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Build this universal
temperature controller
Ever wanted to control the temperature of
liquid in a tank or perhaps the air in an
enclosure? This low cost controller will do
just that. It will hold the temperature at any
setting from near freezing to near the boiling
point of water.
By BRANCO JUSTIC & JEFF MONEGAL
When you think about it, there
are quite a few applications for a
universal temperature controller.
For example, you might want to
control the temperature in a
tropical fish tank or the temperature of etchant when making printed
circuit boards. Perhaps you are a
keen "home brewer" and want
to make the best beer. Again, this
unit will closely control the
temperature of your " brew" .
On the farm , it could hold the
temperature of a small hatchery at
a set level. We're sure readers will
come up with a lot of other uses.
To ensure complete safety, the
60
SILICON CHIP
circuit works on low AC voltages
(anywhere between 20 and 40VAC),
with 32VAC shown as the nominal
input on the circuit.
The controller is housed in a neat
black plastic case measuring 185 x
125 x 50mm. It has rounded corners
and clips together. On the front
panel, it has a single heater control
and two LEDs, one to indicate that
the desired temperature has been
reached and the other to indicate·
that the heater element is actually
on - ie, it shows the heater element cycling on and off.
There is also a 6.5mm jack socket
to connect a temperature sensor
which is a negative temperature
coefficient (NTC) thermistor. This is
placed in the medium (liquid or air)
to be controlled. We'll talk about
this later.
On the back panel, there are two
pairs of banana plug sockets. One
pair is for the 32VAC input while
the other is for the leads to the heating element.
The required temperature setting, as determined by the front
panel knob, will be controlled to
within ± 1 °C, once the "Temp Correct' ' LED indicator comes on.
Circuit details
This temperature controller has
been designed by Oatley Electronics. The circuit is perhaps a little unusual (and even overdesigned) in some aspects but uses
cheap and readily available components. In fact, the full kit for this
project is so cheap there would be
little point in trying to buy the individual parts separately, even if
you have some of them on hand.
Now let's look at the circuit
details.
Power for the circuit comes from
a 32V transformer (not supplied)
which is fed to diode Dl and the
470µF capacitor Cl to produce
about 45V DC. This is then fed to a
regulator circuit consisting of transistor Ql, diode D2, zener diode
ZD1 and .associated resistors and
capacitors.
ZD1 is a 15V zener which provides a constant voltage at the base
of Q1, a BD679 Darlington power
transistor. Ql functions as an emitter follower, producing a regulated
voltage of close to 13.6V at its
emitter.
The 100µF capacitors C3 and C4
provide extra filtering for the DC
supply to the base of Ql. D2 also
helps to improve the filtering but it
really is gilding the lily.
Op amp IC1 is the heart of the
circuit. It is connected as a high
gain (x 10,000) inverting amplifier
but really functions as a comparator. It compares the voltage across
A negative temperature coefficient (NTC) thermistor is used as the
temperature sensor. These feeds back information to the controller so that
temperatures can be kept within ± 1 °C. The thermistor is housed in a plastic
tube which is sealed to prevent corrosion of the leads.
R12
100<:l
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C7
10
+
A
16VWJ
C9
100
D3
1N4148
+
16VWJ:
R16
56k
R7
10M
02
2N2907A
C
C10
.01!.
D4
1N4148
C5
1D
.,.
':'
+
16VWJ
03
':'
2N2907A
C
C11
01:t
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B
EOC
VIEWED FROM
BELOW
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EC B
,;
A2
':'
D1
1N4004
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+v
32VAC
+
31~~11~~~1 m
CJ
TEMPERATURE CONTROLLER
+
C4
+
400mW
-:-
J
'":"
Fig.1: op amp IC1 is wired as a high gain amplifier but really functions as a comparator. When
the thermistor (RTtttl cools down, IC1 's pin 6 output switches low and enables oscillator IC2b
via inverter IC2a. The output pulses from IC2b then trigger Triac Q4 via IC2c/Q2 and IC2d/Q3
to turn on the heater. D1, Qt and their associated components provide a regulated 13.6V supply.
