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Measure and control temperature
over a wide range with this . . .
By JOHN CLARKE
High-Temperature
Thermometer/Thermostat
Need to measure or control temperature over a very wide range?
Now you can do it with this compact unit which hooks up to
a K-type thermocouple. It drives a relay which can be used to
precisely control the temperature in ovens, kilns, autoclaves, solder
baths or at the cold end of the spectrum, fridges and freezers. It is
based on an Analog Devices AD8495 precision instrumentation
amplifier with thermocouple cold junction compensation.
N
OW WE KNOW that some digital
multimeters can measure temperatures with a K-type thermocouple
but that’s all they can do; they cannot
control the temperature in an oven etc.
In other words, they do not provide
an adjustable thermostat function.
In all the above examples, our new
High-Temperature Thermometer/
Thermostat can be used to measure
40 Silicon Chip
and control the temperature at the
same time. That’s because it has a relay
output that opens or closes at a preset
temperature.
The switched output can be used directly or in conjunction with a higherrated relay to control power to the element of a heater or the compressor of
a refrigerator. For heating, the power
can be switched on when the tempera-
ture is below the preset temperature
and switched off when it is above.
Alternatively, for cooling, power can
be switched on when the temperature
goes above the preset and off when it
goes below. The preset temperature for
this thermostat action can be adjusted
between -50°C and 1200°C.
It is important that the thermostat
function does not cause rapid on and
siliconchip.com.au
Features & Specifications
Main Features
•
•
•
•
•
•
•
•
•
K-Type thermocouple probe
Ground referenced or insulated
probe can be used
Measures -50°C to 1200°C (depending on probe)
Pre-calibrated temperature measurement
Optional calibration of span and offset adjustment
Thermostat switching at a preset temperature with adjustable hysteresis
High to low or low to high thermostat threshold
Relay output for thermostatic control
Relay contacts rated at 10A (30V AC/DC maximum recommended
switching voltage)
Specifications
Power supply: 12V <at> 100mA
Measurement range: -50°C to 1200°C (probe dependent)
Initial accuracy: ±4°C for -25°C to 400°C measurements (ambient between
0°C and 50°C)
Optional calibration adjustment for span: -4%, +5.27%
Optional calibration adjustment for offset: ±6.2mV equivalent to >±1°C
off switching of the heater, compressor
or whatever is being temperature-controlled. Hence the design incorporates
adjustable hysteresis. This allows a
preset temperature difference to apply
between switching power on and off.
The hysteresis is adjustable from less
than 1°C to more than 9°C.
The temperature is displayed on a
3½ digit LCD and while the unit can
display a temperature from -50°C to
1200°C the actual measurement range
will depend on the particular probe.
Some K-type probes will operate from
-50°C to 250°C, while others will operate from -40°C to 1200°C.
The High-Temperature Thermometer/Thermostat is housed in a small
instrument case and controls on the
front include a power switch and a
switch to select between measured
temperature and the preset thermostat
temperature. A LED indicator is for
power indication and a second LED
shows when the thermostat relay has
switched on.
At the rear of the case is the power
input socket for a 12V DC supply and
a socket for the K-type thermocouple.
Additionally, there is a terminal connector inside the case for connection
to the thermostat relay contacts. The
common (C), normally open (NO) and
normally closed (NC) contacts are
available for connection.
Inside the case there are jumper
siliconchip.com.au
Thermostat set point range: adjustable from -50°C to 1200°C
Thermostat hysteresis: adjustable from <1°C to >9°C
Cold junction compensation: optimised for 0-50°C ambient temperatures
links to select whether the thermostat
relay switches on above or below the
preset temperature for the thermostat.
There are also jumper selections to
select whether the Thermometer/
Thermostat is built pre-calibrated or
where the temperature calibration can
be accurately adjusted.
K-type thermocouple
As mentioned above, this design
uses a K-type thermocouple which
comprises a junction of two dissimilar
wires; in this case it uses an alloy of
chrome and nickel (called Chromel)
for one wire and an alloy of aluminium, manganese, silicon and nickel
(called Alumel) for the second.
These two wires are insulated and
make contact at the temperature probe
end only. The other end of the wires
are usually connected to a 2-pin plug.
Basically, a thermocouple’s operation relies on the principle that the
junction of two dissimilar metals
produces a voltage that is dependent
on temperature. A K-type thermocouple produces a voltage output that
typically changes by 40.44µV/°C.
This change in output per is called
the Seebeck coefficient and it refers
to the output change that occurs due
to the temperature difference between
the probe end and the plug end of the
thermocouple.
In practice, the Seebeck coefficient
for the K-type thermocouple varies
with temperature and is not precisely
40.44µV but this is a good average
value over the temperature range from
0°C to 1200°C.
If we know the temperature at the
plug end of the thermocouple, we can
calculate the temperature at the probe
since we also know the Seebeck coefficient. For example, if the plug end is
held at 0°C, the output will increase
by 40.44µV for every 1°C increase.
Similarly, the output will decrease
by 40.44µV for every 1°C drop in
temperature.
In practice, we do not keep the plug
end of the thermocouple at 0°C; it’s not
practical. Instead, we compensate the
thermocouple output by measuring the
temperature at the plug end and then
adding 40.44µV for every 1°C that the
thermocouple plug end is above 0°C
or subtracting 40.44µV for every 1°C
that the plug end is below 0°C.
