Fullscreen HTML5Med Res (??MB)HTML4

A high-power dimmer for incandescent lamps Need a dimmer for a large domestic or stage application? This unit will dim an incandescent or halogen lamp load of up to 2400 watts. It can also dim 12V transformer-driven halogen lamps or be used for fan speed control. Design by MARQUE CROZMAN 24  Silicon Chip Low power dimmers for loads up to 500 watts or so are readily available and quite cheap at around $20. But if you want to dim much larger loads than this the cost of a commercial dimmer becomes quite expensive and can range up to several hundred dollars. Why pay that much when you can save money by building this version and incorporate extra features as well? For example, this circuit can be remotely controlled by a 0-10V DC signal. This means that the dimmer itself can be in­stalled out of the way while three wires at low voltage can run to the dimmer potentiometer. This can then be placed in a con­venient location. Alternatively, you could incorporate a local/remote switch so that the dimmer could be directly con­trolled by the knob on its case or via the remote potentiometer. Furthermore, these options can always be incorporated later if you don’t need them right now. The dimmer is housed in a rugged diecast aluminium case measuring 170 x 121 x 55mm. The case provides heatsinking for the Triac as well as external protection for the circuit. As already noted, it can dim up to 2400 watts of lamps which may be made up in any combination. Minimum recommended lamp load is 40 watts. Let’s now have a look at the circuit diagram which is shown in Fig.1. This looks fairly complicated but is essentially a phase controlled Triac, similar to that in any commercial light dimmer. The major difference between this circuit and most 300-500W commercial 10 25VW A2 D3 24V CASE PIN2 IC1b E 4.7k 2 3 1 1 1k IC1b 2 2 3 4 5 6 D2 1N914 7 100  B 8 1k +15V ZD1 10V 400mW PIN14 IC2 +15V 100k SET MIN BRIGHTNESS VR2 10k 10k E 0.1 C 220  100k 100k +15V 5.6k IC2d 5 1 SET MAX BRIGHTNESS VR3 50k A N 240VAC F1 10A 820k IC2c 12 7 13 IC2b 6 10k 8 IC2a 10 LM324 11 4 9 D4 2x 1N4004 BR1 DB104 100 50VW IN GND REG1 7815 OUT CASE A1 I GO C E 7 13 14 11 IC1f IC1e 12 10 6 IC1d 5 VIEWED FROM BELOW B 2  4 IC3 MOC3021 A K .033 250VAC 6 1 4 14 3 10k 10k 2.4kW LAMP DIMMER G 0.1 L1 : 19T, 1mm DIA ENCU ON A PHILIPS 4330 030 60271 TOROID +15V A E CASE N GPO 2.4kW MAX 0.1 250VAC L1 G TR1 BTA41A A2 A1 0.1 250VAC 22  1W 240VAC 470  390  A LED1  K 2.2k 0.5W +4.6V DIMMMER VR1 50k LIN D1 1N914 Fig.1 (right): the full circuit of the dimmer. Most of the circuitry runs at low voltage & is isolated from the 240VAC mains via the transformer & the optocoupler IC3. 4.7k 9 IC1a 40106 As with any dimmer circuit, the power to the lamps is varied by Q1 BC547 Circuit principle 100k dimmers is that most of the circuit is isolated from the 240VAC mains supply by virtue of an optocoupler and a transformer. The heart of the circuit is the Triac, TR1. This is a BTA41A Triac, a 600 volt, 40 amp device which has been selected to cope with the high surge currents when switching on an incan­d­ escent lamp load totalling 2400 watts. Typically, the surge current at switchon can be 10 to 15 times the normal load cur­ rent; ie, the surge current could be 100-150 amps and last for several milliseconds. The Triac must also be able to cope with the high fault currents that flow when high power lamps blow their filaments. To explain, when a lamp blows its filament the now loose sections can flay around and come in contact with the stem supports. When that happens a high fault current can flow which is not extinguished until the stem fuse blows. Clearly, the Triac must be rugged to cope with this. 680  • • • • • IC1c • Features 2400W maximum lamp load 40W minimum lamp load Industry standard 0-10V dimming control Dims transformer-driven halogen lamps 10A mains supply fuse Adjustable