Transcript Low Voltage Lighting A Technical Introduction

Low Voltage Lighting
A Technical Introduction
By Nigel Cumberland IEng. MIIE
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1. The definition of low voltage lighting ?
2. The Basic electric lighting circuit
3. The Elements of a lighting system
3.1 Bulbs
3.2 Power Supplies and Dimmers
3.3 Switches
4. Calculations for low voltage lighting design
5. Safety
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1. The definition of low voltage lighting ?
In the subject of low voltage lighting, low voltage is defined as being
24Volts or 12Volts. This is much lower than the mains supply voltage
which is now classified as being 230V AC for the UK supply network.
Because low voltage lighting systems operate at these reduced voltages
it makes them relatively safe when compared to mains supply lighting
systems.
Main Supply Voltage = 230V AC (Alternating Current)
Low Voltage Lighting = 12V or 24V
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2. The Basic Electric Lighting Circuit
Conductor
Current I
Power supply
Voltage V
Power supply
Lamp
Switch
Circuit Diagram
Lamp
Actual Circuit
The Power Supply supplies a Voltage V to the circuit. This causes a
Current I to flow around the circuit through the bulb. The bulb
presents a Resistance R to the circuit.
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Electricity (in very loose terms) is the movement of electrons
around a circuit
The Voltage (also known as Electro Magnetic Force or EMF) is the
force that drives the electrons around the circuit.
The amount of electrons moving around the circuit is measured in
terms of Current.
The flow of the electrons around the circuit is impeded by a
Resistance.
Analogy – A tap, hosepipe and spray nozzle
Water pressure = Voltage
Current = Flow Rate
Resistance = Setting of the Spray Nozzle
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The relationship in a circuit between Voltage, Current and Resistance
V=I x
is governed by Ohms law. Ohms law states that –
R
Voltage = Current x Resistance
From this equation we can derive the following equations –
I=V/R
and
R=V/I
Another useful equation is that of Power P (unit Watt)
P=VxI
Voltage has the unit of the Volt and is signified by the letter V
Current has the unit of the ampere (shortened as amps) and is signified by the
letter I
Resistance has the unit of the Ohm and is signified by the symbol R
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Some Definitions
An electric current
A current results if a negative charge (electrons) moves around an electric circuit. This flow of negative
electric charge around a circuit is referred to as the electric current. The size of the current is measured in
amperes, eg 1 amp or 13 amp.
Resistance
An electric current is made up of the movement along a wire of the tiny particles called electrons. In a
copper wire that is part of an electric circuit, millions of electrons are moving along it from the negative (-)
to the positive (+) terminals of the battery. This flow will be greater in a wide, thick wire than in a narrow,
thin one. This is because thin wires have a greater electrical resistance. They restrict the flow of electrons
and lead to a heating effect - as the electrons move along, heat is generated and the wire warms up.
Conductors
Wire used in an electric circuit is usually made of copper as this carries an electric current very effectively.
We say that copper is a good conductor of electricity. Materials which do not allow an electric current to
flow well are called insulators. Most metals are good conductors and materials such as plastic and rubber
are good insulators.
An electric circuit
An electrical circuit needs to be complete before an electric current will flow. In making circuits using
batteries, wires and an electrical component (eg a bulb or motor), wires must connect the component to the
battery so that an electric charge can flow through the circuit.
A Switch
A switch is a gap in the wires in an electric circuit. If the gap is closed, the electric current will flow around
the circuit and we say the switch is on. If the gap is open, electricity cannot flow and the switch is off.
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3. The Elements of a Lighting System
3.1 Bulbs and Lamps
Bulbs are electrical devices that emit light when a electric current is passed through
them. There are many different types of bulb incorporating various technologies. Each
type can find its own use in the field of lighting.
The types of bulb differ in their physical packaging, their efficiency, their life span and
the type of light that they produce.
The following terms will be useful when looking at bulb and lamp specifications –
Lumen: Measures the total light output of a bulb. The higher the lumen
output, the brighter the bulb.
Watt: The amount of power needed to operate a light bulb. Watts equal
power in, not light out.
Efficiency: Expressed in lumens per watt (lpw). Higher lumens per watt
bulbs are more efficient.
We will consider 3 types of bulb that are suitable for use in low voltage lighting systems.
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Standard Incandescent Bulb
The Standard Incandescent bulb is the ‘classic’ light bulb as
invented by Thomas Edison.
An incandescent bulb glows when electricity passes through a
filament, usually inside a glass bulb containing an inert gas The
filament, of tungsten in the common light bulb, is heated to over
2600°C so that it glows with a white light.
Only 10% of the electricity is converted to usable light.
The other 90% is wasted as heat. Standard
incandescent bulbs are the least efficient bulbs.
The standard incandescent bulb comes in many
different packages. In low voltage form this type of
bulb is generally only available in low output versions.
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Halogen Incandescent Bulb
Halogen Bulbs provide bright white light. Halogens produce up
to 10% more light and last three to four times longer than
standard incandescent bulbs.
Quartz halogen bulbs
Quartz halogen bulbs produce a bright white light and much
heat. Lamps need a lens shield to protect the user from UV light
and from shattered glass should the bulb overheat.
Both types of Halogen lamps are widely used in low voltage
lighting. They offer very high light output for a small size.
Their main disadvantage is that they produce a great amount of
heat.
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Fluorescent Lamp
A lamp that uses fluorescence as its source of light. It consists of a
glass tube containing a low pressure gas, such as mercury vapour. As
a current passes through the gas, collisions between the electrons and
atoms of the gas excite the atoms, which emit ultraviolet radiation
when they return to the ground state. The radiation strikes a phosphor
coating on the inner surface of the tube, causing the phosphor to
fluoresce emitting visible light.