May 2012 41
THERMOSTAT
PRESET
REF1: 2.5V
REFERENCE
VR1
IC2d
COMPARATOR
(IC2a)
A=3
+2.5V
–
IC1
AD8495
OUT
5mV/°C
2
S2
REF
+2.5V
1.25V
1
NO
COM
NC
K-TYPE
THERMOCOUPLE
+
RELAY
RELAY
DRIVER
(Q1, Q2)
1 = THERMOMETER
2 = THERMOSTAT
1/50
DIVIDER
(VR4, LK3-4)
A=1
~ 1.25V
100 V/°C
INHI
INLO
3.5-DIGIT LCD
PANEL METER
(200mV FULL SCALE)
BUFFER
(IC2b, IC2c,VR3
Fig.1: block diagram of the High-Temperature Thermometer/Thermostat. IC1
processes and amplifies the thermocouple’s output and drives the LCD panel
meter and comparator IC2a. Trimpot VR1 sets the thermostat temperature.
For example, if the thermocouple
plug is at 25°C, its output will be
1.011mV (ie, 25 x 40.44µV) lower than
it would be if it were at 0°C. By adding
an extra 1.011mV to the reading, we
obtain the correct result without having to keep the plug end at 0°C.
Note that there are several dissimilar
metal junctions within the connections between the thermocouple plug
and amplifier. These include the
Chromel to copper junction and the
Alumel to copper junction on the PCB
itself. These do not contribute to the
overall voltage reading provided they
are all kept at the same temperature.
As a result, the PCB has been
designed to help maintain similar
temperatures at these junctions by
making the copper connections all the
same size. Once the PCB is installed
inside its case, the inside temperature
should remain fairly constant for all
these junctions.
Note that if the thermocouple lead
needs to be extended, it’s necessary
to use the same K-type thermocouple
wire for the whole length between the
probe and plug.
Signal processing
Refer now to Fig.1 which shows the
block diagram of the High-Temperature
Thermometer/Thermostat. As shown,
the thermocouple signal is processed
using the Analog Devices AD8495 IC.
This is a precision instrumentation
amplifier with K-type thermocouple
42 Silicon Chip
cold junction compensation. Its output
is 5mV/°C.
The amplifier within the AD8495
is laser trimmed for a gain of 122.4.
This gain effectively converts the
40.44µV/°C output of the thermocouple to 4.95mV/°C. The output is
optimised for a 25°C measurement
where a gain of 122.4 gives a result of
123.75mV.
Within the AD8495, a 1.25mV offset is added to the amplified value,
giving a 125mV output at 25°C. For
temperatures other than 25°C, the
combination of the variation in the
Seebeck coefficient over temperature,
the 122.4 gain and the 1.25mV offset
provides an accurate 5mV/°C output
over the range of -25°C to 400°C. For
this range, the output is within 2°C.
Note that the specification panel
shows that the accuracy is ±4°C for
ambient between 0°C and 50°C and
-25°C to 400°C measurements. This
is different to the 2°C error for the
AD8495 because the display is showing a reading via a voltage divider that
is prone to extra tolerance errors.
It’s possible to calibrate the measurement to a finer accuracy if this is
required. Table 1 shows the expected
output from the AD8495 over a wide
range of temperatures and compares
this with the ideal 5mV/°C output.
How it works
Returning now to the block diagram
of Fig.1, the K-type thermocouple con-
nects directly to the AD8495 (IC1) at
the IN+ and IN- terminals. The resulting 5mV/°C output signal from IC1 is
then fed to the non-inverting input of
comparator IC2a and also to position
1 (Temperature) on switch S2. S2
selects between the Temperature and
Thermostat modes of operation.
In order to allow for negative temperature measurements, the output
from the AD8495 is offset by approximately 1.25V. This offset is derived
by a voltage divider connected across
a 2.5V reference (REF1) and buffered
using op amps IC2b and IC2c. The
buffered 1.25V signal is then applied
to the AD8495’s REF (reference) input.
This effectively “jacks up” the
AD8495’s output by 1.25V. As a result, a -50°C measurement now gives
an output that’s theoretically 250mV
below (-5mV x 50) the 1.25V reference
offset (ie, 1V). Without this offset, the
AD8495 would not be able to handle
negative temperature measurements
since its output cannot go below 0V.
Although the offset only needs to
be 250mV to allow for a -50°C measurement, a value of 1.25V is used
because of the LCD panel meter that’s
used to measure the voltage. This
meter requires an input that’s at least
1V above the 0V supply for correct
operation. According to Table 1, the
actual output from IC1 at -50°C is
228mV below the offset voltage. So
using an offset of 1.25V leaves us with
a comfortable 22mV margin above the
critical 1V level.
The 3.5-digit LCD panel meter
used to display the temperature has
a 200mV full scale reading (actually
199.9mV) for a reading of 1999. It’s
basically connected to measure the
voltage between IC1’s output (via a
divider) and the offset voltage. This
effectively removes the offset voltage
from the reading.
To prevent the meter from overranging and to get a reading in °C,
we need to divide IC1’s output by
50. For example, if the temperature
is 1200°C, the voltage between IC1’s
output and the 1.25V offset will be
6V (ie, 1200 x 5mV). Dividing this
by 50 gives 120.0mV and the panel
meter is configured to show 1200 (ie,
no decimal point).
