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Lights and electronics

Index

General info

Light switch wiring

Light bulbs

In an incandescent bulb, current heats the tungsten filament, which glows white hot. To prevent the filament from rapidly oxidizing, the bulb is filled with an inert gas, mainly argon at low pressure. Much of the energy dissipated by the filament is heat; only a little is light. The fact that a bulb uses 100 watts of energy doesn't mean it gives 100 watts of light (typically only 10% or less of the energy consumed by incandescent lamps is actually used to produce light, the rest ends up as heat). To get most light buy the bulbs that give the highest number of lumens for every 100 watts of energy used. Most light bulbs average around 500 lumens per 60 W. A lumen is the unit of luminous flux equal to the light emitted in a unit solid angle by a uniform point source having an intensity of one candela. There is no direct correlation between lumens and watts because other variables may affect the relationship, such as lamp design life and fill pressure

Tungsten halogen lamps try to reduce filament evaporation by including small amounts of bromine in the bulb atmosphere. The bromine forces the tungsten to redeposit on the filament. Halogen lamp life is about twice that of conventional incandescent lamps. Incandescent lamps are available in wattage ranging from 2 to 1500 watts and above. In many cases, the light level generated by a particular luminaire can be increased or decreased simple by switching to a different lamp wattage (do not exceed the maximum lamp power allowed in the luminaire).. This is because incandescent lamps are "resistance smart." The lamp's filament is designed and sized to offer a preset amount of resistance to current flow. This controls the amount of current passing through the lamp. As long as the bulb voltage is right for applied mains voltage, it will work well.

Light bulbs have changed very little over the years. They are the cheapest form of lighting product, but they are also the most expensive and inefficient light source in the long run. The bulb type has effect on the bulb live and light it gives out. Incandescent bulbs bring out warm yellow tones and are recommended for living rooms and dens. Halogen bulbs are the brightest, whitest and more expensive. They cost twice as much as incandescent bulbs but also last twice as long. Halogen bulbs give out more light per watt than "normal" incandescent bulbs (up to almost 2 times more in best cases). Typical halogen light bulbs offer a life span of 1,500 to 2,000 hours.

Gas discharge lamps

Other lamp types

Dimmers

In many situations, luminiares are not used constantly at full power. They are generally required to fade in and out, and to be used at different brightnesses, or intensities, at different times. A device is required to regulate the amount of electrical voltage sent to each luminaire, thereby allowing the intensity of the light to be varied: this is a dimmer.

A light dimmmer allows controlling of a light bulb brightness. The basic idea of dimmer opertion is that it limit the electrical power that gets to the light bulb. Dimmers today come in many styles to control different types of loads. In some old mains powere lighting systems variable transformer is used as light dimmers, but nowadays they are largely replaced by electronic light dimmers with operate using phase control principle (first this kind of SCR based system was publicly demonstrated in 1962 in London). There are also some other dimmer types used in some special applications (variable transformers, simple resistors for vely low power bulbs and PWM controllers for DC lights). Proper matching of control system types to the load is very important. Not all dimmers work properly with all types of loads. Using wrong type of dimmer causes that the dimming does not work well, and in worst case can damage the light dimmer and/or the lamp connected to dimmer. Normal incandescent bulbs can be dimmed with very many dimmer types, but some other light types are harder to dim. Be warned not to dim lights which have motors or control electronics in them (unless they are specifically designed to be dimmed). For example if you dim a typical disco light (one with electronics and motors) it will probably damage the electronics.

One light dimmer regulates one lighting circuit, or channel, allowing the electrical supply sent to the attached lumainire to vary between 0 and the mains voltage (230V or 110V). Each dimmer is designed to work up to a maximum electrical load, called its capacity. Any number of luminaires can be connected to a dimmer, until the capacity is reached. In stage lighting applications the dimmer systems are generally built from many dimmer packs. A dimmer pack comprises a number of individual dimmer modules, housed together for convenience.

Typical light dimmer circuits use Pulse Width Modulation (PWM) to control the brightness of the bulb. It controls the brightness of the bulb by turning the bulb ON for part of time and then OFF for part of time. The brightness of the bulb is a function of the ON time to the OFF time. Dimmers designed to dim DC light bulbs generally work in this way, they just have an adjustable free-running oscillator with variable pulse width to control the light output. Modern mains light dimmers use TRIACs to control the flow to the light bulbs so that only needed part of mains pulse wave enters the light bulb (the PWM operation is syncronous to mains power).

