International Journal of u- and e- Service, Science and Technology
Vol.8, No. 8 (2015), pp.189-198
http://dx.doi.org/10.14257/ijunesst.2015.8.8.19
Design of the Thermistor Tester based on MSP430
Zenglin Zhang and Xiaoqing Yu*
Northwest A & F University, Shaanxi,Yangling, 712100, China
E-mail:zhangzenglin115@gmail.com,yuxiaoqing115@gmail.com
Abstract
With the development of the single-chip microcomputer and integrated circuits,
thermal resistance tester has gradually replaced the traditional temperature test
equipment. The design uses the low power consumption MSP430 microcomputer as the
core processor, heating power is produced by the switch tube FSDM0565RB, EMI filters
is joined in the switch power supply in order to inhibit the influence of high frequency
interference in AC power network on the equipment. Ref voltage of microcomputer
generates through the transformation of voltage conversion chip AMS1117, signal input
port uses voltage follower to minimize signal noise. The system circuit also joined some
protection circuit, prevention of circuit false operation, etc. In the software design,
measuring resistance judgment is achieved through the difference value of comparison
measuring and standard resistance, which can shorten measure time and is very suitable
for batch into production. Test results show that the thermal tester has a good stability
and reliability, and meet the requirements of design.
Keywords: Mirocomputers, thermistors, Sswitching mode, power supplies, filters
1. Introduction
With the development of electronic technology and the wide application of thermistor,
it is the popularity of measuring instruments for ultra-low power consumption single chip
microprocessor and thermistor sensor [1, 2], which have a significant impact on the
industrial economy and people's daily life. The measuring instrument circuit of traditional
methods is complexity, cost is high, and many components in circuit directly affect the
measurement accuracy.
The design is thermal resistance test equipment, which mainly based on MSP430F5
series single chip microcomputer [3]. General resistance measurement methods have R-V
switching voltage measurement method and R-C transformation frequency measurement,
but circuits are complexity and cost are high in these methods. The most application is the
bridge measurement method [4-6], but debugging is more complex. It can avoid the
inconvenience and less dependence on the external environment by using the comparison
method to measure the thermal resistance measurement.
MSP430F5 series single chip microcomputer is applied for the main chip processing in
the design [7, 8]. According to the design requirements, standard thermal resistor is used
for temperature sensor. When the temperature reaches test temperature 150℃, fan works
to cool the measuring system and the temperature measurement control system starts to
sample at the same time. The main task of design is realizes the comparison measurement
between treatment thermistor and standard thermistor, then, the accuracy is controlled
according to the difference of judgment. The main hardware circuits have switch power,
signal input circuit, single-chip microcomputer control circuit, 3.3 v voltage conversion
circuit several parts.
ISSN: 2005-4246 IJUNESST
Copyright ⓒ 2015 SERSC
International Journal of u- and e- Service, Science and Technology
Vol.8, No. 8 (2015)
2. System Hardware
2.1 MSP430 Single Chip Microcomputer Interface Circuit Design
The MSP430F5 series single chip that developed by TI company is a 16-bit super-low
power consumption mixed signal processor [9, 10]. MSP430 is composed of a high gain
reverse amplifier used for oscillator, fine-tuning capacitor size in the circuit will affect the
oscillator frequency, the stability of the oscillator and the quickness of start up, therefore,
capacitor is selected 22 pF under. Crystal oscillator frequency is 20 MHz. P5.0, P5.1 are
reference voltage sides, C53, C54 are filter capacitance with 1F that are used to stable
reference 2.5 v voltage and ensure accuracy of transformation. MSP430 reset uses the
most simple external key reset circuit to realize, single chip microcomputer interface
circuit design is shown in Figure 1.
Figure 1. Design of Microcomputer Interface Circuit
2.2 Thermistor Signal Input Circuit
In the design, the op-amp selects LM358 and uses ±5 v power. ADC12 kernel is one
of 12 A/D converter and be able to store results in the transform memory, the kernel uses
two programmable reference voltage to define converter maximum and minimum value.
When the analog voltage input is higher than or equal 2.5 v, the ADC12 output is full
scale value 0XFFF. ADC12 output is zero when the input voltage is equal or less than 0 v.