Note that, in the full circuit, either
a fixed divide-by-50 attenuator or an
adjustable divide-by-50 attenuator
can be used. The desired attenuator
is selected using jumper links and
siliconchip.com.au
the adjustable one allows for accurate
calibration.
The display can either show the
measured temperature when switch
S2 is in position 1 or the preset temperature (for the thermostat operation)
when S2 is in position 2. VR1 sets the
thermostat temperature. As shown, it’s
connected to a 2.5V reference (REF1)
and the voltage at its wiper drives op
amp IC2d.
As a result, IC2d’s output can
range up to 7.5V, slightly more than
the 7.25V at IC1’s output when the
measured temperature is at the 1200°C
maximum (ie, 1200 x 5mV plus the
1.25V offset). This allows VR1 to set
the thermostat temperature anywhere
from -50°C to 1200°C.
IC2d’s output is fed to the inverting
input of comparator IC2a where it is
compared with IC1’s output. IC2a’s
output thus switches low when the
temperature is below the preset and
high when the temperature is above
the preset. This output then drives
a relay via transistors Q1 and/or Q2.
Links LK5 and LK6 can be selected
so that the relay either switches on
when IC2b’s output goes high or on
when it goes low.
Circuit details
Refer now to Fig.2 for the full circuit diagram of the High-Temperature
Thermometer/Thermostat. As well as
the AD8495 (IC1) and the LCD panel
meter, it includes an OP747 precision
quad op amp (IC2), a 7805 3-terminal
regulator, an LM285-2.5 precision voltage reference, transistors Q1 & Q2 and
various minor components.
IC1 is powered from a 12V DC plugpack supply via switch S1, diode D1
(for reverse polarity protection) and a
10Ω resistor. A 22V zener diode (ZD1)
clamps any over-voltage transients
while 100µF and 100nF capacitors are
used to bypass the supply.
In operation, IC1 draws just 180µA
to minimise internal heating (note:
internal heating would affect the
measurement of the ambient temperature used for the thermocouple
ice-point temperature compensation).
The K-type thermocouple connects
to its IN+ and IN- terminals (pins 8
& 1) via series 47kΩ resistors. These
resistors and their associated 100nF
ceramic capacitors prevent RF (radio
frequency) signals from being detected
by IC1’s sensitive input stages. The
resistors acts as RF stoppers, while the
siliconchip.com.au
Table 1: AD8495 Output vs. Temperature
Thermocouple
Temperature (°C)
Ideal Output <at> 5mV/°C
(mV)
AD8495 Output
(mV)
Display Reading (°C)
±1 Digit
-50
-40
-20
0
20
25
40
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
720
740
760
780
800
820
840
860
880
900
920
940
960
980
1000
1020
1040
1060
1080
1100
1120
1140
1160
1180
1200
-0.25
-0.2
-0.1
0.0
0.1
0.125
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5.0
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
6.0
-0.228
-0.184
-0.093
0.003
0.100
0.125
0.200
0.301
0.402
0.504
0.605
0.705
0.803
0.901
0.999
1.097
1.196
1.295
1.396
1.497
1.599
1.701
1.803
1.906
2.010
2.113
2.217
2.321
2.425
2.529
2.634
2.738
2.843
2.947
3.051
3.155
3.259
3.362
3.465
3.568
3.670
3.772
3.874
3.975
4.076
4.176
4.275
4.374
4.473
4.571
4.669
4.766
4.863
4.959
5.055
5.150
5.245
5.339
5.432
5.525
5.617
5.709
5.800
5.891
5.980
-46
-37
-19
0
20
25
40
60
80
101
121
141
161
180
199
219
239
259
279
299
320
340
361
381
402
423
443
464
485
506
527
548
569
589
610
631
651
672
693
713
734
754
774
795
815
835
855
875
895
914
934
953
973
992
1011
1030
1049
1068
1086
1105
1123
1141
1160
1178
1196
May 2012 43
44 Silicon Chip
siliconchip.com.au
+
–
CON2
10k
10k
10k
1
8
K
10
9
6
5
SENSE
5
100nF*
100 F
IC2c
IC2b
–Vs
3
2
8
7
* CERAMIC
REF
6
IC1
OUT
AD8495
100nF
–IN
+IN
7
+Vs
100 F
10
UNCAL
10
LK1 LK2
CAL
OUT
GND
V+
IN
REG1 7805
+2.5V
K
A
15k
VR1
100k
100 F
+5V
10k
VR4
100
LK3
CAL
UNCAL
LK4
100nF
THERMOSTAT
PRESET
LED1
POWER
470
HIGH TEMPERATURE THERMOMETER/THERMOSTAT
A
+2.5V
A
K
D1
100nF*
VR3
100
REF
OFFSET
10k
100
47k
A
100nF*
ZD1
22V
100nF*
47k
S1
POWER
100nF
20k
IC2d
4
1.8
39
1
THERMOMETER
13
12
100nF
B
10k
A
K
D1, D2: 1N4004
1k
51k
2
THERMOSTAT
470
IC2: OP747
S2
14
10 F
NP
9
7
5
COM
6
INLO
8
RFL
1
K
1
+5V
B
A
IN
E
CON3
A
ZD1
K
2
NO
NC
COM
OUT
K
A
NC
C
BC337
7805
B
A K
LEDS
GND
LM285-2.5Z/LP
Q2
BC337
GND
E
C
RELAY1
.