Light dimmers are quite energy efficent, although they are not ideal. For mains operated light dimmers the efficiency is usually in the range of 90-97% depending on dimmer load and design for a normal triac based light dimmer (phase control). The efficiency of a dimmer module can be determined by loading it to full capacity and then measuring the voltage drop across the dimmer module (you need at True RMS voltmeter between the Line and neutral wires to do the measurement). The SCRs (Silicon Control Rectifiers) or triac that provide current control in the power device, part of the dimmer module, drop about .75 Volt across each junction (there are two). The choke is responsible for the remainder of the voltage drop. The amount of voltage drop across the choke varies proportionally to the connected load. This means that there will always be a dimmer insertion loss of at least 1.5 volts + the voltage drop across the choke. This is true for all SCR / Choke based dimming regardless of manufacturer. This power loss in dimmer needs to be taken into consideration when calculating the amount of Heating Ventilating and Air Conditioning (HVAC) required in a dimmer room of large dimming systems.

Dimming can extend lamp life. Turn to the tungsten halogen section of most lamp manufacturers catalogues and you will find a small graph which correlates things like % of rated lamp voltage against % rated lamp life and colour temperature. If you study these graphs they will show that 5% under voltage (i.e. dimming) will produce a much longer average life without significantly affecting the light output or the colour temperature. Dimmign more than that usually reduces light output considerably and changes the color temperature quite much. When lamp is dimmed a lot, the light output drops very much faster than consumed power (so the efficiency of lamp drops considerably when normal lamp is dimmed).

One inherent drawback of the ubiquitous thyristor dimmer is that when it is switched on, noise and harmonic disturbances are caused due to the rate of change of current. High peak currents are also generated. Resonance in the lamp filament, colloquially called "lamp sing", is generated by the abrupt, high frequency switching of power to the lamp. The solution has traditionally been to fit a large inductor (choke) to the dimmer circuit to reduce lamp sing and radiated emissions.

With resistive loads, such as incandescent lamps, the current and voltage curves match. This will keep every light dimmer happy. Once you start trying to work with reactive loads, such as transformers, motors, or fluorescent lamps, you may get unhappy dimmers unless you use spacial dimmer deisgned to handle inductive loads. With many normal dimmers and a small inductive load, a resistive ballast in parallel will usually keep the dimmer happy.

For household use dimmer switches come in four popular styles: dial, slide, touch pad, and combination light switch/dimmer slide. Since dimmer switches come in different shapes and each operates a little differently, you should always follow the instructions included with the switch for installation and operation. Theatrical applications and rock shows generally use remotely contolled dimmer packs. Each dimmer pack contains many individual remote contolled (usually 0-10V DC or DMX-512 controlled) light dimmer circuits in one case.

Semiconductor relays and TRIAC based mains controlling

Light control interfaces

In almost all stage lighting situations, luminiares are not used constantly at full power. are generally required to fade in and out, and to be used at different brightnesses, or intensities, at different times under control of lighting operator. The actual dimming is done usign the light dimmer, but it needs to be controlled in some way, usually form remote location. The control desk, or simply the desk, is the front-end of the lighting control system, and provides an interface between the dimmers and the operator. The state of each dimmer can be changed from the desk, therby controlling the output from the luminaires.

The control desk must communicate with the dimmers in order for the changes made by the operator to take effect on the stage. There are many communication system in use between the lighting desk and the dimmers. There are both analogue and digital systems in use. Analogue systems use a control signal that varies in voltage or current in direct relation to the required intensity. Digital systems use control signals that send the required control level numerically for each channel. The control information could be communicated to the dimmers by using one control wire for each channel: this is called multicore control. Alternatively, each control channel can be instantaneously measured, and the resulting values sent down a single control wire to the dimmer in sequence. This procedure is called multiplexing.

Digital systems are more reliable than the analogue equivalent, faster and can be used for more accurate controlling. Besides light dimmers digital light control protocols (most often DMX-512) are used to control devices light intelligent lighting (like roboscanners), foggers and many other lighting devices.

Light effect controllers

Light switches controlled by external signal

Strobe and Flash Lights

A stroboscope is a special kind of light that flashes in a very regular way. Each flash of the strobe light is the result of an electrical discharge (sort of like a spark) traveling through the special gas contained in the strobe "flash bulb". The electrical current flowing through the gas causes it to emit light. The same principle is used in electronic camera flash units, with difference that they normally only flash once and the flash energy is is quite large (makes very bright flash). Usually the discharge happens very quickly, lasting only a tiny fraction of a second.

WARNING: Strobes can be DANGEROUS and can induce epileptic fits. Use sparingly and with caution.

Low voltage light circuits

Small LED flashing circuits

Most of the LED flasher circutis can be used as general purpose light controlling circuits if the LEDs are replaced with an optocoupler which is connected to the power controlling electronics (like transistor, relay or triac).

Other light projects

Electronics kits for light controlling

Here is a small list to pages which contain information on ready made electronics kits which might be usable for experimenters who want to build light controlling electronics themselves. Sometimes it is a good idea to use ready made design for the parts which are directly connected to mains voltage. If you would rather build ready made light controlling electronics then it is best to check lighting companies link page and laser links page.

Related pages

http://www.epanorama.net/ <mailto:webmaster@epanorama.net?subject=Feedback on lights.html page>


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