Fluorescent lighting uses about one-fourth the energy of a
comparable incandescent and they generally run cool
when compared to incandescent bulbs.
They are available in limited styles for low voltage
applications and require some additional equipment to
make them operate. This additional equipment is used to
boost the low voltage up to the operating voltage of the 11
lamp which is over 70 Volts.
Light Emitting Diodes - LED
The Light Emitting Diode is a semiconductor that emits
visible light or near infrared radiation when an electric
current is passed through the device. Visible LEDs emit
Green, Yellow, Orange or Red light. Over the last couple
of years it has also been possible to obtain Blue and near
White LEDs.
LEDs have a very long lifetime and can be switched ON
and OFF at very high speeds without any detrimental effect
to the device.
The light output of the LED is very low, but a group of
LEDs may produce a reasonable short range beam of light.
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3.2 Power Supplies and Dimmers
Most low voltage lighting systems are powered from the mains
supply. As the mains supply voltage is 230V AC and we only
require 24 or 12V, then we must reduce the mains voltage.
This reduction is carried out in a device called a transformer
which reduces the voltage using magnetic coupling.
The diagram below shows how a transformer is connected into
the lighting circuit.
230V
AC
12V
AC
Transformer
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When choosing a transformer the following must be taken into account –
1. Input voltage – check that the input voltage is suitable for the mains
supply – eg 230V Ac for the U.K.
2. Output voltage – check that the output voltage is suitable for the bulbs
or lamps – eg 12V
3. Power rating – the transformer must have sufficient output power
capability to supply the lighting system (covered in later topic).
4. Physical shape and size – this must be suitable for the application.
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Examples of Lighting Transformers
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Dimmers
For your lighting system it may be necessary to have the facility to be
able to dim the bulbs. It is very important that the correct transformer be
obtained if this facility is necessary. Some transformers are not suitable
for use with dimmers.
12V
AC
230V
AC
Dimmer and transformer - WARNING ! you must make sure that they
are compatible. Most transformers cannot be dimmed.
230V
AC
12V
AC
Dimmer
The electrical distribution company Newey and Eyre sell a transformer and dimming system that has the
dimmer on the low voltage side of the transformer as above. Other manufacturers may produce similar
equipment.
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3.3 Switches
For your lighting system you may need the ability to switch the bulbs on
and off. This can be achieved by the introduction switches to the circuit.
The following diagrams show a method of switching the complete circuit
off and a method of switching individual lamps off.
Simple low voltage lamp lighting system with ON / OFF switch
switch
230V
AC
12V
AC
Bulbs connected in parallel
Mains plug
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Simple low voltage lamp lighting system with individual ON / OFF switches
for each bulb
230V
AC
Mains plug
12V
AC
sw itch
sw itch
sw itch
Bulbs connected in parallel
When choosing a switch the current rating of the switch needs to
be suitable for the application.
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4.0 Calculations for Low Voltage Lighting Design
The main calculation that is required for low voltage lighting
systems is to ensure that the power supply is of sufficient
output to drive the number of bulbs in the system.
If more than 1 bulb is required then these are connected in
parallel as below –
switch
230V
AC
12V
AC
20W
20W
20W
Bulbs connected in parallel
Mains plug
20
switch
230V
AC
12V
AC
20W
20W
20W
Bulbs connected in parallel
Mains plug
Calculation of transformer output rating –
In the above example we have 3 x 20W bulbs connected in parallel.
The total power that the circuit will consume is found by adding up the
individual bulb power ratings –
20 + 20 + 20 = 60W
The transformer must therefore be rated at 60W + 10% = 66W.
(the 10% is used for a safety margin)
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Calculation of Circuit Current
We need to calculate the circuit current to ensure that we have chosen
the correct size for the conductors and switches with the correct
current rating.
From the equation for power P = V x I we can calculate the total
circuit current –
I = P/V
I = 60 / 12 = 5 Amperes (5A)
Current I
230V
AC
12V
AC
I1
I2
I3
switch
switch
switch
20W
Mains plug
20W
20W
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It is also possible to calculate the current that each of the switches will
have to carry. We can calculate the individual currents I1, I2 and I3 as
follows –
Using the equation P=V x I transposed to I= P / V
I1 = 20 / 12 = 1.667A
I2 = 20 / 12 = 1.667A
I3 = 20 / 12 = 1.667 A
To check I = I1 + I2 + I3 = 1.667 + 1.667 + 1.667 = 5A
Current I
230V
AC
12V
AC
I1
I2
I3
switch
switch
switch
20W
20W
20W
Mains plug
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5.0 Safety
The following rules should be followed in order to ensure that a system
is safe –
1. The input to the transformer must be connected to the mains by a
fused plug or fused terminals.
2. Cables or other types of conductors must be able to carry the current
that will be drawn by the total bulb system. Check data sheets for
details.
3. Switches must be able to carry the current that they will be
switching.
4. Bulbs will get very hot – so ensure that there is no risk of damage to
components from heat. Check that there is no fire risk.
5. Transformers and dimmers will also get hot. Ensure that they will
not cause a risk.
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EMC and LVD
Electrical products that are intended for sale in the EU must conform
to documented standards that will enable them to comply with the
requirements of the following directives–
EMC – Electro Magnetic Compatibility directive
LVD – Low Voltage Directives
Compliance to these directives will enable the manufacturer to apply
CE mark to the equipment.
Products for sale in non EU countries are usually subject to other
directives and standards eg –
USA – UL
Canada - CSA
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