.8:8.8
10k
2.2k
D2
K
3.5-DIGIT LCD PANEL METER
VR2 1M
+
ROH
RFH
INHI
100k
LK6
L/H
11
IC2a
10k
10
3
2
E
Q1
BC337
C
H/L
LK5
LED2
A
Fig.2: the complete circuit diagram for the High-Temperature Thermometer/Thermostat. An accurate 1.25V reference is derived from REF1 via IC2b or IC2c and
this is applied to the REF input of IC1 to enable measurements down to -50°C. IC1’s output drives the LCD panel meter via a 50:1 divider and also drives the noninverting input of comparator IC2a. IC2a compares IC1’s output with the thermostat preset temperature, as set by VR1 & IC2d, and drives relay 1 via transistors
Q1 and/or Q2 when the preset limit is reached. Links LK5 & LK6 allow the relay to be driven on either a rising or falling temperature.
SC
2012
+5V
K
4.7k
REF1
LM285
-2.5Z/LP
–
+
CON1
K-TYPE
THERMOCOUPLE
12V
DC
INPUT
+12V
+11.4V
100nF capacitors effectively shunt any
remaining RF signal to ground.
In addition, the negative terminal
of the K-type thermocouple is tied to
ground via a 100Ω resistor. This prevents the probe from picking up noise
and mains hum, which would cause
erratic operation.
Note that the 100Ω resistor is included so that the circuit can be used
with both earthed and insulatedsheath thermocouples. Basically, the
thermocouple probe wires are housed
in a cylindrical metal sheath or rod.
Some units connect the negative thermocouple wire directly to this metal
sheath (an earthed probe), while others
fully insulate the metal sheath from
the thermocouple wires (an insulated
probe).
For an insulated probe, it doesn’t
matter whether the negative terminal
is connected directly to ground or
connected to ground via a 100Ω resistor. That’s because an insulated probe
can connect to a point that’s not at 0V
without affecting the operation of the
probe.
By contrast, an earthed probe does
require the 100Ω resistor. That’s
because the probe could make an
external connection to the 0V supply
rail and this might not be at exactly
the same voltage as the 0V rail inside
the unit.
This type of situation could easily
arise, for example, when measuring
engine heat or brake disc heat in a car
and the unit is being powered by the
vehicle’s battery. In this situation, the
probe point and the internal 0V rail
will be at slightly different voltages
due to current flowing in the vehicle’s
chassis. The difference in voltage may
only be small but the thermocouple’s
output only varies by about 40µV/°C,
so only small variations can mean a
huge error in temperature readings.
The 100Ω resistor eliminates this
problem by preventing significant
current flow between the thermocouple’s negative terminal and the 0V rail
within the thermometer.
Deriving the offset
The 1.25V offset for IC1 is derived
from REF1, a precision 2.5V voltage
reference, via a resistive divider. This
divider comprises four 10kΩ resistors
and a 100Ω trimpot (VR3).
As shown, the 1.25V midpoint of
the 10kΩ fixed resistive divider is fed
to pin 5 of IC2b, while the voltage on
siliconchip.com.au
VR3’s wiper is fed to IC2c. VR3 allows
the offset voltage to be varied over a
small range either side of 1.25V.
IC2b and IC2c are both connected as
unity gain buffer stages. When LK1 is
installed, IC2b provides a fixed 1.25V
offset for IC1 at its REF (pin 2) input.
At the same time, IC2c provides the
variable offset output to the panel
meter at its IN LO input.
Alternatively, if LK2 is installed,
IC2c drives both the reference input
of IC1 and the INLO input of the LCD
panel meter. In this case, the voltage
applied to both IC1’s REF input and the
panel meter’s INLO input are exactly
the same and this is the linking option
to use if you do not want to accurately
adjust the temperature calibration.
LCD panel meter
As stated previously, the LCD panel
meter measures the difference between its INHI (pin 7) and INLO (pin
6) inputs. In this circuit, IC1 drives
the INHI input via one of two 50:1
voltage dividers (one fixed, the other
variable) when S2 is in position 1.
IC1 is capable of delivering in excess
of ±5mA to a load but the fixed 50:1
divider draws just 115µA maximum
when IC1’s output is producing 7.25V
for a 1200°C measurement. This low
current minimises any internal heating of the IC.
The fixed divider is selected using
link LK4. It’s made up using a 51kΩ
resistor in the top section and 39Ω,
1.8Ω and 1kΩ resistors at the bottom.
Assuming the values are exact, the
division ratio is very close to 50:1.
However, resistor tolerances can shift
this to within a range of around 50.05:1
to 49.95:1.
The variable divider shares the 51kΩ
and 1kΩ resistors but uses a 100Ω trimpot in place of the 39Ω and 1.8Ω resistors in the fixed divider. This allows
the divider to be adjusted. It’s selected
by installing link LK3 instead of LK4.
The LCD panel meter itself is based
on an Intersil ICL7106 3.5-digit LCD
analog-to-digital converter (ADC).
Its INLO, COM (common) and RFL
(reference low) pins are all connected
together, ie, they are all fed with the
reference offset voltage at IC2c’s output. In addition, the ROH output is
connected to the RFH (reference high)
input and this sets the panel meter to
200mV full scale.
A 5V supply rail for the LCD is
derived from regulator REG1 (7805).
The OP747ARZ Quad
Precision Op Amp
The OP747ARZ quad precision op
amp specified here has features that are
not found in general-purpose op amps.
