent
power supply
+VCC
red
C1
10K
trimpot
R1
10F
25V
D1
red
LED
fan
black
C2
8
J1
2
Vps
555
timer
Vin
6
12V
3
Vo
R2 1K
R4
C3
0.01F
Q1
R3 1K
1
RT
Rt(T)
5K NTC
thermistor
J1 - J271
Figure 4
Fan control circuit Ckt. 2with a
thermistor driven by a current source
100
0.01F
Q1, Q2 - 2N3904
Q2
Figure 5
Breadboard layout Ckt. 1