JUNE 1989
61
Fig.2: here's how to install the parts on
the PCB and wire up the external
hardware. Note the use of shielded cable
for the connection to the 6.5mm jack
socket. There is no need to isolate the
metal tabs of Ql and Q4 from their
respective heatsinks - just make sure
that the heatsinks don't touch since they
sit at different voltages.
CAPACITORS
D
D
D
D
No.
1
3
2
1
Value
0 .1µF
.01 µF
.001 µF
680pF
IEC
1 OOn
1On
1n
680p
EIA
104K
103K
102K
681K
RESISTORS
D
D
□
□
D
D
□
D
D
No.
1
1
1
2
4
2
5
3
1
Value
10MO
1MO
100k0
56k0
10k0
4 .7k0
1 kO
1000
4.70
the NTC thermistor, RTHl, with the
reference voltage set by the temperature controller, VR1.
When the thermistor cools down,
the voltage across it increases. This
causes the voltage at pin 2 to rise
above that at pin 3. When this happens, IC1 's output switches low.
The output line from IC1 is filtered with R8 and C6 and then fed
to Schmitt trigger stage IC2a. This
inverts the output from IC1 and
enables oscillator stage IC2b.
62
SILICON CHIP
4-Band Code
brown black blue gold
brown black green gold
brown black yellow gold
green blue orange gold
brown black orange gold
yellow violet red gold
brown black red gold
brown black brown gold
yellow violet gold gold
5-band code
brown black black green brown
brown black black yellow brown
brown black black orange brown
green blue black red brown
brown black black red brown
yellow violet black brown brown
brown black black brown brown
brown black black black brown
yellow violet black silver brown
IC2b is a Schmitt trigger oscillator with the frequency set by Rl 1
and C8. When its pin 2 is high (corresponding to a low output from
IC1), the oscillator runs at about
500Hz. IC2a also drives the two
LEDs via lkO resistors. When
IC2a's output is high, the " Heater
On" LED is alight and the "Temp
Correct" LED is out. When IC2a's
output is low, the "Temp Correct"
LED lights and the "Heater On"
LED goes out.
The rest of the circuit is used to
trigger the Triac Q4.
Transistors Q2 and Q3 drive the
gate of the Triac via C14, a O. lµF
capacitor. Q2 "sources" current into the gate via C14 and charges it in
one direction and then Q3 charges
C14 in the other direction. IC2c and
IC2d, together with diodes D3 and
D4 and associated components,
form a complementary drive circuit
for Q2 and Q3 which ensures that
there is a "dead time" between one
The PC board is secured inside the case using machine screws and nuts. Smear the mating surfaces of Ql, Q4 and their
heatsinks with heatsink compound before bolting the assemblies together and take care with component orientation.
transistor turning off and the other
turning on.
This "dead time" is good insurance because if the two transistors happen to turn on together they
will provide a direct conduction
path between the positive and
negative supply rails and thereby
destroy themselves.
Construction
As mentioned previously, the
Temperature Controller is housed
in a dip-together plastic case. Inside is a printed circuit board
which accommodates all the
circuitry.
After checking the board for any
track defects such as shorts or open
circuits, you can start the board
assembly by installing the resistors,
diodes and small capacitors. Make
sure that the polarity of the electrolytics and diodes is correct.
Now solder in QZ and Q3 and the
two IC sockets. Note that the IC
sockets have a slight "nick" at one
end which matches the moulding of
most ICs, so as to indicate correct
orientation.
You can now solder in Darlington
transistor Ql and the Triac (Q4).
Note that both these units are laid
flat and mounted on U-shaped heatsinks which are each secured to the
board by one screw and nut. No
mica insulation is needed between
the heatsink and the transistor (or
Triac) which means the heatsinks
are "live" at about + 45DC [for Ql)
or 32VAC (for Q4}. For this reason,
the two heatsinks must not touch.
When you have completed the
board, the two ICs can be inserted
into their respective sockets and
the board can then be set to one
side. The case now needs to be drilled for the board mounting holes and
the front and rear panels need to be
drilled for the various pieces of
hardware. You may also care to
label the controls and this can be
done with white Letraset.
Now install the hardware items
into the case, then install the PC
board and complete the wiring.
Check your work carefully against
the wiring diagram.