The final conversion results of input analog voltage satisfy the formula:
N
ADC
 4095 
Vin  Vr 
Vr   Vr 
In the system software programming, the reference voltage selects Vr+=2.5 v, Vr-=0 v,
the Vm should satisfy 0<Vm<2.5, namely, 3.3×R28/(R26+R28)<2.5 V,
3.3/(R26/R28+1)<2.5, R26>0.32R28. Due to the test minimum temperature of the
thermistor under test is 50±1℃, thermistor resistance maximum is 148.6 KΩat this time,
so R26Ω=51 KΩ. Thermistor signal input circuit design is shown in Figure 2.
Figure 2. Design of Thermistor Signal Input Circuit
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Copyright ⓒ 2015 SERSC
International Journal of u- and e- Service, Science and Technology
Vol.8, No. 8 (2015)
Here, the R25 and R26 are protection resistance, resistance are 100 kΩ. R22, R23 and
C110, C111 constitute a low-pass filter to ensure that the input signal is effective.
2.3 Fan and Electric Heating Control Circuit
P4 and P5 are two 3 pin plug and used to install the fan and heater, respectively. K1
and K2 are two relay parameters, relay is controlled by switch circuit composed of P1.0,
P1.1 and triode, control circuit is shown in Figure 3.
Figure 3. The Control Circuit
2.4 The LCD Display Circuit
V
3
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N
G
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U17
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L
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SO-24
1
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K
A
B
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1
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0
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7
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E
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W
R
5
S
R
4
O
V
3
D
D
V
2
VSS
1
L
5
1
0
1
7
LCD1602
P
9
P
R13A
K
B
-
D
C
8
1
7
1R/0.5W
L
9
1
6
R17
SO-24
0
3
5
4
R16
K
1
3
+
SN74LVCC4245A
4
3
2
2
V
5
2
1
+
U18
LCD display circuit is shown in Figure 4, U17、U18 are two pieces of LCD driver
chip, respectively, P7 is the LCD display.
Figure 4. LCD Display Circuit
2.5 Switching Power Supply
Switching power supply uses pulse width modulation in the system, switching
frequency is 50 KHz. Rated input voltage is 220-240V 50/60Hz, DC output +5 v, +24 v,
-5 v supply for fan, heater and op amp, respectively, power is 50 w.
2.5.1 The Input Circuit of Switching Power Supply
Input circuit including insurance circuit, low pass filter and rectifier filter circuit
several parts, as shown in Figure 5. Insurance circuit is composed of F1 insurance tube
that fusing rapidly when the insurance tube current exceeds rating 5 A to prevent the
circuit components damaged due to overheating.
Copyright ⓒ 2015 SERSC
191
M
2
2
2
4
2
1
1
Y
C
X
1
L
6
0
68uF/400V
1A/400V
1
F
2
1
M
2
2
D
N
G
D
N
2
G
Y
C
6
0
2
K
0
1
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K
8
6
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1
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N
5
D
4
1
T
R
N
I
_
~
-
A
5
/
V
0
5
2
+
~
+
1
J
C
A
L2801
W
S
_
C
A
2
C
1
1
2
1
2
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C
D
K
0
8
6
C
4
7
4
R
2
International Journal of u- and e- Service, Science and Technology
Vol.8, No. 8 (2015)
Figure 5. The Input Circuit of Switch Power
Here, TNR1 is varistor, EMI filter circuit consists of C1, R1, R2, L1, CX, CY1, CY2.
CY1, CY2 can inhibit the surge current and protect diode. D1 is rectifier bridge pile and
constitutes a bridge rectifier circuit. C2 is the power filter capacitor, rectifier filter circuit
directly rectifier, filter for 220 v, 50 Hz AC voltage into about 300 v DC voltage.
