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Build a temperature
controlled soldering station
Can’t afford one those fancy new temperature
controlled soldering stations? Never mind, build
this one instead & save a bundle. It features a
grounded tip to prevent damage to delicate MOS
devices & is fully adjustable from 100°C to over
450°C.
Design by JEFF MONEGAL
Temperature controlled soldering
irons are highly desirable but many
electronics enthusiasts can’t afford
them. Now you can build your own
and learn about the technology at
the same time. It uses a high quality
replacement heating element which
comes with a thermocouple built into
the barrel. A LED bargraph indicator
shows the temperature setting and
another LED shows when the heater
element is on.
The principle of this temperature
controlled soldering iron is simple.
It uses a transformer with a 24V
secondary and a Triac to switch the
heating element on or off, depending
on the feedback from a thermocouple
mounted in the soldering iron barrel.
The heart of the project is the standard
replacement soldering element which
has four wires, two for the element
and two for the thermocouple connections. This replacement soldering
element is available from Dick Smith
Electronics (Cat. T-2008) and is priced
at $19.95.
In essence, the circuit has two
parts, one for show and one for go.
The part for show is IC3, the LM3914
LED display driver and the associated LEDs in the bargraph. The rest
of the circuit, the part that actually
does the work, uses two op amps
in an LM324 quad op amp package,
a transistor and an optocoupler to
drive the Triac. Now let’s refer to the
circuit diagram of Fig.1 to see how it
all comes together.
The thermocouple TH1 is connected to pin 3 of op amp IC1a which is
configured as a non-inverting amplifier
October 1994 65
+2.4V
K
LED13 A
POWER
ON
K
47
470
47k
10
4
3
IC1a
2 LM324
100k
1
+8V
470
16VW
LED2
HEATER
ON
4.7M
VR1
25k
5
+0.75V
6
IC1b
7
11
8.2k
1
OUT
K
470
1
6
IC2
MOC3021
TRIAC1
BT139-600 A2
2
10k
47k
B
1k
470
25VW
A
BR1
W04
+18V
IN
GND
1k
+3V
560
D1
1N914
TH1
REG1
7808
+7.4V
A
LED1
D2
1N914
VCC
470
4
C
G
A1
0V
12V
24V
HEATER
ELEMENT
Q1
BC548
E
4.7k
E
N
240VAC
A
VCC
9
B
E
A
K
LEDS 3-12
VIEWED FROM BELOW
10 K
5
K
A
LEDS 1, 2
AND 13
C
3
LED12
11
6
12 K
LED10
13
14 K
I GO
A1
A2
G
2.7k
IC3
LM3914
7
4
8
2
15
16 K
4.7k
LED8
LED6
17
18 K
LED4
1
COLD
A HOT
K
A
K
A
K
A
K
A
K
LED11
A
LED9
A
LED7
A
LED5
A
LED3
A
TEMPERATURE CONTROLLED SOLDERING IRON
Fig.1: the circuit uses the feedback from a thermocouple inside the soldering
iron’s barrel to control the switching of a Triac. The Triac is not phase
controlled but turns on or off depending on the temperature control VR1.
with a gain of 48. The 470Ω resistor
provides a current of 5mA through
the thermocouple and the resulting
small voltage developed across the
thermocouple is added to the voltage
generated due to the “Seebeck effect”;
ie, the voltage generated by a junction
of two wires of dissimilar metals.
The voltage at pin 3 of IC1a is amplified (by 48 times) and the output
at pin 1 is fed, via a filter network
consisting of a 100kΩ resistor and 1µF
capacitor, to pin 6 of IC1b. This second
op amp is connected as a comparator.
It compares the voltage from pin 1
of IC1a with the preset voltage from
potentiometer VR1, the temperature
set control.
When the voltage at pin 5 of IC1b is
above that at pin 6, the output at pin 7
66 Silicon Chip
is high and this causes transistor Q1 to
turn on. Q1 then turns on the internal
LED in optocoupler IC2 and this turns
on the Triac, to heat up the soldering
iron element. The collector current of
Q1 also passes through LED2 and this
serves as an indication that the heater
element is cycling.
