Lighting controls
Every building has a lighting system that is responsible for 10 -15% of the total energy consumption.
Lighting controls give the flexibility to design a space for multiple uses and easy access. Controls used with high-wattage incandescent are especially effective for saving energy, but they should be considered for use with any lights.
Switching is the main operation in the lighting control systems. We present some possibilities to connect and switch the bulbs. As in all systems, we can identify two kinds of elements: energetic elements and informatics elements.
The first of these switches are relative high power circuits, the main objective is to implement the commutation. In the second system, the important thing is the information exchange between the control centre sensors and actuators.
Animation 1 Lighting control systems
The main elements of a switching system energetic part could include:
- Electro-mechanical switching elements, like relays, buttons, and circuit breakers;
Animation 2 The functionality of the relay
Mechanical switches
Switches are devices used to allow electric current to flow, when closed, and to prevent current flow, when opened. They include some mechanical elements like springs, beams, and contacts. Cooper, silver, gold or platinum are the contact materials that realize the commutation of electric circuit. Common switch types include:
- Pushbutton;
- Toggle;
- Rotary coded DIP;
- Key lock;
- Slide;
- Reed relay.
a) A pushbutton switch is a mechanical switch defined by the method used to activate the switch. The activation method is typically in the form of a plunger that is pushed down to open or close the switch.
Figure 1. Push and switch buttons
b) A toggle switch moves or swings to make or break the circuit;It implies a maintained contact (or a mechanical system, that memorize the position of contact until a new button is pushed/ actuated) and momentary contact types.
Figure 2. Toggle switch button
c) A rotary coded DIP (Dual In-line Package) switch is activated by means of a rotating shaft that can have several stop positions. For each position of the shaft (the input) the switch generates an output binary code.
A DIP switch is an electronic package (circuit board) consisting of a series of switches. Typically used in the configuration of computers and computer peripherals (for example, circuit boards, modems etc.), rotary switches move in a circle, and can stop in several positions along its range.
Figure 3. Rotary switches
d) A key lock switch has a key as the activation means. The key is rotated, and can stop in several positions in its range.
Figure 4. Key lock switch
e) In a slide switch a slider moves linearly (slides) from position to position.
Figure 5. Slide switch
f) Reed switches are magnetically activated switches. They are typically manufactured with two ferromagnetic reeds (contact blades), which are sealed in a glass capsule that presents a low pressure in its inner atmosphere. In the presence of a magnet, the blades (contacts) are commutated.
Figure 6. Reed switches
The configuration of switch contacts can differ. Pole and throw configurations for switches can be single pole single throw (SPST), single pole double throw (SPDT), double pole single throw (DPST), or double pole double throw (DPDT).
Important electrical switch specifications to consider when searching for switches include maximum current rating, maximum AC voltage rating, maximum DC voltage rating, and maximum power rating. Other important parameters to consider include the terminal type, construction materials, common features, and environmental conditions.
Electronic switching devices
Thyristors or silicon controlled rectifiers (SCRs): give a small current signal from the gate to the cathode, which initiate a uni-directional current flow from the anode to the cathode. A SCR is turned on when making its gate positive to its cathode, thus causing current to flow through the gate.
When the gate voltage reaches the threshold voltage, VGT, and the resulting current reaches the threshold current, IGT. Within a very short time, known as the gate-controlled turn-on time tgt, the load current can flow from anode to cathode. If the gate current consists of a very narrow pulse, say less than 1µs, its peak level will have to increase for progressively narrower pulse widths to guarantee triggering.
When the load current reaches the thyristor’s latching current, IL, the load current flow will be maintained even after removal of the gate current. As long as an adequate load current continues to flow, the thyristor will continue to conduct without the gate current. It is said to be latched “on”. If the current lowers until the IL, the conduction through thyristor is interrupted, it is said to be close, or “off”.
Figure 7 Thyristor structure (model and symbol)
The equivalent scheme is presented in figure 8:
Figure 3.22 Equivalent scheme of a thyristor
Triacs: a triac can be viewed as two SCRs connected in parallel, in opposite directions, with a unique gate. The device can conduct current in both directions when a positive pulse, similar to the tyristors, is applied to the gate.
Figure 9 Triac structure (model and symbol)
MOS transistors: the voltage applied to the gate controls the flow of electrons from the source to the drain. A positive voltage applied to the gate (in the case of channel MOSFET transistors) attracts electrons to the interface between the dielectric gate and the semiconductor.
These electrons form a conducting channel between the source and the drain called the inversion layer. No gate current is required to maintain the inversion layer at the interface since the gate oxide blocks any carrier flow.
As a net result, the voltage, which is applied to the gate, controls the current between drain and source. In order to control the current on the channel, the voltage applied on gate must be more that the threshold level, which is different for each transistor.
Figure 10 Internal structure of a MOSFET transistor
IGBT: The Insulated Gate Bipolar Transistor or IGBT combines the high DC current gain of a MOSFET with the high current handling capability and high blocking voltage of a BJT in a surprisingly simple structure as shown in the previous figure. At first glance the vertical structure looks like the one of a regular bipolar transistor.
However, a closer look reveals a p+ substrate rather than an n- substrate. To further analyze this structure we use the equivalent circuit, which contains the p-n-p BJT as formed by the bottom three layers as well as the n- MOSFET underneath the gate electrode.
One should note that the p-type collector of the p-n-p BJT and the n-type source of the n-MOSFET share the same metal contact. Also, the drain region of the n-MOSFET is the buried n-type layer, which is the n-type base of the p-n-p BJT.
The electrons originating from the n+ source flow laterally underneath the gate and then flow down in the buried n-type region, thereby supplying the gate current of the p-n-p BJT. Since the gate current is provided locally, the emitter current will be concentrated around the same area.
Figure 11 Internal structure of an IGBT (model and principle)
The collector would be grounded while the positive voltage is applied to the emitter. Therefore this device can be connected in a switching circuit just like an n-p-n BJT with the important distinction that no gate current is required to maintain the on-state current.
All the devices, presented above, are used for the commutation of bulbs or other lamps. By regulating the control signal’s phase is possible to vary the intensity of light. The devices that can do it are named light dimmers .
Dimmers allow to use one light for many purposes and to modify instantly the atmosphere of a room with a simple adjustment. They will also save energy when used at lower levels on light intensity. Look for full-range dimmers so that you can vary the light continuously from zero to full brightness.
Dimmers can be used with incandescent lights, including low-voltage systems, and with many fluorescents lamp too. Except for some new screw-base compact fluorescents, fluorescent lights must have their own dimming ballasts and should never be installed in sockets for dimmable incandescent. Dimming incandescent lamps should extend their life; however, if halogen lamps are dimmed frequently, manufacturers advise operating them occasionally at full light output.
There are several choices of wall-mounted dimmers: toggle, rotary, sliding, solid-state touch, and new integrated systems with remote controls that can recall previous lighting levels. If several high-wattage incandescent lamps are to be controlled at one point, add a hard-wired dimmer. For plug-in lamps that do not have a dimming switch, you can purchase an adapter for a socket or cord dimmer.