2.5.2 Pulse Oscillation Circuit
6
5
4
3
C
2
4
2
H
u
0
1
47uF/35V
V
5
+
6
0
2
47uF/35V
C15
47uF/35V
-5V
L12
C12
2
1
H
H
u
0
1
6
0
2
1
L13
0
6
0
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1
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u
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1
7
3
2
1
C11
D11
8
5
5
D
N
G
A
D
N
5
R
1000uF/16V
5
0
2KF
8
0
Y
2KF
6
5
0
8
0
7
5
0
3
5
0
8
PC817
PGND
DIP-4
2
U
0
TO-92
1.2K
TL431
F
n
7
4
8
0
5
0
8
5
9
R
F
n
0
1
0
U
C13
K
8
1
0
8
0
8
R
K
1
7
R
47uF/35V
8
C
C
10nF/1KV
5
0
8
0
R
+
F
n
0
1
C
6
3
D
3
V
B
F
4
6
C
SR560
G
FR103
C
N
C
C
D
N
G
2
L11
+
D
Vstr
+
FR103
6
D
1
FR103
+
6
0
1
u
0
2
1
1000uF/16V
9
10nF/1KV
TO220-F6L
FSDM0565RB
D12
2
4
C
1
U
470uF/35V
+
C16
C10
+
流
整
1
+
C14
TO220
1
1
1
L10
+
D10
F10LC20U
1
T
T2834_12
4
R
100K/1W
3
68K/1W
R
1
D
2
P
Pulse oscillation circuit including switching transformer primary winding, adjust tube,
starting circuit, oscillating circuit, pulse width adjustment, error amplifier circuit, etc., is
mainly composed of three loop circuits. Figure 6 is a simplified circuit of the pulse
oscillation circuit, the switch tube FSDM0565RB, switch transformer T1, capacitor C7
composed of oscillation circuit, which function is produce high frequency oscillation and
output rectangular pulse.
Figure 6. The Simplified Diagram of Pulse Shock Circuit
2.5.3 The Output Circuit of the Switch Power Supply
The output circuit of the switch power supply is shown in Figure 7, 7-8, 9-10,11-12
are secondary output winding of the switch transformer, C10, L10, C14 composed of ∏
type filter and change the high frequency pulse of high frequency rectifier output into a
smooth DC. Because it is the high frequency filter, filter element value and volume are
small.
The switch tube during exit saturation to stop, the same name end of the switch
transformer secondary winding will change potential to make the rectifier diode D10, D11
positive conducting, the magnetic energy of secondary coil 7-8, 11-12 can be converted
into electrical energy and supply power to the load, and charging for C10, C11 at the
same time. During the switch tube saturation and conductivity, the induced voltagethe of
secondary coil 7-8, 11-12 makes D10, D11 cut-off, C10, C11 capacitor began to
discharge and supply power to load through the accumulation energy.
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2
6
5
4
3
C
2
4
2
H
u
0
1
47uF/35V
V
5
+
6
0
2
47uF/35V
-5V
L12
C12
47uF/35V
H
6
u
0
1
0
2
1
L13
6
2
0
H
u
0
1
D
N
G
A
D
N
1000uF/16V
Y
D
N
G
10nF/1KV
C
6
SR560
G
C11
D11
8
5
L11
+
+
1
0
7
3
1
FR103
+
6
0
2
1
C16
H
u
0
1
1000uF/16V
9
D12
2
1
2
470uF/35V
+
C15
C10
+
1
+
C14
TO220
1
1
1
L10
+
D10
F10LC20U
1
T2834_12
T
1
D
P
International Journal of u- and e- Service, Science and Technology
Vol.8, No. 8 (2015)
Figure 7. The Output Circuit of Switch Power
2.5.4 Voltage Control Circuit
4
2
H
u
0
1
47uF/35V
V
5
+
6
0
2
47uF/35V
C15
47uF/35V
-5V
L12
C12
u
0
1
H
H
u
0
1
6
0
2
1
L13
6
0
2
1
H
u
0
C11
D11
8
5
R
D
N
5
5
0
8
0
2KF
Y
C
2KF
6
5
0
8
0
7
5
0
8
3
5
0
8
PC817
2
PGND
DIP-4
U
0
TO-92
1.2K
9
R
TL431
F
n
7
4
5
0
5
0
8
F
n
0
1
0
U
C13
K
8
1
0
8
0
8
R
K
1
7
R
47uF/35V
8
C
C
10nF/1KV
5
0
8
0
R
+
F
n
0
1
1000uF/16V
6
3
D
C
C
V
B
F
3
6
4
C
SR560
G
FR103
5
C
N
D
N
G
2
L11
+
D
Vstr
+
1
0
7
3
1
6
1
FR103
+
6
0
2
1
1000uF/16V
9
TO220-F6L
FSDM0565RB
D12
2
1
2
1
U
470uF/35V
+
C16
C10
+
1
+
C14
TO220
1
1
1
L10
+
D10
F10LC20U
1
T2834_12
T
It can be included from U0=(T1/T)×Ui, the high and low of the output voltage can be
adjusted as long as the adjust switch pulse duty ratio, namely the control switch tube
conduction time. The longer switch tube conduction, the higher the output voltage, the
lower conversely. Voltage control circuit is mainly composed of sampling comparison,
error amplifier, pulse width adjustment and other parts, which is shown in Figure 8.