First, it features a low offset voltage
of 100µV maximum and the input bias
and offset currents are in the very low
nA range. Second, it can handle input
voltages ranging from the ground supply rail up to within 1V of the positive
supply. And third, the output can reach
close to each supply rail.
Taken together, these characteristics
make the op amps ideal for this circuit.
REG1’s input and output rails are
both filtered using 100µF electrolytic
capacitors, while LED1 in series with
a 470Ω current-limiting resistor provides power indication.
This regulated 5V supply also drives
the 2.5V reference (REF1), this time via
a 4.7kΩ resistor. As well as providing a
source for the offset voltage, the resulting 2.5V rail is also fed to the top of
VR1 which sets the thermostat preset.
VR1 is connected in series with a
15kΩ resistor across this supply and
its wiper provides an output which
ranges from 326mV up to 2.5V. IC2d
amplifies this by three, as set by the
20kΩ and 10kΩ resistors in the feedback path. The resulting voltage at the
output of IC2d can range anywhere
from 978mV up to 7.5V and that more
than covers the possible voltage range
from IC1, for temperatures ranging
from -50°C to 1200°C.
As described previously, op amp
IC2a is wired as a comparator. It monitors IC2d’s output and compares this
with IC1’s output. IC2a thus switches
its output high when the measured
temperature is above the preset temperature or low when the measured
temperature goes below the preset
(ignoring hysteresis).
Trimpot VR2 (1MΩ) and the 100kΩ
and 470Ω resistors provide hysteresis.
With VR2 set at 1MΩ, the hysteresis is
at its minimum and there is less than
1°C hysteresis. At the other extreme,
with VR2 set for 0Ω, the hysteresis is
more than 9°C.
Relay driver circuit
IC2a drives transistor Q1, which
in turn drives Q2, when link LK5 is
inserted. Alternatively, if LK6 is selected, Q1 is bypassed and IC2a drives
Q2 direct.
May 2012 45
1M
100
47k
100
100nF
100nF
22V
ZD1
100
10
LK1 LK2
LK3 LK4
1.8
51k
39
1k
15k
0V
+5V
100nF
A
S2
LED2
LED1
VR4
10k
REF1
2.2k
A
VR3
TEMPERATURE
THERMOMETER
/THERMOSTAT
100nF
LM285
-2.5Z/LP
470
HI
47k
BC337
10k
100k
10k
12150112
EPYT K
RHIGH
ETE M O MRE HT
100nF
100nF
20k
10k
IC2 (UNDER)
VR2
100nF
10
470
VR1
4.7k
100 F
4004
100 F
S1
100 F
100k
100nF
IC1 (UNDER)
ROH
RFH
RFL
InHi
InLo
COM
COIL
10k
REG1
7805
D1
Q1
RELAY1
4004
D2
LK5 LK6
10k
Q2
10k
10k
CON3
–
Thermocouple
K type
+ CON1 –
LOW
© 2012
BC337
10 F
NP
+
TO THERMOCOUPLE
SOCKET
NO
COM
NC
CON2
12VDC IN
RELAY
CONTACTS
13 12 11 10 9 8 7 6 5 4
2 1
3.5-DIGIT LCD PANEL METER (REAR)
Fig.3: follow this diagram to build
the unit but note that the first job is
to install surface-mount devices IC1 & IC2 on the underside of the PCB (see
below). You can omit the relay, CON3, S2 and transistors Q1 & Q2 if you intend
using the unit as a thermometer only and don’t need the thermostat function.
(UNDER SIDE OF PCB)
1
IC1
04105121
K TYPE
THERMOMETER
1
IC2
46 Silicon Chip
Fig.4 (left): this diagram and
the above photo show how
surface-mount devices IC1
& IC2 are mounted on the
underside of the PCB. Make
sure that both devices are
correctly orientated (pin 1 is
identified by a small dot on
the device body) and follow
the step-by-step procedure
described in the text to solder
them into position.
These two links select whether the
relay turns on for a low-to-high temperature transition (LK6 in place) or a
high-to-low transition (LK5 in place).
When LK6 is in circuit, Q2 turns on
when IC2a’s output goes high (ie, when
the temperature rises above the preset)
and this turns on relay 1. The relay
subsequently turns off again when
IC2a’s output switches low (ie, when
the temperature falls below the preset).
Conversely, when LK5 is in circuit,
Q1 inverts the logic. In this case, Q2
and the relay are normally on since
Q2’s base is pulled high. However,
when IC2a’s output switches high (as
the temperature rises above the preset),
Q1 turns on and pulls Q2’s base to
ground. As a result, Q2 and the relay
turn off and remain off until the temperature falls below the preset again.
LED2 lights whenever the relay
switches on to indicate that the set
temperature threshold has been reached. The associated 2.2kΩ resistor limits the current through LED2, while
diode D2 protects Q2 from damage
by quenching the back-EMF voltage spikes generated when the relay
turns off.
The relay provides both the usual
common (COM), normally open (NO)
and normally closed (NC) contacts, so
it can also drive a load on or off depending on the selection of the NO or
NC contacts. So it may seem that links
LK5 and LK6 are not really necessary
to reverse the switching sense.
However, there are reasons why you
may wish to select whether the relay
is normally powered or not, especially
when the relay contacts are required to
switch a heating or cooling operation.
One reason is that less current is
drawn by the circuit when the relay
is off and you might want to choose
the link and contact configuration that
draws the least power.
