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. . . dimming with the power of a PIC Pt.1: By JOHN CLARKE This single-channel fully automatic high-power light dimmer has a host of control features because it is driven by a PIC microcontroller. It will drive incandescent lamp loads up to a total of 2400 watts. 26  Silicon Chip www.siliconchip.com.au T HE SILICON CHIP Touch/Remote Controlled Dimmer, described in the January and February 2002 issues, was a low power device, suitable for lamp loads up to 250W. That’s OK for dimming the lights in your lounge room or bedroom but useless for dimming high power stage lights or a bank of lights in a hall or church. For that purpose you need a high power dimmer and that is the reason for this completely new design. It is specially de­signed to drive the high power lamps used in stage lighting, up to a total of 2400W. It has all sorts of control features such as preset brightness levels, dimming rates, flash on and off buttons and so on. Our last high power dimmer, featured in the August 1994 issue, was a fairly basic design with just a slider knob to control the brightness. This new design has no slider knob but can dim up or down manually or automatically and has LED bar­graphs to indicate the brightness levels, dimming rates and more. Features The SILICON CHIP Automatic Light Dimmer is housed in a rugged diecast metal case measuring 170 x 120 x 55mm. We used a diecast metal box for two reasons: first because stage light dimmers often have a rugged life and second, the case provides heat­sinking for the Triac which is the power control device at the heart of the circuit. At one end of the case is the 240VAC mains cord, a 3-pin mains socket for the lamp, a power switch and a fuse holder. On the front panel are two LED bargraphs, a large LED brightness indicator and no less than eight switches of various sorts. Along the bottom edge of the control are three rocker switches, two of which are spring-loaded centre-off types. Want to dim the lights up or down? Use the DIM switch in the lefthand corner. Push it up to go brighter; down for dimmer. Want to flash the lights to full brilliance? Push the FLASH switch on. Want to flash them off? Push the FLASH switch off. This can be done at any time, regardless of other settings or modes. Dimming can be manual or automatic, depending on the set­ting of the Automatic/Manual Dimming switch www.siliconchip.com.au SPECIFICATIONS Maximum lamp power ��������������������2400W Minimum lamp power ���������������������60W (lower power lamps may flicker) Phase angles ���������������������������������5.8° for maximum brightness and 174° for minimum brightness Auto Dimming rates ������������������������0, 0.5s, 1s, 1.5s, 2s, etc in 39 steps up to 40s maximum Dimming steps �������������������������������102 typical beyond initial preheat setting up to full brightness Dimming display �����������������������������39 levels Triac gate drive period ��������������������80µs in the right-hand corner. When dimming automatically, pushing the DIM switch UP lets the lamp(s) brighten up to the preset brilliance. Pushing the DIM button DOWN, dims the lamp(s) back down to zero. Brightness can be preset to one of 39 brightness levels with the LEVEL UP/DOWN rocker switch. Brightness levels are indicated on a 20-LED bargraph. Yes, we know we said there are 39 brightness levels? So how do you indicate 39 levels with 20 LEDs? The trick is that we use 20 LEDs to indicate 20 levels from maximum to minimum but the intermediate brightness levels are indicated with two adjacent LEDs – its harder to describe than to use. So as the level is increased, we get one LED, then two adjacent LEDs, then the top one of that adjacent pair, then the next adjacent pair and so on. This one-two-one LED sequence indicates 39 levels. The same 20-LED bargraph can also show the Flash brightness setting which is preset using the DIM/FLASH switch, in conjunc­tion with the up/ down rocker switch to the left of the bargraph. Filament preheat When high power lamps are initially switched on, their cold filaments have a very low resistance and so they have very high surge currents. This is bad enough at switch-on but if a lamp is to be repeatedly flashed on, as it can be with this dimmer, then the repetitive surge currents can destroy the Triac and also blow out the filament of the lamp itself. To reduce this problem, the lamp filament is always run with a low value of “preheat” cur­rent, typically with the filament glowing a dim red. Preheat setting is done by pressing the DIM UP, LEVEL UP and Store Settings switches all together. We will discuss this later in this article. The actual lamp brightness is indicated in two ways on the display. Firstly, there is a large 10mm LED which glows according to the lamp brightness. Second, the 20-LED bar- MAIN FEATURES • • • • • • • • • • High power lamp control Maximum lamp brightness preset Minimum lamp brightness preset for filament preheating Automatic or manual dimming between brightness presets Separate flash on and flash off Flash brightness preset Dimming rate programmable from instant through to 40 seconds A and B dimming rate selection Lamp brightness indication Automatic dim up and dim down indication April 2002  27 in the two bargraphs are in “dot” mode – ie, single LEDs glowing – rather than “bar” mode. When dimming automatically, dimming can be stopped at any time by momentarily pressing the