maximum brightness Adjustable minimum brightness RF interference suppression 7.5kV optocoupler isolation between control circuitry and 240VAC mains for safety. +15V • • • August 1994  25 This view shows how the completed PC board is mounted in the case, along with the GPO & the mains terminal block. The front panel controls are connected to the board via a 7-way pin header. switching on the Triac early or late in each mains half-cycle. For high power operation, the Triac is triggered on late in each mains half-cycle so that the effective voltage fed to the lamp load is low. Similarly, for high power operation or full on, the Triac is triggered early in each mains half cycle so that virtually the full mains voltage is applied to the lamp load. This method of power control is referred to as “phase con­trol” because we vary the phase of the Triac trigger pulses with respect to the mains waveform. Most small dimmer circuits use a Diac or similar capacitor discharge device to trigger on the Triac but this circuit is more complex, mainly to provide isola­tion between the control circuitry and the 240VAC mains supply. Circuit description Before we dive into a full description of the circuit shown in Fig.1, let’s identify some of the key sections. First, at the top righthand corner is the Triac itself which feeds the lamp 26  Silicon Chip load via a standard 3-pin mains socket. In the lower right­hand corner is the low voltage supply which uses a 24V transformer feeding a bridge rectifier and 3-terminal regulator. In the top lefthand corner is the ramp generator (IC1a & IC2a) while below that, in the bottom lefthand corner, is the 0-10V DC control circuitry. Now that you are oriented, let’s start with the low voltage supply involving the 24V transformer. As already noted, this feeds a bridge rectifier (BR1) and a 100µF capacitor to drive a 3-terminal regulator REG1, which produces a 15V WARNING! While most of the circuitry operates at low vol­tage, this is a mains operated circuit and must be regarded as potentially lethal when power is applied to it. This project is not one for beginners and should only be attempted by construc­tors who have previous experience with mains powered circuits. DC supply. The reason for using the relatively high transform­er voltage of 24VAC has to do with the ramp synchronisation. Two diodes, D3 and D4, feed the rectified but unfiltered DC to a network at the input of IC1a which consists of a 4.7kΩ resistor, diode D1 and a 2.2kΩ resistor. As a result of this network, the voltage at the input of Schmitt trigger IC1a will be at +15V for most of the time but will drop to +4.6V at the beginning of each mains half-cycle. This waveform is inverted and squared up by IC1a to pro­duce a series of narrow positive pulses synchronised to the 50Hz mains supply. This pulse train drives the base of transistor Q1 which discharges the capacitor at its collector every 10ms. In between each discharge the capacitor is charged via the 100kΩ resistor connected to the +15V rail. The resulting sawtooth waveform is buffered by op amp IC2a and inverted by op amp IC2b and then fed to pin 13 of op amp IC2c which is connected as a comparator. Op amp IC2d is fed by the 50kΩ dimmer potentiometer VR1 which is fed with +10V from zener diode 7 D3 3 K 2 LED1  1 D1 Fig.2: this diagram shows the additional circuitry required for the remote dimming facility. It is connected to the main circuit via a 7-way header plug and socket on the PC board. ZD1 1 IC3 1k ZD1. Thus the input from VR1 can range anywhere from zero to 10V DC, depending on the desired lamp brightness. The DC voltage from VR1 is buffered by op amp IC2d which has an adjustable gain of less than unity (ie, it is an attenuator). The voltage from IC2d is fed to pin 12 of comparator IC2c which compares it with the 100Hz sawtooth voltage at pin 13. The result is a variable width pulse train