A 4.7MΩ resistor is connected between pins 5 and 6 of IC1a to give the
comparator a degree of hysteresis; ie,
positive feedback. Thus, when pin 6
rises above pin 5 (due to increased
voltage from the thermocouple TH1),
the output at pin 7 flicks low and
because of current flow through the
4.7MΩ resistor, pin 5 is actually pulled
slightly lower than it would otherwise
be. Thus the voltage at pin 6 has to
drop further than it otherwise would
before the output at pin 7 flicks high
again. This “hysteresis” action prevents the circuit from rapidly hunting
on and off.
Temperature indication
IC3, the LM3914 dot/bar display
driver, is used to give an indication
of the temperature setting. Its pin 5
input is tied to pin 6 of IC1b, while
the pin 6 input is tied to the +3V side
of VR1. When the soldering iron is
cold, the voltage at pin 5 of IC3 is only
a few millivolts. This means that the
first LED in the 10 LED bargraph will
not light until the iron has reached a
significant temperature above cold. In
other words, if the iron is not warm
enough to melt solder then no LEDs
in the bargraph will light.
The arrangement of the power transformer and rectifier is a little unusual.
The transformer is a multi-tap unit
(DSE Cat. M-1991) with the 12V and
REAR PANEL
SOLDERING
IRON
SOCKET
EARTH
LUG
MICA
INSULATOR
TRIAC1
A1 A2 G
EARTH
(GREEN/
YELLOW)
1
MAINS
CORD
2
ACTIVE
(BROWN)
BROWN
BLUE
24V
12V
PRIMARY
PRIMARY
NEUTRAL
(BLUE)
0V
POWER TRANSFORMER
LEDS 2-13 MOUNTED ON COPPER SIDE OF BOARD
K
2 1
TRIAC1
IC1
LM324
Q1
470uF
1uF
47k
1
1k
47uF
470
470
8.2k
10k
4.7k
4.7M
100k
470uF
10uF
1
K
A
LED13
47k
IC2
MOC3020
1k
BR1
D2
VR1
REG1
7808
12V
OV
TRANSFORMER
SECONDARY
A
LED2
A
A
A
A
A
A
A
A
A
A
560
LED3 K
LED4 K
LED5 K
LED6 K
LED7 K
LED8 K
LED9 K
LED10 K
LED11 K
LED12 K
470
2.2k
IC3
LM3914
4.7k
1
LED1
K
D1
A
Fig.2: this wiring diagram must be followed carefully, particularly the details of the mains cord
termination & the wiring to the Triac.
October 1994 67
24V taps being used. 12VAC is fed to
the bridge rectifier and the resulting
DC is smoothed by the 470µF 25VW
electrolytic capacitor. The is fed to a
7808 8V 3-terminal regulator which
supplies most of the circuit.
24V AC supplies the soldering iron
element via the Triac. Note here that
the 0V tap on the transformer does
not connect to the earth of the circuit
but that the 0V rail, depicted by the
familiar earth symbol, does connect
to the 240VAC mains Earth via the
mains cord.
The barrel of the soldering iron is
earthed via the thermo
couple connection. We’ll discuss this point later
in the text.
Construction
Most of the circuit components are
mounted on a PC board measuring
132mm wide by 65mm deep. This
board is slot mounted vertically in
an Arlec case measuring 140mm
wide, 70mm high and 130mm deep.
The sides of the PC board need to be
tapered slightly at top and bottom
to make sure it fits snugly into the
case. The case comes with a plastic
front panel and steel rear panel and
these will require drilling before the
assembled PC board can be mounted.
But let’s talk about board assembly
first. The component overlay diagram
can be seen in Fig.2 which shows all
the wiring.
Before you begin any soldering,
check the board thoroughly for any
shorts or breaks in the copper tracks.
These should be repaired with a small
artwork knife or a touch of the soldering iron where appropriate.
If all is OK, you can start assembly
by inserting all the resistors and capacitors. Remember to watch the ori-
The PC board is mounted vertically inside the case & the transformer must be
placed so that its terminals do not contact components on the board. Note how
the indicator LEDs are mounted so that they sit flush with the front panel.
entation of the electrolytic capacitors.
Next, insert the diodes, bridge rectifier
and transistor Q1. If you want to use
sockets for IC1, IC2 and IC3 then insert
them now. Finally, insert and solder
the regulator. It is laid flat on the topside of the PC board and secured with
a screw and nut.
