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Lab Experiment No. 7 Cooling Fan Control Circuit

I. IntroductionThis lab experience involves a project rather than an experiment. The project is to build and test circuits that use a

thermistorto control temperature by activating a cooling fan.

II. The Thermistor: Theory of OperationA 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]

1 1

o

BT T

t t oR T R T e

(1)

In this equation, T is the ambient temperature in K, To is the reference or nominal temperature (usually 300.15K or

27C), 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 6K 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

1100

t t o o

TCRR T R T T T

(2)

where TCR is the temperature coefficientin %/C. If the TCR is positive, the thermistor is referred to as apositive

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

to3%/C.

III. Fan Control CircuitsThere 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 5K NTC thermistor RTis connected to a 10K 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 (V in) 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, Vin is expressed as

1

t

in CC

t

R TV V

R T R

(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) D 1. The

LED provides visual indication of the fans condition. The voltage transfer curve (VTC) of the control circuit isshown in Figure 3. At the targets nominal operating temperature (To), Rt(To) is about 5K 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 targets temperature begins to increase, Rt(T) begins to

decrease causing Vin to decrease as well. When Vinreaches 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

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Vin reaches VTH, Vo immediately drops to zero volts which turns Q1 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 p-

channel junction field-effect transistor (PJFET) J1and the 10K 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 InstrumentsThe components and instruments required for this lab are listed below.

Components:

Resistors:

100 1K (2) 10K trim pot NTC thermistor

Capacitors:

0.01F (2) 10F

Active devices:

IC: 555 timer NPN BJT: 2N3904 (2)

Red LED PJFET: J271

Instruments:

Power supply Multimeter

Agilent E3620A Agilent 34401A

Additional:

12V cooling fan Breadboard

Tool box Hook-up wire

V. Project ProcedureBoth 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 Vin 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 Vin as thetemperature of the thermistor decreases to nominal. As Vin exceeds 8V, the fan and LED should turn off.

(g) Repeat (c) through (f) for Ckt. 2 shown in Figure 4.

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VI. References[1] M. Sapoffand 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 McGraw-

Hill Companies, Inc., New York, NY, 2002. (ISBN 0-07-232084-2)

50 25 0 25 50 75 100 125 1500.1

1

10

100

1 103

B = 3900K

B = 4100K

Temperature (C)

Resistance(Kohms)

Figure 1

RT curve for a typical thermistor

Vps

12V

2

8

6

1

3555

timer

fan

D1

red

LED

R4

C3

R3

C2

Q1

Q2

R1

R2

Rt(T)

10K

trimpot

5K NTC

thermistor

1K1K

0.01F 0.01F

100

Agilent

power supply

Vin

Vo

Q1, Q

2- 2N3904

red

black

C1

10F

25V

RT

+VCC

Figure 2

Fan control circuit Ckt. 1 with a

thermistor in a voltage divider

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Vo

Vin12V

8V4V

0V

0V

12V

hysteresis band

fan on

fan off

increasing decreasing

VTH

VTL

Figure 3

Control circuit VTC

Vps

12V

2

8

6

1

3555

timer

fan

D1

red

LED

R4

C3

R3

C2

Q1

Q2

R1

R2

Rt(T)

10K

trimpot

5K NTC

thermistor

1K1K

0.01F 0.01F

100

Agilent

power supply

Vin

Vo

Q1, Q

2- 2N3904

red

black

C1

10F

25V

RT

+VCC

J1

J1

- J271

Figure 4Fan control circuit Ckt. 2with a

thermistor driven by a current source

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Figure 5

Breadboard layout Ckt. 1