corresponding to the dimmer setting; ie, wide pulses for a high brightness so that the Triac is triggered early in each 1k 100k VR3 100k 1 HEADER 22  1W 0.1 250VAC .033 250VAC 0.1 250VAC MOC3021 Fig.3 (right): the part layout diagram for the PC board. This should be used in conjunction with the wiring diagrams of Figs.4 & 5. Note the wire link between ZD1 and the adjacent 220Ω resistor. Note also that the two pads immediately above the 7-pin header are vacant. D2 1 4.7k 680  A 10uF 4.7k 2.2k 10k 5.6k 10k 1 100k VR2 D4 POWER TRANSFORMER IC2 LM324 820k Q1 10k 100  OPTIONAL REMOTE CONTROL 100uF BR1 4 10k 0.1 5 IC1 40106 1 S1 VR1 50k LIN 100k 24V 220W XLR 1 PLUG 6 390  3 470  2 REG1 (MOUNTED ON CASE) PRIMARY 50k LIN XLR 2 PANEL SOCKET 3 7 L1 F1 TRIAC1 ACTIVE NEUTRAL mains half-cycle and narrow pulses for a dim setting. The pulses from IC2c are then buffered by four paralleled inverters (IC1c,d,e & f) which drive the opto­ coupler IC3. In turn, the optocoupler triggers the Triac which drives the lamp. At this stage you have most of the picture of the circuit operation but there are some details yet to be discussed. For example, the remaining inverter in the 40106 hex Schmitt trigger package, IC1b, is connected to G A2 A1 LOAD the output of IC2c and is used to drive indicator LED1 via a 1kΩ resistor. This LED then provides a rough indication of the brightness setting of the dimmer since it is driven from the same pulses as are used to trigger the Triac. A 7-pin header socket on the board provides for local or remote operation. On the dimmer panel, a 50kΩ slider pot (VR1) provides the dimming control. Alternatively, switch S1, an addi­tional 50kΩ linear potentiometer and a 3-pin XLR socket can provide for remote dimming, as shown in Fig.2. RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  4 ❏  4 ❏  1 ❏  2 ❏  1 ❏  2 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 Value 820kΩ 100kΩ 10kΩ 5.6kΩ 4.7kΩ 2.2kΩ 1kΩ 680Ω 470Ω 390Ω 220Ω 100Ω 22Ω 4-Band Code (1%) grey red yellow brown brown black yellow brown brown black orange brown green blue red brown yellow violet red brown red red red brown brown black red brown blue grey brown brown yellow violet brown brown orange white brown brown red red brown brown brown black brown brown red red black brown 5-Band Code (1%) grey red black orange brown brown black black orange brown brown black black red brown green blue black brown brown yellow violet black brown brown red red black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown orange white black black brown red red black black brown brown black black black brown red red black gold brown August 1994  27 CORD GRIP GROMMET Fig.4: this diagram shows all the off-board wiring & the primary & secondary connections for the power transformer. Note that the earth leads from the power cord & power socket must be soldered to an earth lug which is securely bolted to chassis. MAINS CORD REG1 (MOUNTED ON CASE) 100k 10k 820k VR3 100k 100  IC1 40106 470  1 390  100k 1 1k ZD1 IC3 D2 4.7k 220  SECONDARY PRIMARY POWER TRANSFORMER 5.6k 10k 1 100k VR2 10uF 1k 100uF Q1 10k IC2 LM324 0.1 24V 1 HEADER 7 22  1W 0.1 250VAC .033 250VAC 0.1 250VAC MOC3021 L1 F1 A N G A2 A1 5 O/P 4 3 1 TRIAC1 2 A VR1 EARTH (GREEN/ YELLOW) K LED1 EARTH LUG NEUTRAL (BLUE) ACTIVE (BROWN) N E PANEL MOUNT POWER SOCKET A Note that the potentiometer must be a linear type otherwise the dimming charac­teristic will not be smooth and progressive. Brightness adjustments Adjustments are provided in the circuit for maximum and minimum 28  Silicon Chip brightness settings. First, VR2 provides the minimum brightness setting, when the main potentiometer VR1 is at is zero setting. VR3 sets the gain of op amp IC2d and thereby sets the maxi­mum brilliance setting. It is set by taking VR1 to its maximum setting and then noting the lamp brilliance. VR3 is then rotated clockwise to note if there is any increase