All of the LEDs except LED1 are
soldered to the copper side of the
PC board. Their lead length should
be about 18mm to allow them to
protrude through holes in the front
panel. 3mm LEDs are used for the
bargraph while the others can be
3mm or 5mm types. The 25kΩ potentiometer VR1 is also mounted on
the PC board and secured with a nut
and lockwasher. Before it is mounted,
its shaft should be cut to a length of
RESISTOR COLOUR CODES
❏
No.
❏ 1
❏ 1
❏ 2
❏ 1
❏ 1
❏ 2
❏ 1
❏ 2
❏ 1
❏ 3
68 Silicon Chip
Value
4.7MΩ
100kΩ
47kΩ
10kΩ
8.2kΩ
4.7kΩ
2.7kΩ
1kΩ
560Ω
470Ω
4-Band Code (1%)
yellow violet green brown
brown black yellow brown
yellow violet orange brown
brown black orange brown
grey red red brown
yellow violet red brown
red violet red brown
brown black red brown
green blue brown brown
yellow violet brown brown
5-Band Code (1%)
yellow violet black yellow brown
brown black black orange brown
yellow violet black red brown
brown black black red brown
grey red black brown brown
yellow violet black brown brown
red violet black brown brown
brown black black brown brown
green blue black black brown
yellow violet black black brown
ing a mica washer and bush and smear
the mounting surface lightly with
heatsink compound. Check with your
multimeter, switched to the “Ohms”
range, to confirm that the metal tab of
the Triac is actually isolated from the
metal panel.
Make sure that the mains wiring
is as depicted in Fig.2. Both the rear
panel and the transformer case must
be connected to the 240VAC mains
earth, while the Active and Neutral
wires should be terminated in a 2-way
insulated terminal block. The 240VAC
connections to the transformer should
be fitted with heatshrink sleeving to
make them safe.
Soldering iron assembly
Make sure that the mains cord is securely anchored using a cordgrip grommet &
note that the rear panel & the transformer case must be connected to the mains
earth. Bind the wiring that runs between the rear panel & the PC board using
cable ties as shown in this photograph.
about 25mm. This will allow sufficient shaft to protrude through the
front panel and have a knob fitted.
Drilling the case
Quite a few holes need to be drilled
in the case. On the front panel, you
will need to cut a slot for the 3mm
bargraph LEDs, plus holes for the
other two LEDs and the potentiometer
shaft. The latter hole can be 8-10mm in
diameter.
The base of the case needs to be
drilled for the transformer mounting
screws and a screw for the insulated
2-way terminal block. On the rear
panel, you will need holes for the
cordgrip grommet (for the mains cord),
for the Triac, for the earth solder lug
and for the 4-pin screw-in soldering
iron socket.
When drilling the base of the case
for the transformer, you will need to
offset it so that its voltage terminals do
not foul components on the PC board.
Check the relevant photo in this article
to clarify this point.
Mount the Triac on the rear panel us-
Where to buy the parts
A kit of parts for this project will be available from CTOAN Electronics.
This will comprise the PC board plus all on-board components, Triac,
mains cord and moulded plug. The soldering iron element, transformer
and case are available from any Dick Smith Electronics store. The cost
of the kit is $33.00 plus $5.00 for postage & packing. Ctoan Electronics
will also be selling built and tested PC boards for $58.00. A repair service
will also be available.
Contact CTOAN Electronics at PO Box 1031, Jimboomba, Qld 4280.
Phone (07) 297 5421.
Note: copyright of the PC board associated with this project is retained
by CTOAN Electronics.
There are several approaches you
can take to make the complete soldering iron. The specified soldering iron
element, DSE Cat. T-2008, comes with
the thermocouple already embedded
in the metal barrel so you only have
to connect the four wires, two for
the thermocouple and two for the
heater element, to a 4-way cable and
plug to match the socket on the rear
of the con
troller’s case. The tricky
bit comes in making or obtaining a
suitable handle.
If you already have a defunct soldering iron, you may be able to adapt
its handle to the specified element.
Alternatively, you could buy a cheap
iron such as the model T-2100 from
Dick Smith Electronics. You could
then discard its element and replace
it with the temperature element under
discussion.
The prototype pictured in this article was made from an old paint roller
handle, with a short length of dowel
inside to provide something for the
screws to be anchored in. Make sure
that what ever you use as a handle
will not melt because the base of the
heating element gets quite warm.
