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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

Current I

Voltage V Lamp

Conductor

Power supply

Switch

Circuit Diagram Actual Circuit

Lamp

Power supply

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 is governed by Ohms law. Ohms law states that –

V=I x RVoltage = 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

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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.

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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 lamp which is over 70 Volts.

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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.

230VAC

12VAC

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.

230VAC

12VAC

Dimmer and transformer - WARNING ! you must make sure that theyare compatible. Most transformers cannot be dimmed.

230VAC

12VAC

Dimmer

The electrical distribution company Newey and Eyre sell a transformer and dimming system that has thedimmer 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.

Mains plugBulbs connected in parallel

Simple low voltage lamp lighting system with ON / OFF switch

230VAC

12VAC

switch

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Mains plugBulbs connected in parallel

Simple low voltage lamp lighting system with individual ON / OFF switchesfor each bulb

230VAC

12VAC switch switchswitch

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 –

Mains plugBulbs connected in parallel

230VAC

12VAC

switch

20W 20W 20W

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Mains plugBulbs connected in parallel

230VAC

12VAC

switch

20W 20W 20W

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)

Mains plug

230VAC

12VAC switch switchswitch

Current I

I1 I2 I3

20W 20W 20W

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Mains plug

230VAC

12VAC switch switchswitch

Current I

I1 I2 I3

20W 20W 20W

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

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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