Low Voltage Lighting A Technical Introduction

1
Low Voltage LightingA Technical Introduction

• By Nigel Cumberland IEng. MIIE

2

• 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
• Calculations for low voltage lighting design
• 5. Safety

3
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
4
2. The Basic Electric Lighting Circuit
Power supply
Lamp
Circuit Diagram
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.
5
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
6
The relationship in a circuit between Voltage,
Current and Resistance is governed by Ohms law.
Ohms law states that VI x 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 V x I 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
7
Some Definitions An electric currentA 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. ResistanceAn 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. ConductorsWire 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
circuitAn 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 SwitchA 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.
8
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.
9
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 2600C 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.
10
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.
11
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 lamp which is over
70 Volts.
12
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.
13
(No Transcript)
14
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.
15

• When choosing a transformer the following must be
  taken into account
• Input voltage check that the input voltage is
  suitable for the mains supply eg 230V Ac for
  the U.K.
• Output voltage check that the output voltage is
  suitable for the bulbs or lamps eg 12V
• 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.

16
Examples of Lighting Transformers
17
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.
18
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.
19
When choosing a switch the current rating of the
switch needs to be suitable for the application.
20
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
21
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)
22
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)
23
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 PV 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
24

• 5.0 Safety
• The following rules should be followed in order
  to ensure that a system is safe
• The input to the transformer must be connected to
  the mains by a fused plug or fused terminals.
• 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.
• Switches must be able to carry the current that
  they will be switching.
• 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.

25
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