DIM UP/ DOWN switch in the op­posite direction to the dimming. So if the lamp is in the process of dimming up, dimming can be stopped by momentarily pressing the DIM DOWN switch. If this switch is held down or pressed again, then the dimmer will begin dimming down. Similarly, if dimming down in auto mode, dimming can be stopped by momentarily pressing the DIM UP switch. Dimming rate Scope 1: this somewhat distorted mains sinewave is straight out of a power point. While nominally 240V AC, 50Hz, in this case it’s actually 250V AC and the frequency is just a tad low (neither of which is unusual). Scope 2: this scope shot shows the power being made available to the load very late in the half cycle so that it effectively receives just under 40V. In this case, the lamp would be barely glowing. graph indicates actual brightness and preset brightness levels. Actual lamp brightness is shown with a flashing LED in the bargraph, while the DIM or FLASH preset levels are shown by a constant LED. If the two levels are the same, the indicating LED will shimmer rather than glow constantly. By the way, all the LED indications 28  Silicon Chip PLEASE NOTE! The scope waveforms in this article are shown to explain the operation of the circuit. DO NOT try to reproduce these waveforms yourself – it is much too dangerous. Automatic and manual dimming occurs at a preset RATE. You can set two dimming rates (A and B) with each one ranging from instantaneous to 40 seconds, in 0.5s increments, as displayed on the right-hand bargraph. The topmost LED indi­cates the longest dimming time and therefore the slowest rate. When automatic dimming is in progress, the topmost LED in the Rate 20-LED bargraph display flashes for dimming up while the lowest LED flashes when dimming down. Some controls cannot be used when dimming is in process. These are the Level Up/Down, Rate Up/Down and Store Settings switches. These switch­ es are locked out of service during dimming to prevent any possible lamp flickering which may happen if there is any attempt to operate several functions at the one time. The A and B dimming rate settings, the Dim and Flash pre­ sets and the minimum level preheat setting can be stored so that these settings will be remembered when the dimmer is used next time, after being switched off. This is done by pressing the “Store Settings” switch. In fact, it is important to press this switch after the minimum level filament preheat has been set so that this will always be set correctly.During this time, the LED bargraph dis­play momentarily goes off as an acknowledgement that storage has taken place. When ever the dimmer is first switched on, the lamp bright­ness is set to fully off and no filament pre­heating is applied. This means that no power is supplied to the lamp. The dimmer will begin to provide power to the lamp www.siliconchip.com.au Fig.1 the block diagram of the Auto Dimmer circuit. The key device is the PIC16F84A-20/P microcontroller. It accepts inputs from the transformer and switches and provides outputs to switch a Triac and drive the LED displays. as soon as the Flash or Dim switches are pressed. Phase-controlled Triac As with any light dimmer, the circuit uses a phase-controlled Triac to set the lamp brightness. The principle is virtually the same as outlined in the January 2002 article on the Touch-controlled Dimmer. For those readers who did not see that article, we will go through the details again. Our mains electricity supply is a 240VAC 50Hz sinewave which goes positive for 10ms, back through zero and then negative for 10ms. This returns to zero and again goes positive. Normally a lamp is connected across this supply whenever it is switched on. In a dimmer circuit, we delay apwww.siliconchip.com.au plying power to the lamp during each half-cycle of the mains waveform and switch it off each time the voltage goes through zero to effectively provide less power and so dim the lamp. This timed switching of the power is performed by a Triac which can be triggered on by a short pulse at its gate. The Triac will then only turn off when the current through it drops below a certain threshold value. In practice, when driving a resistive load this means that the Triac switches off when the mains vol­tage is near 0V. The accompanying oscilloscope waveforms, repeated from the January 2002 issue, show how it works. The first oscilloscope waveform (Scope 1) is the mains sinusoidal voltage measured on the Active output of a power point. Note that the mains voltage shown here is closer to 250VAC and it is by no means unusual to have such a high voltage. The second oscilloscope waveform (Scope 2) shows the waveform applied to the lamp when it is dimmed to a low brightness. In this case, the lamp is powered about 150° from the start of each mains half-cycle and is switched off at 0V. Note that the lamp voltage is applied for both positive and negative excursions of the mains active and the RMS voltage is around 39V. The third oscilloscope waveform (Scope 3) shows the lamp voltage when the dimmer is set for close to maximum brightness. Now the voltage is switched on early in each mains half-cycle so that almost the full mains waveform is applied. Again the lamp is switched off at 0V. The RMS voltage is now a lot higher, at 242V. The circuit for the lamp dimmer obtains this phase control by dividing April 2002  29 Scope 3: this waveform shows triggering very much earlier in the cycle, so