5.4. light-dimmers
Solid-state light dimmers work by varying the "duty cycle" (on/off time) of the full AC voltage that is applied to the lights being controlled.
For example, if the voltage is applied for only half of each AC cycle, the light bulb will appear to be much less bright than when it gets the full AC voltage, because it gets less power to heat the filament.
Solid-state dimmers use the brightness knob setting to determine at what point, in each voltage cycle, to switch the light on and off.
Typical light dimmers are built using SCR and the exact time when the SCR is triggered relative to the zero crossings of the AC power is used to determine the power level. When the SCR is triggered it keeps conducting until the current passing through it goes to zero. By changing the phase at which you trigger the triac you change the duty cycle and therefore the brightness of the light.
Triacs and thyristors are sensitive to over currents. When dimming normal light bulbs, short circuits are quite probable because of burnt filament. For this reason, light dimmers must have their own fuse, which protects them against failures.
Dimmer circuits normally use coils that limit the rate of current rise. Typically, filtering in light dimmers causes the current rise time (current rises from 10% to 90%) to be in range of 30 to 50 microseconds. This gives acceptable results in typical dimmer applications at home (typically this limitation is made using 40.100 mH coil).
The coil itself does not usually solve the whole problem due to the inductor’s self-capacitance: they normally resonate below 200 kHz and look like capacitors to disturbances above the resonance frequency. That's why there must be also capacitors in order to suppress the interference at higher frequencies.
Normal light dimmers are designed to only dim non-inductive loads like light bulbs and electric heaters. Normal light dimmers are not suitable to dim inductive loads like transformers, fluorescent lamps, neon lamps, and halogen lamps with transformers and electric motors. There are special dimmers available for those applications.
Fully loaded halogen transformers usually dim quite well. Some of the electronic transformers are made dimmable and work well with traditional light dimmers. The ones which are not meant to be dimmed can be damaged by the dimming and even damage your dimmer.
Fluorescent light bulbs (including compact fluorescents) are more energy-efficient than regular bulbs due to the different method they use to produce light. Regular bulbs (also known as incandescent bulbs) create light by heating a filament inside the bulb; the heat makes the filament white-hot, producing the light that you see. A lot of the energy used to create the heat that lights an incandescent bulb is wasted.
A fluorescent bulb, on the other hand, contains a gas that produces invisible ultraviolet light (UV) when the gas is excited by electricity. Most compact fluorescent lamps (CFL) have ”integral" ballast, built into the light bulb, whereas most fluorescent tubes require separate ballast independent of the bulb.
Both types offer energy-efficient light. A dimming compact fluorescent light bulb is specially designed for use with dimming switches. Regular compact fluorescent bulbs should not be used with dimming switches, since this can shorten the bulb’s life.
Until recently, screw-in CFLs were not compatible with conventional household dimmer switches. Using them on dimmer circuits created a fire hazard. However, new screw-in products that have been introduced can be used with dimmer switches, allowing for even more energy savings. To prevent any hazard, it is best to check the package for compatibility with dimmers, or for a safety warning.
Generally, electronic loads, like switching power supplies, cannot be dimmed. The power supply might get damaged because it has never been designed to operate on other waveforms than sine wave (other waveforms can cause current spikes). The dimmer can be damaged by the high current surge that a switching power supply takes. This happens when the triac on dimmer starts to conduct in the middle of the phase.
A typical touch dimmer has the following circuit parts:
- A special timing circuit, which senses if the contact on the touch plate was long or brief. In operation, a momentary touch of the sensor plate with the fingers (50 - 400 ms) will toggle the light ON or OFF depending on its previous state.
- A memory circuit, which stores the intensity level of the lights.
- A circuit which generates the pulses necessary to vary the light intensity
Professional voltage controlled dimmers. Remote control light dimmers typically use 0-10V control signals for regulate the lamp brightness. In this case 0V means that the lamp is off and a 10V signal means that the lamp in fully on. A voltage between those values adjusts the phase when the TRIAC will fire.
The circuit works so that the comparator output in low when the input voltage is higher than the ramp voltage. When the ramp signal voltage gets lower than the input voltage the comparator output goes high, which causes the current to start to flow through resistor to opto-coupler making the triac connect. Because the ramp signal starts at every zero crossing from 10V and goes linearly to 0V at the time of one half cycle the input voltage controls the time when the triac is triggered after every zero crossing.
5.5. Advanced light systems control using a microprocessor in the phase controlling
If you want a digital control of light dimmer you can use a simple microcontroller to do the controlling phase. The microcontroller has start by reading the dimmer set value through some specialized interfaces (commercial digital dimmers use DMX512 interface). The control value is typically 8 bit numbers where 0 means that light is off, and 255 that light is fully on.
The microcontroller can easily generate the necessary trigger signal using the following algorithm:
- Convert the light value to software loop count number
- First wait for a zero crossing
- Run a software loop which waits the necessary time until it is time to trigger the triac
- Send a pulse to the triac circuit to trigger the triac to conduct
Software loop is a quite simple and useful method if the time needed to execute each microprocessor command is definite. Another possibility is to utilize microcontroller timers:
- You can generate an interrupt at every zero crossings and at every timer count.
- At every zero crossing the microcontroller loads the delay value to the timer and starts counting.
- When the counter time has elapsed it generates an interrupt. The timer interrupt routine sends a trigger pulse to the triac circuit.
Figure 3.26 Phase controlling using a microprocessor
The digital system, managed by a microcontroller can drive the dimmer. In this sense a preset information that specifies the voltage level applied on the bulb is necessary to be transmitted using the serial interface of controller (SI) or using a potentiometer (P) that are connected to an analogue input line. A zero crossing detector circuit (ZCDC) gives to the controller the time reference for the phase control of command signal.