Take care when making connections
to the soldering iron element. All
connections must be well anchored
and should be well insulated to eliminate any possibility of shorts. The
two white wires are for the heating
element while the other two are for
the thermocouple: green is the direct
connection to the barrel while black is
the positive output which ultimately
connects to pin 3 of IC1a.
When all wiring is complete, thoroughly check it all against the circuit
and wiring diagrams of Fig.1 and
October 1994 71
PARTS LIST
1 PC board, code CE/94, 75mm
x 130mm
1 soldering iron element (Dick
Smith Cat. T2008)
1 soldering iron handle (see text)
1 multitap transformer, DSE Cat
M-1991 or equivalent
1 Arlec plastic case, 140 x 70 x
130mm, DSE Cat. H-2516
1 3-core mains flex & moulded
3-pin plug
1 cordgrip grommet to suit mains
cord
1 knob
1 4-way mic plug, DSE Cat.
P-1824
1 4-way mic socket, DSE Cat.
P-1834
1 25kΩ linear pot (VR1)
The soldering iron is connected to the controller via a 4-way microphone plug
& socket. Two of the leads are for the heater, while the other two leads go to
the thermocouple. Make sure that all leads to the soldering iron are securely
anchored, to avoid any possibility of shorts.
setting – the iron
should get hotter.
At the minimum
setting the first LED
may not be on. The
indicator only serves
as an indication that
the temperature is rising, falling or steady.
It is not meant to
accurately indicate
tip temperature.
Fig.4: isolate the Triac from the rear panel using
On the prototype,
a mica washer & insulating bush, as shown
it was found that a
in this mounting diagram. Smear all mating
good soldering temsurfaces with heatsink compound before bolting
perature was obtain
the assembly together, then use your multimeter
ed when two of the
(switched to the “Ohms” range) to confirm that
green LEDs were on.
the metal tab of the Triac is indeed isolated from
The red LEDs are
the metal panel.
meant to indicate
a very hot tip and
on our units the tip
Fig.2. You are now ready for the big actual
ly changed colour when all
moment. Connect the soldering iron LEDs were on.
and switch on the power and watch
If everything did not happen as it
for anything abnormal such as sparks, is supposed to then switch off and go
fire or explosions. Both the power and back over your work. Disconnect the
heater LEDs should come on. After a iron and connect a 100Ω pot is place
few seconds you should be able to feel of the thermocouple (ie, between pin
the iron barrel getting hot.
3 of IC1a and 0V). By rotating the pot
Rotate the temperature control fully you can simulate the rising and falling
anticlockwise. The heater LED should of the iron temperature.
cycle on and off with the temperature
You can also check out the various
being quite low. It will of course be too voltages on the circuit to see if they
hot to touch but may only just melt are correct. The voltage at pin 1 of
solder. Now increase the temperature IC1a should rise and fall with the
72 Silicon Chip
Semiconductors
1 LM324 quad bipolar op amp
(IC1)
1 MOC3021 optocoupled Triac
trigger (IC2)
1 LM3914 dot/bar display driver
(IC3)
1 LM7808 8V 3-terminal
regulator (REG1)
1 BC548 NPN transistor (Q1)
1 BT139-600 Triac (see text)
1 W04 bridge rectifier (BR1)
2 1N914 silicon diodes (D1,D2)
2 5mm red LEDs (LED1,2)
3 3mm yellow LEDs (LED3,4,5)
5 3mm green LEDs (LED6-10)
2 3mm red LEDs (LED11,12)
1 5mm green LED (LED13)
Capacitors
1 470µF 25VW electrolytic
1 470µF 16VW electrolytic
1 47µF 16VW electrolytic
1 10µF 16VW electrolytic
1 1µF 16VW electrolytic
Resistors (0.25W, 5%)
1 4.7MΩ
2 4.7kΩ
1 100kΩ
1 2.7kΩ
2 47kΩ
2 1kΩ
1 10kΩ
1 560Ω
1 8.2kΩ
3 470Ω
Miscellaneous
Hook-up wire, nuts, bolts, solder.
rotation of the 100Ω pot. Whenever
the voltage at pin 2 is below pin 3,
Q1 should turn on, as indicated by
SC
the heater LED.
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