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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.