Dimming Fluorescent Lamps

It has always been possible to dim fluorescent lamps, all that is required is to reduce the lamp current within the lamp after the discharge arc within the lamp has struck. This used to mean using preheat transformers to keep the cathodes hot enough to release electrons and controlling the lamp current by using different inductors or a combination of an inductor and a phase shift dimmer.

Modern electronic control gear has made it much simpler to add dimming control to fluorescent lighting. Cathode preheating is built in to the control gear and the sophisticated circuitry allows the lamps to run down to 1 % of normal lamp power; previously the lamp arc became unstable at less than 10 % of normal input power.

Lamps too have changed. The modern krypton filled, T8 and T5 lamps are a far cry from argon filled T12 lamps. Although they are still mercury discharge lamps, environmental concerns have lead to the mercury dose within the lamp being reduced to a minuscule amount.

Have you ever wondered why lamp manufacturers quote 100 hour lumen figures for fluorescent lamps? This is because they depend on the customer to finish off the manufacturing process.

To control the mercury vapour pressure within the arc tube ensuring a stable discharge, the free mercury within the lamp must migrate to a special area of the arc tube called the "cool spot". This process normally occurs within the first 100 hours of the lamp's life. This migration process only occurs when the lamp is running at its design output. This is the reason that we always recommend that dimming luminaires be run at full output for 100 hours after installation before being dimmed.

Failure to carry out this lamp conditioning before dimming means that the control gear struggles to try and keep the arc discharge going, leading to flickering lamps and short lamp life. The installer must also pass on this information to the user. Whenever new lamps are installed in dimming luminaires, they must be conditioned at full output for 100 hours before dimming.

There are two different types of dimming control gear on the market, each with its own advantages and disadvantages:

Analogue

The main method of analogue control uses a low voltage in the range 1-10 Volts dc produced by the ballast. A separate pair of control cables goes from the controller to luminaires in the controlled group. Because this is a direct current system it is important to ensure that all connections are made with the correct polarity. One crossed over pair will stop the whole system from working usually with all lamps stuck at a dimmed level.

Digital (Addressable and Non-Addressable)

Digital control systems can be split down in to three different signals

• SwitchDIM / Touch and Dim
• DSI (Digital Serial Interface)
• DALI (Digital Addressable Lighting Interface)

Digital dimming works by sending a series of pulses along a pair of control cables. As the signal is digital then it does not suffer from interference from the mains cables so it can be installed in the same containment without the need for a screen round the conductors. The digital systems are not polarity sensitive. When using a non-addressable system, the control pair runs between the controller and luminaires in each dimming group. In an addressable system, the same control pair can be linked to every luminaire and every controller.

This control gear can dim fluorescent lamps from 3 % to 100 % lamp power. It can not turn the lamps off through the control signal. The power to the ballast has to be turned off to extinguish the lamps. Care should be taken to ensure the maximum switch capacity is not exceeded.

If control from more than one point is required then a change over switch between controllers is required, only allowing one position at a time to have control of the luminaires.

This method of control offers smooth dimming and is suitable for single small rooms. It works by the ballast sensing a mains signal at its control input. These systems use a momentary, non-latching switch to provide the control input.

The ballast must have an unswitched supply to activate the control circuitry. Mains voltages will be present in the luminaires even though the lamps may not be illuminated.

Depending on which manufacturer of control gear is used depends on the maximum quantity of ballasts that can be used.

A short push will turn the lamps either on or off. A long push will dim the lamps gradually up or down.

DALI is a ballast protocol and not intended to control motors, blinds or other types of loads.

This is at the forefront of control for architectural lighting as each ballast can be individually addressed. It can be accurately set from 0 % to 100 %.