Sensor lead
Now you will need to make up the
sensor lead for the thermistor. For
this you need a 6.5mm jack plug and
a length of light duty twin flex or
shielded cable. Connect the jack
plug to one end of the cable and the
NTC thermistor to the other. The
thermistor can be protected from
damage by fitting a length of heatshrink tubing over it.
With all assembly complete, plug
in the sensor cable and apply 32
volts AC to the input of the controller. Measure the DC voltage between the collector (centre pin) of
JUNE 1989
63
PARTS LIST
1 plastic instrument case , 185
x 125 x 50mm
1 PC board, code OETC89,
102 x 85mm (available from
Oatley Electronics)
1 1oon wirewound
potentiometer (VR 1 )
1 instrument knob to suit pot
1 6800 NTC thermistor (Rrn 1 )
1 8-pin IC socket
1 1 4-pin IC socket
2 U-shaped transistor heatsi11 ks
(included in kit)
1 6 . 5mm jack socket
1 6 .5mm jack plug (for
thermistor cable)
4 banana jack sockets (2 red,
2 black)
4 banana plugs
2 5mm LED mounting bezels
Semiconductors
1 SC151 D, E or BTl 39-600
15-amp Triac (Q4 )
1 80679 NPN Darlington
transistor (01)
2 2N2907 A NPN transistors
(Q2 , Q3)
1 TL071 op amp (IC 1)
1 4093 quad Schmitt NANO
gate (IC2)
1 1 N4004 silicon power diode
(01)
3 1 N4 1 48 signal diodes (0 2 ,
03 , 04)
1 15V 400mW zener diode
(ZD1)
1 4 . 7V 400mW zener diode
(ZD2)
1 5mm red LED
1 5mm green LED
Capacitors
1 4 70µF 63VW electrolytic
1 1 001,tF 35VW electrolytic
2 1 OOµF 1 6VW electrolytic
1 0. 1µF disc ceramic
3 .0 1µF disc ceramics
2 .001 µF disc ceramics
1 680pF disc ceramics
Resistors ( ¼ W,
1 10MO
1 1MO
1 1 OOkO
2 56k0
4 10k0
5 %)
2 4.7k0
5 1k0
3 100!2
1 4 . 7!2
Miscellaneous
Hook-up wire , solder, screws ,
nuts, washers, heatsink compound.
64
SILICON CHIP
You can use either shielded cable or light-duty twin flex to make up the sensor
lead. One end of the cable goes to the thermistor leads while the other goes to
a 6.5mm jack plug.
where the fault lies. If you have no
voltage from Q l it is likely that you
have r ever se connected the zener
diode, ZDl .
Now check that you can get the
red LED to light by rotating the control knob clockwise. Rotating it
anticlockwise should extinguish the
red LED and light the green LED.
Heater elements
The NTC thermistor looks like a
ceramic capacitor but is labelled 68rl
10%. It's resistance varies with
temperature.
Ql and the Al terminal of Q4 [this
is OV in the circuit). It should be
about -t- 45V. Now measure the voltage at the emitter of Ql. It should
be about + 13.6V.
If these voltages are not correct
check the circuit around Ql to see
If you have to make up heating elements for temperature contr olling
a liquid, the best approach is to use
modified electric jug elements.
These are cheap, come with
preformed leads, and it is easy to
take off resistance wire to obtain
the exact ohms value you r equire.
If the liquid to be heated is corrosive, the heating element should
be sealed in a Teflon bag. We 'll
have more to say on this topic in a
future issue.
~
Where to buy the parts
A complete kit of parts for this project is available from Oatley Electronics, 5 Lansdowne Parade, Oatley West, NSW 2223. Phone (02) 57 9
4985. Address all mail orders to Oatley Electronics, PO Box 89 , Oatley
NSW 2223 .
The kit can be purchased as follows:
The complete kit, as per prototype but Without instrument case , is
$29 .90 . The instrument case is $8 .00. A short form kit with the PC
board , 80679 transistor and NTC thermistor is also available for
$12 .00. Postage and pac king for any of these combinations is $ 2 .50.
A suitable 32V 4 A transformer is also available for $26.90 plus freight.
Limited numbers are available.
If you have trouble getting your Temperature Controller to work , post it
to Oatley Electronics at the above address. They will fix it for you at a
minimum service charge . For more details, phone them on (02 ) 5 79
4985.
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