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By RICK WALTERS Regulated Supply For Darkroom Lamps Don’t let variations in the mains supply ruin your photographic prints. This regulated power supply will keep the halogen lamp in your enlarger at its correct colour temperature. Maintaining an enlarger lamp at its correct colour tempera­ture is important when doing darkroom work, especially if you expect to obtain consistent results. In particular, the colour temperature of the lamp is critical for colour prints, although you can often get away with small variations for black and white prints. 54  Silicon Chip Unfortunately, many a darkroom session can be made frus­trating by small variations in the lamp output due in turn to variations in the mains supply. These variations are quite normal and can be due, for example, to heaters or air-conditioners cycling on and off or to some other cause. When this occurs, the lamp output changes and this affects both its colour temperature and exposure times. To overcome this problem, some readers have asked us to design a mains stabiliser but these are expensive and impractical for the hobbyist to con­struct. This Halogen Lamp Supply will effectively do the same job at a fraction of the cost. It provides a well-regulated 12V supply for the halogen lamp in the enlarger and varies its output by just 2mV for mains input voltages ranging from 195VAC to 280VAC. The unit is also very easy to operate. The front panel carries just a mains rocker switch, a power indicator LED and a toggle switch to turn the lamp on and off. Alternatively, the enlarger lamp can be turned on and off via a remote switch con­nected to a terminal block on the rear panel. Fig.1: the circuit is based on a TL494 PWM controller (IC1). This controls the output voltage by varying its output pulse width at pins 9 and 10. The circuit (see Fig.1) is basically a regulated 12V power supply capable of supplying up to 10A. It uses a mains transform­er to feed a full-wave bridge rectifier and this then supplies an unregulated filtered DC voltage to a switching regulator circuit. In this type of regulator, the output switching devices (power Mosfes) are either on or off and so their losses are quite low. In fact, the bridge rectifier gets much hotter than the output devices. Circuit details Let’s now take a look at the circuit in greater detail. As shown in Fig.1, the primary of the transformer is protected by a 3A slow-blow fuse which has been specified to handle the high inrush current. The two 18V secondary windings of the transformer are con­nected in parallel to provide the required current and these feed the bridge rectifier. This in turn supplies the rectified DC to two 4700µF filter capacitors which are both needed to cope with the high ripple current. They are followed by a 15A fuse which is only included to provide output short circuit protection. The output of the fuse is fed directly to one side of the lamp and to REG1, a 12V regulator which supplies a stable voltage to the rest of the circuit. We could probably have omitted the regulator as IC1 has its own inbuilt reference but as we are looking for a rock steady output, we decided to include it. The heart of the circuit is IC1, a TL494 pulse width modu­ lation (PWM) controller. Inside this device is an on-board oscil­lator, a reference regulator, two error amplifiers, several com­parators and a pair of output driver transistors. You can find out more about this device by referring to the Motor Speed Con­troller article in the June 1997 issue (a block diagram of the device was published on page 28). In simple terms, the TL494 PWM controller operates as fol­ lows. Its oscillator runs at 20kHz (as set by the RC components on pins 5 & 6) and it produces a pulse train at its outputs at this frequency. The width of the pulses is varied (ie, pulse width modulated) and the ratio of the “on” time to the “off” time controls the voltage applied to the load which in this case is the enlarger lamp. A fraction of the output voltage is fed to one input of one of the error amplifiers (pin 2 of A1), while the other input (pin 1) is connected to a reference voltage. If the output voltage rises slightly, the error amplifier senses this change and alters the output on-off ratio to bring the output voltage back to the required level. This is done by reducing the “on” time at the device out­puts (pins 9 & 10). The converse applies for a falling output voltage. Pins 9 & 10 of IC1 are simply the emitters of the two output transistors, November 1997  55 This close-up view shows the assembled PC board with one Mosfet fitted. Note that the board was modified after this photo was taken and some parts shown here have been deleted from the final design. connected here in parallel. Their collectors at pins 8 & 11 are connected to a +12V supply rail derived from 3-terminal regulator REG1. This means that the internal transis­tors operate as emitter followers and each time they turn on, they pull the bases of Q1 & Q2 to +12V. As a result, the emitters of Q1 & Q2 which are wired as complementary emitter followers, together with the gates of Q3 & Q4, swing