Another reason is that you might
want to ensure fail-safe operation if
power is cut to the circuit. By using
the COM & NO contacts to do the
switching, you can ensure that power
is not provided for heating or cooling
if the power to the Thermometer/
Thermostat fails.
Construction
The assembly is straightforward
with all parts except the probe socket
and the LCD panel meter mounted on
a PCB coded 21105121 (117 x 102mm).
This is housed in a plastic instrument
siliconchip.com.au
The thermocouple socket is connected to an
adjacent screw terminal block via two short leads.
Alternatively, the screw terminal block could be omitted
and a couple of flying leads soldered direct to the PCB.
case measuring 140 x 110 x 35mm.
Begin by carefully checking the PCB
for any defects. Check also that the
hole sizes are correct for each component to fit neatly. The corner mounting
holes and the regulator mounting hole
should all be 3mm in diameter.
Our prototype used a double-sided
PCB and Fig.3 shows the parts layout.
The first step is to install IC1 and IC2.
These are both surface-mount devices
(SMDs) and mount on the underside
of the PCB – see Fig.4.
To install these, you will need a
fine-tipped soldering iron, some fine
solder and some quality solder wick. A
magnifying lamp or at least a magnifying lens will also be handy.
It’s best to install IC2 first. This is
the 14-pin device with the wider pin
spacings. First, place the PCB copper
side up and apply a small amount of
solder to the top-right pad, then pick
the IC up with tweezers and position
it near the pads. Check that it is orientated correctly (ie, with its pin 1 dot
positioned as shown on Fig.4), then
heat the tinned pad, slide the IC into
place and remove the heat.
Now check the IC’s alignment
carefully using a magnifying glass. It
siliconchip.com.au
should be straight, with all the pins
centred on their respective pads and a
equal amount of exposed pad on either
side. If not, reheat the soldered pin and
nudge the chip in the right direction.
Once its position is correct, solder
the diagonally opposite pin, then
recheck its position before soldering
the remaining pins. Don’t worry too
much about solder bridges between
pins at this stage; they are virtually
inevitable and can easily be fixed.
The most important job right now is
to ensure that solder flows onto all the
pins and pads.
Once you’ve finished, apply a thin
smear of no-clean flux paste along
all the solder joints and remove the
excess solder using solder wick. You
should then make a final inspection
to ensure that there are no remaining
solder bridges and that the solder has
not “balled out” onto a pin without
flowing onto the pad.
If there are still bridges, clean them
up with more flux and solder wick.
Once IC2 is in place, you can install
IC1 in exactly the same manner.
Through-hole parts
The larger through-hole parts can
now be installed on the top of the
PCB. Start with the resistors and diodes, then install zener diode ZD1,
the MKT and ceramic capacitors and
the electrolytics. It’s a good idea to
check the value of each resistor using
a multimeter before installing it.
Take care with the polarity of the
electrolytics, the diodes and the zener
diode. They must be orientated as
shown on Fig.3.
Transistors Q1 & Q2 and the LM3852.5 precision voltage reference (REF1)
can go in next. REG1 can then be installed. This mounts horizontally with
its tab against the PCB, so you will
have to bend its leads down at right
angles to match its mounting holes.
Secure its tab to the PCB using an M3
x 6mm screw and nut before soldering
its leads. Don’t solder the leads before
securing the tab; you could crack the
copper tracks at the mounting screw
is tightened if you do.
Trimpots VR1-VR4 are next on the
list. These must all be mounted with
the adjustment screw to the right. Follow with the three 3-way pin headers
for links LK1-LK6, then install the
6-way and 2-way polarised headers
for the LCD panel meter connections.
May 2012 47
The cable gland on the rear panel allows an external lead to be fed into the case and connected to the relay contacts at
CON3. The LCD is secured to the front panel by running a couple of beads of silicone adhesive or hot-melt glue down
the vertical inside edges.
Be sure to orientate these headers as
shown, ie, with their vertical tabs
towards the panel meter.
Once they’re in, you can install the
two LEDs but first you have to bend
their leads down through 90° some
9mm from their bodies.
The best way to do this is to first cut
a cardboard spacer 9mm wide. This
is then be used as a template when
bending the LED leads. Make sure
that each LED is correctly orientated
before bending its leads – the (longer)
anode lead must be on the right when
looking at the lens.
Having bent their leads through 90°,
the two LEDs must be installed with
their leads 5mm above the PCB. This
is best done by pushing them down
onto a 5mm spacer, then soldering the
leads to the PCB pads.
Switches S1 & S2 are right-angle
types and are mounted directly on the
PCB. Push them down onto the board
as far as they will go before soldering
their leads. The PCB assembly can then
be completed by installing the relay,
48 Silicon Chip
the DC socket (CON2) and the 2-way
and 3-way screw terminals.
Connecting the panel meter
The panel meter is wired to the
6-way header plug and to the 2-way
header plug using short lengths of ribbon cable. These leads can be obtained
by separating an 8-way ribbon into
6-way and 2-way strips.
Cut these strips to 50mm in length,
then strip about 2mm of insulation
from the individual wires at one end
and crimp them to the header pins.
The pins can are then inserted into
the headers.
The other ends of these leads can
then be stripped and soldered to the
LCD panel meter pins. Check carefully
to ensure that each wire goes to the
correct pin on the panel meter and
that there are no shorts between them.