in brilliance and then backed off slightly. The idea is to set it so that the maximum setting of VR1 does in fact give the maximum brilliance. The settings of VR2 and VR3 will interact so it will be necessary to adjust each XLR PANEL SOCKET ON/OFF SWITCH A (OUTPUT) A 2 7 3 240VAC 50k LIN LAMP DIMMER LOAD 1 N N E 5 Fig.6: this diagram shows how the dimmer could be wired up in a permanent installation. The dimmer itself could be installed in the ceiling, while the low voltage potentiometer connections & 10A switch could be on a standard architrave plate. Note that this installation can legally only be performed by a licensed electrician. 6 S1 4 3 1 2 A VR1 Fig.7: this diagram shows the mounting details for the insulated tab Triac. No mica washer or plastic bush is required. K LED1 Fig.5: this diagram shows the wiring of the remote control ver­sion with all connections made via a 150mm length of rainbow cable & a 7-way header plug. in turn several times to finalise the settings. High voltage circuitry So far, virtually all of the circuit description has ap­ plied to the low voltage portion but the components associated with the Triac need explanation. First, the 22Ω resistor and 0.1µF capacitor comprise a “snubber” circuit which allows the Triac to commutate correctly (ie, switch off reliably) at the end of each mains half-cycle when the load is inductive. This would be the case when dimming 12V transformer driven halogen lamps. The optocoupler is also provided with snubber protection and this takes the form of the 390Ω and 470Ω current feed resis­tors which are tapped off by the .033µF 250VAC capacitor. One of the drawbacks of this type of dimmer circuit is the very fast switching of the Triac. This produces switching transients which range up to 30MHz or more, resulting in a buzz­ ing sound when received by radios. To eliminate this problem, RF suppression is provided by inductor L1 which is in series with the load socket, together with the 0.1µF capacitor across the load. L1 and the 0.1µF capacitor comprise a low pass filter which is a very effective at reducing the amount of radiated interfer­ence. Note that a critical aspect of L1 is that it is wound onto an iron powder toroid. This gives an inductor with a relatively low Q-factor, ensuring that oscillations caused by the fast switching of the Triac are well damped. Construction The specified Triac is an insulated tab device which is mounted directly to the case for good heatsinking. Note the plastic cable tie which secures the interference suppression toroid (L1) to the PC board. The new dimmer is housed in a diecast aluminium case meas­ uring 171 x 120 x 55mm, as noted above. Most of the circuitry is mounted on a PC board measuring 96 x 79mm and coded 10107941. This also has the transformer mounted on it, as can be seen in the component overlay diagram of Fig.3. Note that this is slight­ly August 1994  29 This inside photo shows all the wiring, including the wired remote control facility. Note that there are slight differences between the board in this photo and the diagram of Fig.3. different from the PC board shown in the photos. 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. Mount the diodes, resistors and wire links first. Note that one row of resistors is installed “end on” to save board space. You can use the clipped off resistor leads for the wire links. Now mount the 3AG fuse clips, the capacitors and the two trim­pots. Note that VR2 is 10kΩ while VR3 is 50kΩ; don’t inadvertent­ly swap them around. This done mount the transistor and the integrated circuits and make sure you install them with the correct orienta­tion which is shown by the notch at the pin 1 end. Fig.8: this is the full size etching pattern for the PC board. 