that the lamp receives almost all the available power. In this case, the lamp would be at virtually full brilliance. up each half cycle (180°) of the mains waveform into 250 discrete sections. Thus, each discrete section is equiv­ alent to 0.72° (180/250). The overall range of phase control in the dimmer circuit is restricted to a minimum count of 8 (5.8°) and a maximum count of 241 (174°). Block diagram Fig.1 shows the general arrangement of the dimmer circuit. The key device is the PIC16F84A-20/P microcontroller. It accepts inputs from the transformer and switches and provides an output to switch the Triac. Its other outputs drive the LED displays. IC1 operates from a 20MHz timebase and this clocks a timer which counts up until it reaches 40µs. The output then clocks the brightness counter which counts from 0 through to 250 for each 10ms half-cycle of the mains voltage. This is locked to the mains waveform via the zero voltage negative edge detector which resets the brightness counter to zero each time the mains voltage drops to zero. An important part of this circuit is the feedback from the brightness counter back to the internal timer. This is required to lock the internal timer rate to the brightness counter and adjusts so that the counter is just on the verge of counting to 250 at the occurrence of the zero voltage signal from the 30  Silicon Chip mains. Any deviation from this locked arrangement will pro­duce flickering in the phase controlled lamp. The current required lamp brightness is stored in the brightness level register and this value is compared with the brightness counter value using an exclusive comparator. The exclusive comparator output drives the optocoupled Triac driver (IC4) when both the brightness counter and the brightness level register are the same value. Input signals from switches S1-S8 provide the controls to set the dimming brightness, flash brightness, dimming rate and so on, as described above. Input response logic decides what action to take when one of these switches are pressed. Circuit diagram The circuit for the Automatic Dimmer is shown in Fig.2. As already noted, IC1, the PIC16F84A-20/P microcontroller, is the heart of the circuit. This IC runs at 20MHz, by virtue of the 20MHz crystal (X1) connected to pins 15 & 16. It needs to run at this speed in order to perform all the necessary functions of driving the LED displays and monitoring the switches without this interfering with providing the trigger pulses to the Triac. The two bargraphs, comprising LEDs 1 to 40, are driven via IC2, IC3 and transistors Q1-Q5. However, while the LEDs are physically arranged as two 20-LED bargraphs, they are connected in a matrix of five rows and eight columns. They are driven in multiplex fashion, under the control of IC1, IC2 and IC3. Each of the five transistors drives the commoned anodes of its row of LEDs via a 47Ω resistor. The eight columns are each driven via a Darlington transistor in the cathode driver (IC3). Each of the eight base inputs in IC3 is driven by 4017 coun­ter IC2 which is clocked by IC1. Only one column is driven at a time and the required LEDs in that column are driven by the row drivers, Q1-Q5. Each time IC2 is clocked by IC1, one of its eight outputs goes high to drive IC3 to display the next column. After the last column is lit, IC2 is clocked again so that the “8” output goes high. This output is not connected to the circuit and so all the columns (and all LEDs) are off. Next, IC1 checks switches S1-S4 to see if they have been operated. It does this by pulling the RB3-RB7 lines (pins 9-13) low in turn, to check if its pin 18 is pulled low via a switch and diodes D1-D5. So for example, if RB7 is brought low and S1 is open, pin 18 of IC1 will remain high via the 10kΩ pullup resistor. If the switch is closed, the low RB7 output will pull pin 18 low via diode D1. The diodes ensure that the RB7RB3 outputs are not shorted together if more than one switch is closed. Note that bringing the RB7-RB3 lines low will also drive transistors Q1-Q5, so it is important that the columns are off. This is why IC1 can only check these switches when the (unused) “8” output of IC2 is high. After this switch test, IC2 is reset via a high RB1 signal from IC1. Now the “0” output is high to drive column 1 again. Switches S5 to S8 are tested for closure using the high outputs from IC2 and the RA0 input, pin 17, of IC1. Normally pin 17 is held low via a 10kΩ resistor. If the “0” output (pin Fig.2 (right): the full circuit details of the Auto Dimmer. Microcontroller IC1 controls optoisolator IC4 which in turn controls Triac1 to vary the lamp brightness. It also drives transistors Q1-Q5 and IC2 to switch the LED displays. www.siliconchip.com.au www.siliconchip.com.au April 2002  31 This is the view inside the prototype with the wiring almost completed. The full assembly details will be published in Pt.2 next month. 3) of IC2 is high and switch S5 is closed, then RA0 will be pulled high via diode D6. If the switch is open then RA0 will remain low. Diodes D6-D12 ensure that IC2’s outputs are not shorted together if more than one switch is closed. Triac drive The RA3 and RA4 outputs of IC1 drive IC4, the MOC3021 opto­coupled Triac driver, via a 220Ω resistor. When these outputs are high, the LED inside IC4 is off. When these pins go low, the LED is driven and this activates the internal Triac between pins 4 and 6 to drive the gate of Triac1, a BTA41600B. The gate drive current comes via 360Ω and 470Ω resistors from the 240VAC mains Active line The .047µF capacitor is included as a “snubber” to prevent false switching of the Triac by transients on the mains Active. The gate drive pulse to Triac1 is set at 80µs which is sufficient time to ensure that it latches on for the duration of the mains half cycle. 