The interrupt generated by the ZCDC will be treat by the controller triggering a timer/counter circuit that can be preset. It will generate a delay and when the time interval has finished the corresponding interrupt service routine generates a control signal to the commutation devices placed into the energetic circuit. The waveform of energetic circuit is shown in figure 3.26.
The mean voltage in a period is given by the following equation:
(3.17)
where: 
(3.18)
(3.19)
where 
, and 
represent the command angle, R- load.
Reverse phase control is a new way to do light dimming. In reverse phase controlling, the idea is to turn on the switching component to conduct at every zero crossing point, and turn it off at the adjustable position in the middle of the AC current phase.
Then, the turn-off point timing controls the power to the load. The waveform is exactly the reverse of what is usual in traditional light dimmers. Because the switching component must be turned off at the middle of the AC phase, traditional SCR and triacs are non-suitable components. Possible components for this kind of controller would be transistors, FETs, IGBTs and GTO-thyristors. Power MOSFETs are quite suitable and they have been used in some examples of dimmer circuits.
5.6. Description of two used light dimmer circuits
1 kW 230V AC light dimmer circuit
This circuit is a quite typical triac based dimmer circuit with no fancy special features. It is designed only to operate with non-inductive loads like standard light bulbs and to dim light bulbs in the 50-1000W ranges.
Figure 3.27 Schematics of a simple dimmer (1KVA/230Vac light dimmer)
In this circuit potentiometer P1 is used for controlling the dimmer setting. The trimmer P2 is used for setting the dimming range (how much light can be dimmed maximally). When the circuit is tuned up, P2 should be adjusted so that when P1 touch its maximum resistance setting (light most dimmed) the light bulb should be just dimmed completely off.
This adjustment assures us that the dimmer circuit dims smoothly from zero to the maximum setting. If P2 is tuned to an excessively dimmed preset position, the circuit does not dim nicely up from the light off setting and the operation when P1 is in its maximum value is unpredictable. If you have adjusted P2 to an excessively low value, you just cannot dim the light bulb completely off (sometimes this can be an intentional setting, for example in theatrical lighting where preheat is used).
Advanced dimming systems
Lighting dimmers use phase-control - you switch on at a point on the supply voltage waveform after the zero crossing, so that the total energy input to the lamp is reduced. The time between zero crossing and switching is controlled by external control interface, which is most often a 0-10V DC control voltage, or a digital DMX512 interface.
Figure 3.28 A simple voltage controller dimmer
This circuit can control loads of up to 2A (460VA). The circuit is basically a normal light dimmer circuit, but the potentiometer is replaced with a LDR resistor, which changes its resistance depending on the light level. In this circuit, a LED powered from control voltage source is used for shining variable intensity light to the LDR, so it is imperative to make sure that LDR does not receive light from other sources.
This circuit is basically very simple and not very sensitive on what LDR is used as R2. The disadvantage of this circuit is that the control is not very linear and the different dimmers built around this circuit can have quite varying characteristics (depending mainly on the LED and LDR characteristics).
The control voltage is optically isolated from the dimmer circuit connected to mains. If safety isolation is needed, then it is important to have enough distance between the LED and LDR, or to use a transparent isolator between them to guarantee good electrical isolation. If the dimmer sensitivity is not suitable for the circuit described above, then you can adjust the value of R1 to get the control voltage range you want.
5.7. Description of the lighting control system
A lighting control system can include:
- The wires that form the cabling at 220 V AC (the network). It connects the light that you need at ac power supply.
- Keypads, the buttons or the relays that transmit commands to light fixtures and a controller that is programmed to cue the lights automatically, based on certain conditions, such as the time of day.
- Occupancy (motion) sensors are typically used as part of a security system to detect intruders. They have proved to be an excellent way to save energy, especially in bathrooms and bedrooms where lights are frequently left on. Sensors can operate automatically to turn lights on when movement is detected, then off after a specified period of no motion, or they can have manual on or off switches. Some models feature dimmers that reduce light to a preset level rather than turn completely off when there is no movement; others come with photo sensors that turn lights on only when the light level is below a preset point and motion is detected.
- Light (Photocell) sensors, or photo sensor, measures the light level in an area and turns on an electric light when that level drops below a predefined minimum. They are especially effective with lights that stay on all night long, such as some outdoor fixtures or night-lights. If a light does not need to remain on throughout the night, the use of a timer or motion detector is the response to daylight and dusk. Also, light sensors can automate the switch of lights.
- Home control. Some lighting control systems are so smart that they can operate nearly every electronic component in a house, including motorized drapes, thermostats and cueing up an entertainment system.
- Timers are an inexpensive way to control the period of time for a light to stay on inside the home or outdoors. They can be located at a light switch, at a plug, or in a socket. Some models are turned on manually and set to turn off after a designated number of minutes or hours. Others can be programmed to turn on and off at specified times.
- Central controls are used to monitor home lighting and to operate switches, sensors, and dimmers as desired throughout the house. Often they are integrated with security systems, telephones, and cable television. They can save energy, but depending on their complexity, may be quite expensive.
- Wireless Lighting Control. These wireless systems use either standard electrical wiring or radio-frequency airwaves to carry on, off, bright and dim commands from pushbutton transmitters, also called keypads, to smart dimmer switches. The absence of special cabling and sophisticated microprocessors makes wireless lighting control systems less expensive than wired-in low-voltage lighting control systems.