Figure 8. The Stabilized Voltage Control Circuit
Resistor R5, R6 constitutes the voltage sampling circuit and TL431, PC817 constitutes
voltage comparison circuit, TL431 is integrated voltage regulator block which has three
terminals are reference terminal R, anode, A cathode K, respectively. It uses shunt voltage
method, namely anode terminal grounding, the reference voltage terminal connects R5
partial pressure, the cathode terminal connects photocell as adjustment terminals. The
output of voltage value is:
Vout （1 
R5
） U Re f
R6
，
Because the circuit requires stable 5 v voltage value: R5=R6=1.2 KF. The R5 partial
pressure is input of the sampling voltage.
3. System Software
System software design mainly includes: main program module, the basic time delay
module, clock setting module, AD conversion module, LCD display module, etc. System
is firstly initialized, mainly is the chip initialization, the initial time of clock circuit is set
up, waiting for buttons command through the query way, the entire system begins to work
if the button is pressed. For programming, the main program mainly realize the
comparison of the sampling data, display control, key control, collection complete
judgment, data archiving, etc. The main program flow chart is shown in Figure 9.
Copyright ⓒ 2015 SERSC
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Vol.8, No. 8 (2015)
开始
初始化
否
按键标志位
P2.4=0
是
否
加热到180℃
是
加热停止风扇工作
开始采样
否
采样64组
是
两组数据做差值
运算
符合标准
否
显示NG
是
显示PASS
Figure 9. The Flow Chart of Main Program
3.1 The Timer Interrupt Program
Interrupt time interval is controlled by timer A, an interrupt is generated every 240 ms,
A/D conversion samples once until the complete of 64 data acquisition. Interrupt cycle
produces program is followed:
void Timer1Init(void)
{TA1CTL |= MC_1+MC_2 + TASSEL_2;
TA1CTL |= ID_2;
TA1CCR0 = 0xEA60;
CLRB(TA1CTL,TAIFG); TA1CTL |= TAIE;
__bis_SR_register(GIE);}
Interrupt entry procedures are follows:
# pragmavector=TIMER1_A1_VECTOR
__interrupt void Timer1(void) { }
3.2 The Heating Module Settings
After the port initialization, the main program comes into the heating module to heat
thermistor. The heating temperature is 180 ℃. When the thermistor resistance is heated to
180 ℃, thermistor is 7 kΩ and the MCU voltage value is: u=3.3/(50+7)×7=0.405v，
NADC=4095×0.405/2.5=73.71=0X49. Procedures is follows:
do { P2OUT |= 0X01; }
while(ADC12MEM1 < 0X49);
P2OUT |= 0X00;
P2OUT |= 0X02;
Here, P2.1 is heating control port and effective at high level, ADC12MEMI is
temporary address of AD conversion A0 data. When heating temperature reaches 180 ℃,
fans began to heat dissipation and measure.
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International Journal of u- and e- Service, Science and Technology
Vol.8, No. 8 (2015)
3.3 The Data Processing Program
When interrupt data acquisition reaches 64 group, processing program starts to average
64 data. Because the temperature difference control of the thermistor can affect on the
fixing system control when it is more than 4 ℃ , the resistance of the temperature
difference 4 ℃ is 500 Ω. Accuracy control functions:
d 精度 
3.3
（⊿  0.5） 3.3⊿
d 精度 
50.5  ⊿
50  ⊿ ,
82.5
⊿  100.5⊿  2525 ，
2
Here, ⊿ is the thermistor resistance under 50 ℃~180 ℃. The smaller d value, the
higher control precision. When the voltage difference value is 0.006 v through calculation,
the resistance is judged as NG, ⊿ NADC= 4095×0.006/2.5=9.8≈10=0XA. Procedures
is follows:
while(1){
if (nADC_Flag )
{nADC_Flag=0;
for (i=0;i<64;i++)
{pBuf_1[i]=ADC_BUF_Temp_1[i];
pBuf_2[i]=ADC_BUF_Temp_2[i]; }
sum_1=0; sum_2=0;
for(i=0;i<64;i++) {sum_1+=pBuf_1[i];
sum_2+=pBuf_2[i]; }
sum_1 >>= 6; sum_2 >>= 6;
fval=(float)(sum_1-sum_2);
if (fval<0) fval = -fval;
4. Conclusions
Before drawing PCB, all components encapsulation need to be added Some packaging
can be directly found in Protel standards component library, but some components
encapsulation need to be drawed, such as FSDM0565RB, PC817, TL431, T2834,
ASM1117 and single chip microcomputer MSP430F5438 and so on.