30  Silicon Chip The transformer is bolted to the board using screws, nuts and lock­ washers and then its primary and secondary leads are soldered in. Note that the secondary wires are not depicted on the overlay diagram of Fig.3 but they are shown on the wiring diagram of Fig.4. This was done for clarity. The iron powder toroid (Philips 4330 030 60271) is wound with 19 turns of 1mm diameter enamelled copper wire. Strip the wire ends for soldering and space the turns evenly around the core. When soldered to the board, secure the toroid with a Nylon cable tie – see photos. Finally, you can mount the 3-terminal regulator and the Triac. The regulator is mounted on top of the board in the con­ventional way while the Triac leads are soldered to the underside of the board so that its metal tab can be bolted to the floor of the diecast case. Case assembly At the time of writing this article we do not know whether kits will be offered with pre-punched metalwork. If not, there will be quite a lot of drilling and filing to be done to prepare the case. You will need to drill and cut the holes for mounting the board, 3-terminal regulator and Triac, the Earth solder lug, the 2-way insulated terminal block for the mains cable, the hole for the cordgrip grommet, the flush-mount mains socket and the dimmer potentiometer. Note that all screw holes in the underside of the case and the lid should be countersunk. Four adhesive rubber feet should be fitted to the bottom of the case to avoid scratching table surfaces. The slider requires a slot 2mm wide and 50mm long. As well, if you require the optional remote facility, you will also need to drill or punch holes for the XLR socket and switch. The front panel artwork shown in Fig.9 shows how the front panel compon­ents are laid out. Use a photocopy of the artwork as a drill­ing template. The PC board is mounted on four 6mm pillars on the base of the case. Before installing it, you should attach the 250VAC 10A rated hook-up wires which will connect to the AC socket and to the insulated terminal block. Use brown for the Active lead and Blue for Neutral. Having mounted the board, the The 3-terminal regulator (REG1) is heatsinked by bolting it directly to one end of the case. Do not use an insulating washer here, as the tab of the regulator actually grounds the low voltage side of the circuit to the case. 3-terminal regulator and Triac can be bolted to the case. Note that the metal surface must be smooth and free of metal swarf. Use a light smear of heatsink compound under the metal tab to improve heat transfer. Note that mica washers are not required for either of these semiconductor devices. In fact, the tab of the 3-terminal regulator actual­ly grounds the low voltage side of the circuit to the case. The specified Triac, on the other hand, is an insulated tab device, so no mica insulation kit required. The mains cable should be secured in the case with a cordgrip grommet and its yellow/green wire should be attached to the earth solder lug. An earth wire from the mains socket runs to the same solder lug. All the wires to the lid of the case are run as a multi-strand (rainbow) cable to the 7-pin header socket on the PC board. If you require the optional remote facility, you will need to wire the front panel as shown in the diagram of Fig.5. The header plug comes un-assem­ bled as the plastic shroud together with a strip of pins. Carefully strip back and tin seven strands of a 150mm length of rainbow cable. With the pins still attached as a strip, crimp each pin onto the tinned wires before soldering. The pins can then be separated from the strip and pushed into the plastic shroud. Push until the locking spring on each pin becomes seated in the header. Once assembled, the rain­bow cable can be wired to the front panel components. When all the wiring is complete, check your work carefully against the circuit of Fig.1 and the wiring diagrams of Figs.3 & 4. Now you are ready to apply power but do not local remote input remote Max. load 2400W Fuse rating 10A (inside case) DANGER 240 VOLTS AC INSIDE lighting dimmer Fig.9: full size artwork for the front panel. August 1994  31 PARTS LIST 1 PC board, code 10107941, 96 x 79mm 1 sealed diecast aluminium case, 171 x 121 x 55mm 1 Philips toroid, Part No. 4330 030 60271 1 M2854 24V CT transformer 1 Clipsal 10A flush mount GPO socket 1 7-way single-in-line PCB header & socket (0.1-inch spacing) 1 female 3-pin XLR socket 1 SPDT round rocker switch 1 black slider pot knob 1 LED mounting bezel 1 self-adhesive front panel 1 earth lug 1 terminal block 4 adhesive rubber feet 1 1-metre length 1mm diameter enamelled copper wire 1 150mm-length 7-way rainbow cable 1 cordgrip grommet for mains cable 1 10A 240V AC 3-core mains cable & moulded plug 2 PCB mount 3AG fuse clips 1 10A 3AG fuse 9 3mm dia. x 10mm countersunk machine screws 3 3mm dia. x 25mm countersunk machine screws 12 3mm hex nuts & washers 4 6mm standoffs 1 50kΩ linear slider pot (60mm travel) (VR1) 1 10kΩ horizontal trimpot (VR2) connect a load at this stage. Fit the 10amp fuse, put the lid on the case and apply power. Move the slider up and down and observe the LED. It should brighten and dim in accordance with the control setting. If that happens, you are practically finished apart from setting the minimum and maximum brightness settings. To do this, you must connect a lamp load of 40 watts or more and take the lid off the case to do the adjustments. Warning: this circuit is potentially lethal due to the presence of 240VAC on the Triac and associated components. With the power on, set the dimmer pot to the minimum set­ting and adjust trimpot VR2 so that the lamp filament 32  Silicon Chip 1 50kΩ horizontal trimpot (VR3) Semiconductors 1 40106 hex Schmitt inverter (IC1) 1 LM324 quad op amp (IC2) 1 MOC3021 (IC3) 1 BTA41A Triac (TR1) 1 BC547 NPN transistor (Q1) 2 1N914 diodes (D1,D2) 2 1N4004 diodes (D3,D4) 1 10V 400mW or 1W zener diode (ZD1) 1 7815 15V regulator (REG1) 1 DB104 bridge rectifier (BR1) 1 5mm red LED (LED1) Capacitors 1 100µF 50VW electrolytic 1 10µF 25VW electrolytic 2 0.1µF MKT polyester 2 0.1µF 250VAC metallised polycarbonate (0.4-inch lead spacing) 1 .033µF 250VAC metallised polycarbonate (0.4-inch lead spacing) Resistors (0.25W,1%) 1 820kΩ 1 680Ω 4 100kΩ 1 470Ω 4 10kΩ 1 390Ω 1 5.6kΩ 1 220Ω 2 4.7kΩ 1 100Ω 1 2.2kΩ 1 22Ω 1W 2 1kΩ Scope photo 1 – ramp generator waveforms: top, waveform at pin 8 of IC1a; bottom, waveform at the collector of Q1. Scope photo 2 – comparator wave­ forms: top, waveform at pin 8 of IC2a; bottom, waveform from pin 14 of IC2c. Miscellaneous Heatsink compound, cable ties is at red heat. Now set the dimmer control to its maximum setting and adjust VR3 so that the lamp just gets to maximum brightness when VR1 is brought to its maximum. The settings of VR2 and VR3 will then have to be repeated because they do interact. Finally, attach the lid to the case and you are finished. What if it doesn’t work? The first point to check is the DC voltage as marked on parts of the circuit of Fig.1. You can use the case as the negative connection point for your multimeter. Note that the DC output of IC2d should vary in response to the setting of the potentiometer VR1. If you have an oscilloscope, you can check for the presence of the waveforms shown in Scope photo 3 – Triac waveforms: top, waveform at A2 of the Triac; bottom, waveform at pin 2 of IC3. Note that the mains waveform is flattened due to external causes. the accompanying scope photographs If not, you can still check for the presence of trigger pulses at the outputs of IC2c, IC2d and the paralleled outputs of IC1. This can be done because your multimeter can measure the average DC level of the pulses. At maximum setting, the pulsed DC output will measure close to +15V while at minimum it should be close to 0V. Failing that, check your soldering very carefully. The main cause of failure in SC projects is bad solder­ing.