In fact, it’s a good idea to slip a short
length of heatshrink over each wire
before soldering it and then pushing
over the soldered joint to insulate it
from its neighbours.
Jumper links LK2 & LK4 should now
be installed and either LK5 or LK6.
Install LK5 if you want the relay to
switch on when the temperature drops
below the preset. Alternatively, install
LK6 if the relay is to switch on when
the temperature rises above the preset.
Final assembly
Fig.5 shows the front and rear panel
artworks. You can purchase finished
panels from SILICON CHIP or you can
download the artworks in PDF format
from our website.
Mounting the panel meter
The LCD panel meter is mounted by
sliding into its front-panel slot (which
is open at the top). Check that the top
of the meter sits flush with the top of
the panel. If it protrudes slightly, it will
be necessary to make the slot slightly
deeper until it does sit flush.
The meter is secured in place by
running a bead of silicone sealant or
hot-melt glue along the two vertical inside edges, adjacent to the front panel.
siliconchip.com.au
+
its mounting slot from the rear (terminal screws facing up) and fitted with
the supplied clip to hold it in place.
Once that’s done, the rear panel can
be slipped into the case and two short
wires run between the thermocouple
socket and the screw terminal block
on the PCB.
The lid can now be test fitted to
make sure everything is correct. Note
that it will be necessary to file the
RELAY CONTACTS
10A MAX & 30V MAX
K-Type Thermocouple
Relay
Temperature
Power
SILICON CHIP
THERMOMETER/
THERMOSTAT
Thermostat
RELAY
OUTPUT
12V DC
.
Once the meter is in place, the front
panel and the PCB assembly can be slid
into the case. The PCB is then secured
to the base using four self-tapping
screws that go into integral mounting
bushes. That done, the leads from the
panel meter can be plugged into the
headers on the PCB.
The rear panel carries a cable gland
(for the relay outputs) and the thermocouple socket. The latter is fed through
Fig.5: these front and rear panel
artworks can be copied and used as
drilling templates. Finished panels
are also available from SILICON CHIP.
two ridges at the front of the lid down
where they meet the panel meter.
Testing
To test the unit, first apply power
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
siliconchip.com.au
No.
1
1
2
1
1
8
1
1
1
2
1
1
2
1
Value
100kΩ
51kΩ
47kΩ
20kΩ
15kΩ
10kΩ
4.7kΩ
2.2kΩ
1kΩ
470Ω
100Ω
39Ω
10Ω
1.8Ω (5%)
4-Band Code (1%)
brown black yellow brown
green brown orange brown
yellow violet orange brown
red black orange brown
brown green orange brown
brown black orange brown
yellow violet red brown
red red red brown
brown black red brown
yellow violet brown brown
brown black brown brown
orange white black brown
brown black black brown
brown grey gold gold
5-Band Code (1%)
brown black black orange brown
green brown black red brown
yellow violet black red brown
red black black red brown
brown green black red brown
brown black black red brown
yellow violet black brown brown
red red black brown brown
brown black black brown brown
yellow violet black black brown
brown black black black brown
orange white black gold brown
brown black black gold brown
not applicable
May 2012 49
Parts List
1 PCB, code 21105121, 117 x
102mm
1 plastic instrument case, 140 x
110 x 35mm
1 12V DC 500mA plugpack
1 3.5-digit LCD panel meter (Jaycar
QP-5570 or similar)
1 front-panel label or 1 front-panel
PCB, code 21105122
1 rear-panel label or 1 rear-panel
PCB, code 21105123
1 K-type thermocouple probe (Jaycar QM-1292 -50°C to 250°C,
QM-1283 -40°C to 1200°C)
1 K-type thermocouple probe socket (Element14 Cat. 708-6386)
1 SPDT 10A 12V relay, Jaycar SY4050 or equivalent (RELAY1)
2 SPDT PCB-mount toggle switches (S1,S2) (Altronics S1421 or
equivalent)
1 PCB-mount 2.5mm DC socket
(CON1)
1 2-way PCB-mount screw terminal
block, 5.08mm spacing (CON2)
1 3-way PCB-mount screw terminal
block, 5.08mm spacing (CON3)
1 cable gland for 3-6.5mm diameter
cable
1 2-way polarised pin header,
2.54mm spacing
1 6-way polarised pin header,
2.54mm spacing
1 2-way header sockets to match
above header
1 6-way header sockets to match
above header
2 3mm LED bezels (optional)
3 3-way pin headers, 2.54mm spacing (LK1-LK6)
3 jumper shunts
4 No.4 x 6mm self-tapping screws
1 M3 x 6mm pan-head machine
screw
1 M3 nut
1 100mm length of 0.8mm tinned
copper wire
1 50mm length of 8-way ribbon
cable
Semiconductors
1 AD8495ARMZ precision thermocouple amplifier with
cold junction compensation (IC1)
(Element14 Cat. 186-4707)
1 OP747ARZ quad precision single
supply op amp (IC2) (Element14
Cat. 960-4405) (IC2)
1 LM285Z/LP-2.5 micropower
voltage reference diode (REF1)
(Element14 Cat. 966-5447; Jaycar ZV1626)
1 7805 5V 3-terminal regulator
(REG1)
2 BC337 NPN transistors (Q1,Q2)
1 22V 1W zener diode (ZD1)
2 1N4004 1A diodes (D1,D2)
1 green 3mm LED (LED1)
1 red 3mm LED (LED2)
Capacitors
3 100µF 16V PC electrolytic
1 10µF 50V non-polarised electrolytic
4 100nF ceramic
4 100nF MKT polyester
Trimpots
1 1MΩ top-adjust multi-turn trimpot
(code 105) (VR2)
1 100kΩ top-adjust multi-turn trimpot (code 104) (VR1)
2 100Ω top-adjust multi-turn trimpots (code 100) (VR3,VR4)
Resistors (0.25W, 1%)
1 100kΩ
1 2.2kΩ
1 51kΩ
1 1kΩ
2 47kΩ
2 470Ω
1 20kΩ
1 100Ω
1 15kΩ
1 39Ω
8 10kΩ
2 10Ω
1 4.7kΩ
1 1.8Ω 5%
Note: PCBs for this project are
available from SILICON CHIP.