PCB can be drawed after encapsulation are added into schematic diagram. Circuit
board adopts double, the size and shape of the circuit board should be considered. The
automatic wiring easys to cause the interference of high frequency on signal lines, if the
top and the bottom line are parallel, capacitive coupling is easiley conduced and lead to
signal interference. Design uses manual wiring, and the simple circuit part adopts
automatic wiring, PCB circuit board is shown in Figure 10.
Figure 10. PCB Circuit Board
In the design, thermal resistance tester is a kind of high precision, wide range high
speed impedance measurement instrument. Thermal resistance tester based on single chip
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Vol.8, No. 8 (2015)
microcomputer reading is convenient and precision is high. The thermal resistance tester
has high measurement precision, fast speed, wide range, etc. At any scale range, it has the
same test speed and stable performance.
ACKNOWLEDGEMENTS
The authors wish to thank the National Engineering Research Center for Water-Saving
Irrigation, which partially supported this research through the “ China Postdoctoral
Science Foundation funded project ” (2014M552495) and the “ Twelfth Five-Year"
National Science and Technology Support Program” (2011BAD29B08)，The authors
are also grateful to the anonymous reviewers for their valuable feedback.
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[1]
Authors
Xiao Q. Yu, received the B.S. degree from Department of
Information Engineering, Lanzhou University of Finance and
Economics, Lanzhou, China in 2006. She received her M.S. degree
and Ph.D. degree from Department of Mechanical and Electric
Engineering and Department of Water Resources and Architectural
Engineering, Northwest A & F University, Shaanxi, China in 2009
and 2013, respectively.
Currently, she is pursuing postdoctor degree from Department of
Water Resources and Architectural Engineering under the
supervision of Prof. Wen T. Han. Her current research interests are in
Agricultural Water-Soil Engineering and Wireless Sensor Networks.
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Wen T. Han, received his B.S. degree from Department of
Mechanical and Electric Engineering, Northwest Agriculture
University, Shaanxi, China in 1996. M.S. and Ph.D. degree from
Department of Mechanical and Electric Engineering, Northwest A &
F University, Shaanxi, China in 1999 and 2004, respectively.
Working Experiences:
2005-present: researcher, Institute of Soil and Water Conservation
of Chinese Academy of Sciences Northwest A & F University,
National Engineering Research Center for Water Saving Irrigation at
Yangling.
2004-2005: Assistant Professor, Department of Mechanical and
Electric Engineering, Northwest A & F University.
2001-2004: A lecturer, Department of Mechanical and Electric
Engineering, Northwest A & F University.
Research Interests:
Information monitoring of crop and environment; intelligent
control for precise irrigation; water distribution Simulation of
sprinkler irrigation; Development of nozzle
Currently, he published academic papers more than 30, including
SCI and EI articles 16; the national invention patent 6.
Zeng L. Zhang, received his B.S. degree from Department of
Mechanical and Electric Engineering, Harbin institute of Technology,
Harbin, and M.S. degree from Department of Mechanical and
Electric Engineering, Northwest A & F University, Shaanxi, China in
2000 and 2007, respectively.
Currently, he is a teacher in Department of Mechanical and
Electric Engineering, Northwest A & F University, Shaanxi. He is
pursuing Ph.D. degree under the supervision of Prof. Pu T. Wu. His
current research interests are in Agricultural Water-Soil Engineering
and Wireless Sensor Networks.
Copyright ⓒ 2015 SERSC
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