32  Silicon Chip Triac1 is a BTA41-600B, a 600V 40A device which has been specified to cope with the very high surge currents which occur when switching a 2400W incandescent lamp load. Typically, the surge current at switch-on can be 10-15 times the normal load current; ie, the surge current could be 100150 amps and could last for several milliseconds. WARNING! Part of the circuitry used in this Automatic Light Dimmer operates at 240VAC (see Fig.2) and is potentially lethal. Do not touch any part of this circuit while the unit is plugged into the mains and do not operate the circuit outside its earthed metal case. This project is for experienced constructors only. Do not build it unless you are entirely familiar with mains wiring practices and construction techniques. The Triac must also be able to cope with the very high fault currents that occur when high power lamps blow their fila­ments. When this happens, the broken sections of the filament can establish an arc between the stem supports and this arc current continues until the stem fuse blows. Considering that this arc current can be many hundreds of amps, the Triac has to be very rugged. EMC filtering The rapid switching of the Triac, combined with high cur­rents, means that this circuit can generate a lot of interfer­ence. So we have included a two-stage filter network comprising L1 & C1 and L2 & C2. The 4.7MΩ resistor across the Active and Neutral output discharges the capacitors when power is off to prevent these from being left charged. The first stage in the filtering uses an powdered iron toroid for the 40µH inductor L1. This type of inductor is quite lossy for frequencies above about 1MHz and so in conjunction with the 0.1µF capacitor, it attenuates much of the electromagnetic interference www.siliconchip.com.au Fig.3: this simple circuit is used to derive the low-voltage AC and DC supply rails for the dimmer. (EMI) caused by the rapid switching of the Triac. However, it is not a good filter below 1MHz, particularly for frequencies between 10kHz and 100kHz which must also be attenuat­ed to prevent EMI above the allowable limits. This is where the second filter comes into play. It com­prises a toroidal core with two windings. The core has a high permeability ferrite material so that we can obtain a much higher value of inductance without an excessive number of turns on the toroid. However, the combination of high inductance and high load current means that the core is easily saturated by the magnetic flux generated when current flows through the wind­ings. This is why we have two windings on the core; so that any flux generated by one winding is opposed by the second winding. This means the net magnetic flux in the core adds up to zero and so saturation does not occur. However, if we say that the flux generated by one winding is cancelled by the flux in the second, then how does the filter work? Clearly, flux cancellation does occur for the low frequency part of the load current but for the high frequencies, which are bypassed by C2, flux cancellation does not take place and so the twin windings give a high effective inductance for the interfer­ence frequencies we are trying to get rid of. LED brightness indication As mentioned previously, we use a large LED on the front panel to mimic the lamp brightness. This is driven by the RA2 output of IC1 which goes low when the Triac is driven to light the LED via a 470Ω resistor. It is then switched off at the end of each mains half-cycle. Note that the drive to LED41 does not occur for the filament preheat period, where the lamps are effec­tively off. Low voltage power for the circuit comes from transformer T1, as shown on the circuit of Fig.3. Its centre-tapped secondary feeds diodes D13 and D14 and the 470µF capacitor filters the DC which is fed to the 7805 3-terminal regulator, REG1. This provides the +5V rail for the ICs and the LEDs. Most of the circuitry is isolated from the mains by trans­former and the optocoupler IC4. The portion of the circuit in the top right-hand corner of Fig.2 is at 240VAC mains and is poten­tially lethal. Zero crossing detection Because IC1 must provide precisely timed trigger pulses to the Triac, it needs be synchronised to the 240VAC mains waveform. To do this, IC1 monitors the 15VAC waveform from the transformer at its RB0 input, pin 6. This is used to detect the zero crossing point of the mains voltage. The 15VAC signal is filtered with a two-stage RC filter comprising 220Ω & 2.2kΩ resistors and 1µF & 0.1µF capacitors. This rolls of frequencies above about 700Hz to remove transients from the mains. The 100kΩ resistor to the RB0 input is included because there is a diode within IC1 which clamps the voltage when it goes 0.7V below ground. The resistor limits current in the clamping diode. That’s all for this month. Next month we will conclude with all the construction and setting up details. SC UM66 SERIES TO-92 SOUND GENERATOR. THESE LOW COST IC’S ARE USED IN MANY TOYS, DOORBELLS AND NOVELTY APPLICATIONS 1-9 $1.10 10-24 $0.99 25+ $0.88 www.siliconchip.com.au April 2002  33