from 0V to +12V. This means that the gate drive signal is limited to this voltage. Q1 and Q2 are included for another 56  Silicon Chip reason and that is to rapidly charge and discharge the gate capacitances of the Mosfets each time they turn on and off. This improves the switching action of the Mosfets; ie, it speeds up their turn-on and turn-off times and thereby reduces their power dissipation. Each time the Mosfets turn on (ie, when Q1 & Q2 turn on), current flows through them and the lamp to ground. The switching regulator (IC1) then acts to ensure that the average output voltage applied to the lamp is 12V. In order to control the output voltage precisely, the TL494 monitors both sides of the lamp. The filtered output from the bridge rectifier is monitored via 20kΩ and 2.2kΩ voltage divider resistors (R3 & R4), the output of which goes to pin 1 of com­parator A1. The voltage on the other side (at the drains of the Mosfets) is sensed via R1 & R2 (18kΩ and 4.7kΩ) and the sampled voltage fed via a 47kΩ resistor to pin 2 (the other input of com­parator A1). In addition, a voltage is tapped off the +5V reference by VR1 and fed through a second 47kΩ resistor to pin 2. This trimpot is used to set the output voltage. To understand how this works, it’s important to realise that the voltage on pin 2 is always equal to the voltage on pin 1, since these two pins are the inputs of an op amp. This means that if the wiper of VR1 is wound down towards 0V, the voltage at the junction of R1 & R2 must increase so that the pin 2 voltage remains the same as the voltage on pin 1. Conversely, if the wiper of VR1 is wound towards +5V, the voltage at the junction of R1 & R2 goes down to maintain the voltage on pin 2. What happens is that the TL494 varies its output pulse width so that its pin 2 voltage matches its pin 1 voltage. In practice, of course, VR1 is set to a fixed value and so the TL494 maintains a constant average voltage on the drains of the Mosfets and thus across the lamp. Note that the reference voltage for pin 1 of IC1 has been derived from the unregulated DC supply rail. This has been done so that the circuit automatically compensates for mains voltage variations. If the mains voltage varies, then so does the unregu­lated DC supply rail and thus the voltage on pin 1. As a result, the TL494 varies its output pulse width to bring the pin 2 vol­tage into line and keep the average lamp voltage constant. Slow start circuit Switches S2 (local) and S3 (remote) are used to turn the lamp on or off. They work in conjunction with a slow start cir­cuit which has been included to prolong the life of the lamp. If both switches are off, the 1µF capacitor between pins 4 & 14 of IC1 will be discharged due to the 4.7kΩ resistor and diode D1. The voltage on the inhibit pin (pin 4) will thus be equal to the REF voltage on pin 14 (5V) and there will be no output from the TL494. Conversely, when one of the switch­ es is closed, the 1µF capacitor charges via the 100kΩ resistor in parallel with D1. During this time, the voltage on the inhibit pin gradually falls and the output pulse width from the TL494 steadily increases. This means that the lamp voltage rises steadily to 12V, thereby providing a soft start. Construction Most of the parts are accommodated on a PC board coded 10107971 and measuring 145 x 102mm. Before commencing the assem­bly, check the board carefully against the published pattern to ensure that there are no etching defects. Fig.2 shows where all the parts go. No particular order need be followed when assembling the board but it’s best to start with the smaller parts first (resistors, trimpot, diodes and low-value capacitors). Table 1 shows the resistor colour codes but it’s also a good idea to check the resistor values on a digital multimeter. Take care to ensure that the correct transistor is in­stalled at each location and that its orientation is correct. We used an IC socket for the TL494 but this can be considered op­tional. It too must be correctly orientated, as must D1 and the electrolytic capacitors. PC stakes are used for the external connections to the local and remote switches and to LED1. Do not use PC stakes for the other wiring connections though – these points carry heavy currents and the leads should be soldered directly to the PC board. Although the circuit diagram (Fig.1) shows two Mosfets in parallel, one should be sufficient for lamp loads up to about 5A (ie, for lamps rated up to 60W). This Mosfet can be mounted Fig.2: take care to ensure that all polarised components are correctly orientated when building the PC board. You can use just one Mosfet for lamp loads up to about 60W. in either the Q3 or Q4 position and should be fitted with a small U-shaped heatsink (see photo of prototype). If the lamp is rated at more than 60W, then the second Mosfet should also be installed. Note that it will be necessary to splay the fins on one of the heatsinks slightly, so that the second heatsink can be fitted to its Mosfet. Make sure that the metal tabs of the Mosfets go towards IC1. The metal tab of REG1 Table 1: Resistor Colour Codes ❏ No. ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 ❏  1 ❏  2 ❏  2 ❏  1 ❏  2 Value 1MΩ 100kΩ 47kΩ 20kΩ 18kΩ 10kΩ 4.7kΩ 2.2kΩ 1kΩ 4.7Ω 4-Band Code (1%) brown black green brown brown black yellow brown yellow violet orange brown red black orange brown brown grey orange brown brown black orange brown yellow violet red brown red red red brown brown black red brown yellow violet gold brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown yellow violet black red brown red black black red brown brown grey 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 silver brown November 1997  57 Fig.3: be sure to use mains-rated cable for all wiring to fuse F1, the mains terminal block, switch S1 and for the earth connection to S2. LED1 and the wiring to it can be omitted if you use a neon-illuminated mains rocker switch. faces in the opposite direc­tion. Now install the two large 4700µF electrolytic capacitors and fit the bridge rectifier and the chassis-mount fuseholder. These last two items are bolted to the PC board using machine screws and nuts. Orient the bridge rectifier so that its “+” and “-” terminals are located as shown and note that a Powerfin heatsink (normally drilled for a TO-3 transistor) goes between it and the PC board. 58  Silicon Chip Smear heatsink compound over the bottom of the bridge rectifier before bolting it down. You can use one of the existing TO-3 mounting holes, which means that the bridge rectifier will sit slightly off-centre. The PC board assembly can now be completed by installing the wiring to the fuseholder and to the “+” and “-” terminals of the bridge rectifier (BR1). Use heavy duty 10A cable for this job. We used automotive-style spade connectors to terminate the leads on the bridge rectifier and fuseholder terminals. Two more heavy-duty cables, each about 180mm long, should also be soldered to points 3 and 4 on the PC board. Use a red lead for the point 3 connection and a black lead to point 4 (these are the output leads for the lamp and are later run to a terminal block mounted on the rear panel). Drilling the case A standard plastic instrument case with plastic front and rear panels is used to house the circuitry. Fig.3 shows the layout inside the case. As can be seen, the power transformer (T1) is mounted on an aluminium baseplate and this is secured, along with the PC board, to integral standoffs on the base of the case using self-tapping screws. The first step is to mark out and drill the necessary holes in the baseplate. You will need four mounting holes for the case standoffs, a mounting hole in the rear lefthand corner to secure the earth solder lugs, and a mounting hole in the centre to take the power transformer bolt. The hole locations for the standoffs can be easily marked out by first marking the tops of the standoffs with a felt pen and then carefully pressing the aluminium plate onto them. This done, the holes can all be centre punched and drilled to 3mm. By the way, the aluminium baseplate allows the mains wiring and the circuit to be correctly earthed. As such, it is an im­portant safety measure and must not be deleted. When drilling is complete, deburr all the holes and secure the two earth solder lugs in position using a machine screw, washer and nut. An additional locknut should then be fitted so that the earth lugs can not come loose. This done, secure the transformer to the baseplate, then fasten the baseplate to the case standoffs using four self-tapping screws. Note that the transformer is secured using a large bolt, two rubber washers and a large metal washer. One of the rubber washers sits under the transformer, while the second sits under the metal washer at the top. The front and rear panels can now be drilled to accept the various hardware items. The front panel requires a rectangular cutout for the mains switch (S1), plus holes for the power indi­cator LED and toggle switch S2. If you use a mains switch with a neon illuminated rocker, the power indicator LED can be omitted. The cutout for the mains switch can be made by drilling a series of small holes around the inside edge of the cutout area and then knocking out the centre piece. The edges should then be cleaned up using a file and the cutout carefully enlarged until the mains switch just fits. Make sure that the switch is a tight fit and that it cannot accidentally fall out. Moving now to the rear panel, you Parts List 1 PC board, code 10107971, 145 x 102mm 1 160VA toroidal power transformer with two 18V secondaries; Jaycar MT-2113, Altronics M4055 or equivalent 1 plastic case, 200 x 160 x 70mm 1 panel-mount 3AG fuseholder 1 3A 3AG fuse (F1) 1 chassis-mount 3AG fuseholder; Altronics Cat. S6010 or equiv. 1 15A 3AG fuse (F2) 1 3-way mains terminal block 1 4-way mains terminal block 1 mains switch with plastic rocker; Jaycar Cat. SK-0984 or SK0985 (illuminated) 1 mains lead with moulded 3-pin plug 1 miniature toggle switch 1 Powerfin heatsink (DSE Cat. H-3400 or equiv). 1 semi-mini heatsink (DSE Cat. H-3404 or equiv). 