The unit can be used with any K-type
thermocouple, eg, the Jaycar QM1292
or QM1283.
and off when the preset goes just over
or under the measured temperature.
VR2 can now be adjusted to give
the required amount of hysteresis
(clockwise for more hysteresis and
anticlockwise for less).
Calibration
If you wish, the unit can be left uncalibrated in which case its accuracy
will be as shown in the specifications
panel.
Alternatively, if you wish to calibrate the unit for improved accuracy,
the procedure is as follows:
(1) Remove jumper links LK2 & LK4
and install links LK1 & LK3 instead.
(2) Place the thermocouple probe in a
cup of distilled water brimming with
ice (note: the ice also needs to be made
from distilled water to ensure accuracy
and the ice-water mixture has to be
constantly stirred to maintain a 0°C
temperature).
(3) Adjust VR3 so that the thermometer reads 0°C.
(4) Place the thermocouple probe in
boiling distilled water and adjust VR4
for a reading of 100°C at sea level or
deduct 1°C for every 300m above sea
level.
That completes the calibration. The
lid can now be attached to the case and
the unit is ready for use.
Ambient temperature display
and check that the power LED lights.
The display should also show a temperature reading with S2 (Thermostat/
Thermometer) in position 1 (Temperature).
If it does, check the power supply
voltages on the board. REG1’s output
should be close to +5V, while pin 7 of
IC1 should be about 11.4V as should
50 Silicon Chip
pin 4 of IC2. REF1 should have close
to 2.5V across terminals 1 and 2.
Now check that the display shows a
temperature that’s close to the ambient
when the connected probe is exposed
to room air. Assuming it does, switch
S2 to position 2 (Thermostat) and
check that you can adjust the preset
using VR1. The relay should click on
There are a couple options available
if you just want the unit to measure
the ambient temperature.
First, you can use the thermocouple
as the sensor and simply sit it in free
air. Alternatively, you can disconnect
the thermocouple and short its inputs
on the PCB using a short length of wire.
The unit will then display the ambient
siliconchip.com.au
Controlling Mains Voltages
temperature (in °C) as measured by the
AD8495 itself.
Note that this will really be the temperature inside the case rather than the
room temperature. However, this will
be close to room temperature, since
there is little warming inside the case.
If you intend using this project simply as an ambient temperature thermometer or to measure temperatures
up to 199°C only, then the divider
resistors can be changed so that they
divide by five instead of 50. That way,
siliconchip.com.au
As presented in the diagram and photos, the Digital Thermometer/Thermostat is
capable of controlling external loads running at 30V DC and up to 10A. However, it
can control 230VAC loads, provided the relay and the wiring itself is rated for 250VAC
mains operation. This will mean that a larger case must be used to accommodate
the extra wiring and mains input and output sockets (note: the plastic case used here
is not suitable; it’s too small and the back is too flimsy to safely anchor mains cables).
The mains input wiring will need to include a mains fuse and we suggest an IEC
chassis-mount male socket that includes a switch and fuse (eg, Jaycar PP-4003). For
the output mains wiring, use a chassis or panel-mount female IEC socket (eg, Jaycar
PS-4176) or 3-pin mains panel-mount socket (eg, Jaycar PS-4094).
All mains wiring should be run in 250VAC 10A-rated cabling. Cable tie and clamp the
internal mains wires so they cannot possibly come adrift and contact any low-voltage
section of the circuit. It’s a good idea to secure the terminal block wires to the PCB; eg, by
using silicone sealant or a cable tie that loops through a couple of holes drilled through
the PCB adjacent to the terminal block.
A metal enclosure will need to be securely earthed. For a plastic case, any exposed metal
screws used to secure the IEC connector or other parts near to the mains wiring will also
need to be earthed. Nylon screws can be used as an alternative to earthing the screws.
The relay should be an Altronics S-4197 or exact equivalent, with contacts rated for
250VAC operation. Finally, for the 3-way terminal block, CON3, we recommend using a
Weidmuller type (Jaycar HM-3132) so that it has sufficient voltage rating.
the display can show the temperature
with a 0.1°C resolution.
To do this, change the 1kΩ resistor
to 12kΩ, the 39Ω resistor to 750Ω and
the 1.8Ω resistor to a 0Ω resistor (or
wire link). The 100Ω trimpot (VR4)
on the adjustable side of the divider
should be changed to 1kΩ.
Finally, the decimal point in front
of the righthand digit can be displayed
by connecting the LCD panel meter’s
DP3 pin to the +5V supply. The details
are shown on the instruction sheet
SC
supplied with the meter.
May 2012 51
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