3 solder lugs 1 piece of aluminium, 130 x 90 x 1.6mm 1 5kΩ horizontal PC-mount trimpot (VR1) Semiconductors 1 TL494CN switching regulator controller (IC1) will require mounting holes for the cordgrip grommet, the fuseholder and the 4-way terminal strip. The locations of these components can be gauged from the photographs and from Fig.3. Note that the mains cord hole should be carefully profiled to match the cordgrip grommet. Final wiring The hardware items can now all be mounted in the case, ready for wiring – see Fig.3. Note that the mains terminal strip is secured to one of the case standoffs using a self-tapping screw. Exercise extreme care when installing the mains wiring, as your safety depends on it. In particular, make sure that the mains cord is securely anchored by the cordgrip grommet on the rear panel and cannot be pulled out. The Active (brown) lead from the 1 BC639 NPN power transistor (Q1) 1 BC640 PNP power transistor (Q2) 1 or 2 MTP75N05 Mosfets (Q3,Q4) – see text 1 7812 regulator (REG1) 1 1N914 small signal diode (D1) 1 400V 25A bridge rectifier (BR1) 1 red LED and mounting bezel (LED1) (not needed if mains switch has neon-illuminated rocker) Capacitors 2 4700µF 25WV PC electrolytic 3 100µF 25WV PC electrolytic 1 1µF 16VW PC electrolytic 3 0.1µF MKT polyester 1 .0068µF MKT polyester Resistors (0.25W, 1%) 1 1MΩ 1 10kΩ 1 100kΩ 2 4.7kΩ 2 47kΩ 2 2.2kΩ 1 20kΩ 1 1kΩ 1 18kΩ 2 4.7Ω Miscellaneous Heatshrink tubing, red and black heavy-duty hookup wire, light-duty figure-8 cable, mains-rated cable (brown, blue & green/yellow) mains cord goes directly to fuse F1, the Neutral (Blue) lead goes to the mains terminal block, and the Earth lead is soldered to one of the earth lugs on the baseplate. Additional mains-rated leads are then run from the fuse and terminal block to the mains switch (S1). The terminals of the fuseholder and mains switch should be covered with heatshrink tubing to prevent accidental contact with the mains. This involves slipping a length of heatshrink tubing over all the leads before they are soldered to the terminals. After soldering, the heatshrink tubing is pushed over the fuse­holder and mains switch bodies and shrunk using a hot-air gun. The two orange wires from the transformer are the primary leads and these go to the mains terminal block, as shown. The low-voltage secondary November 1997  59 The power transformer is mounted on an aluminium plate which must be securely earthed. Sleeve all exposed terminals on the mains switch and fuse with heatshrink tubing to prevent accidental contact with the mains. leads are much thicker. Twist the red and pink leads together and terminate their ends in a spade terminal. This done, do the same for the white and yellow leads, then connect the transformer secondaries to the AC terminals on the bridge rectifier. Finally, complete the wiring to LED 1, switch S2 and to the rear-panel terminal strip. Note that a solder lug goes over S2’s collar and that an earth lead (mains rated) is run from this collar back to an earth solder lug on the baseplate. This earths the metal parts of the switch body, including the toggle actua­tor. Testing Before switching on, go back over the wiring carefully and check that all is correct. This done, wind VR1 fully clockwise apply power and check 60  Silicon Chip SILICON CHIP This advertisment is out of date and has been removed to prevent confusion. SMART ® FASTCHARGERS Brings you advanced technology at affordable prices Fig.4: check your PC board against this full-size etching pattern before installing the parts. REMOTE SWITCH TO LAMP (12V) FUSE 3A 240VAC Fig.5 (above & right): these two labels can be affixed to the rear panel above the terminal block and next to the mains fuse. Disconnect mains plug from wall outlet before removing fuse As featured in ‘Silicon Chip’ Jan. ’96 This REFLEX® charger charges single cells or battery packs from 1.2V to 13.2V and 110mAh to 7Ah. VERY FAST CHARGING. Standard batteries in maximum 1 hour, fast charge batteries in max. 15 minutes AVOID THE WELL KNOWN MEMORY EFFECT. the voltages at various points on the circuit. You should get about +27V at the output of the bridge rectifier, +2.7V on pin 1 of IC1, +5V on pin 14 and +12V on pin 12 (ie, the output of REG1). If all these voltages are correct, connect a 12V test lamp and a voltmeter across the output terminals and carefully wind VR1 up until you get 12V. You should now be able to measure about 2.4V at the wiper of VR1 and 3.0V at the junction of R1 and R2 although your unit may vary slightly from these figures. Finally, the output voltage should be reset when the enlarger lamp is connected, to make sure it is correct. That’s it – you can now tackle your darkroom printing jobs without being affected by annoying variations in the SC lamp output. NO NEED TO DISCHARGE. Just top up. This saves time and also extends the life of the batteries. SAVE MONEY. Restore most Nicads with memory effect to remaining capacity and rejuvenate many 0V worn-out Nicads EXTEND THE LIFE OF YOUR BATTERIES Recharge them up to 3000 times. DESIGNED AND MADE IN AUSTRALIA 12V-24V Converters, P. supplies and dedicated, fully automatic chargers for industrial applications are also available. For a FREE detailed technical description please Ph: (03) 6492 1368 or Fax: (03) 6492 1329 2567 Wilmot Rd, Devenport, TAS 7310 November 1997  61