Overdriven fluorescent light

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Overdriving a fluorescent lamp is a method of getting more light from each tube than is obtained under rated conditions. It involves taking the light fixture apart and rewiring the insides.

ODNO (Overdriven Normal Output) fluorescent tubes are generally used when there isn’t enough room to put in more bulbs to increase the light. The method is effective, but generates some additional issues.

This technique has become popular among aquatic gardeners as a cost effective way to add more light to their aquariums. While power compact fluorescent lights cost upwards of $60-100 in the US, a twin bulb ODNO light can be set up for $30.

Caution: this article describes techniques some of which are not entirely safe to implement, and the techniques described should not be implemented by anyone unable to verify their specific design is safe.


Contents

[edit] Terminology

Tube drive is described as 1x, 2x, 3x or 4x. However this is not a measure of the power consumed, but rather comes from the practice of parallelling anything up to 4 ballast outputs. 4x operation is in the region of twice rated power.


[edit] Ballasts

Electronic ballasts are usually used for this as they use regulation, protection and current sharing circuitry, enabling them to work in some poorly designed lighting configurations.


[edit] Electronic

There are 2 main methods of overdriving with electronic ballasts:

[edit] Higher power ballast

The ballast is replaced with a higher rated power ballast, and the circuit tweaked to reduce filament current back to normal value.

This is a relatively safe and reliable method, but overdriving by any method brings several safety issues that must still be addressed.


[edit] Parallel ballast outputs

This parallels the outputs of a multi-tube electronic ballast with current limiting.

This approach brings reliability issues, and with ballasts lacking current limiting also fire risk issues, but is popular nonetheless.

Each electronic ballast normally drives either two or four tubes. The ballasts are wired with their outputs in parallel such that a normal two-tube ballast drives a single tube; a four-tube ballast drives either one or two tubes. This re-wiring increases the amount of current the ballast will supply each tube, resulting in increased light output.

Usually extra ballasts are put into the fixture and wired to the sockets the original ballast is no longer powering. For instance, a four-tube fixture with a ballast re-wired to drive two tubes will have a second ballast similarly wired to drive the other two sockets.

The reasons parallelled electronic ballasts work are:

  1. recent US ballasts use current mode programmed control to limit their output (based on legislation for energy efficiency passed in 1992 in the United States), which causes current sharing
  2. the ballasts usually use a resonant output stage, causing synchronisation and quite good isochronation of the 2 or more ballasts.

Identical ballasts are always used when parallelling. Parallelling of ballasts with different design frequencies or starting methods would be a recipe for failure and fire.


[edit] Magnetic

Magnetic ballasts also work well, but they lack the inbuilt control and protection of their electronic counterparts, so an ill designed circuit is more likely to destroy ballast or tube. Correct design and wiring is necessary.

The 2 methods used are use of a single higher power ballast and parallelling ballasts. Parallelled magnetic ballasts require correct connection polarity to avoid ballast destruction.


[edit] Filament current

Most fluorescent lighting ballasts deliver filament current to preheat the tube before striking. Higher power ballasts (whether single or parallelled) deliver higher filament current, which cause premature tube failure and risk of glass shattering in use.

Some filament current should therefore be shunted away from the tube filaments to avoid filament overheating, with consequent safety risks. This is done with any of:

  1. power resistors across each filament (for any ballast type)
  2. in switch start ballasts, with an additional inductor ballast in series with the starter to reduce current to normal during starting.
  3. As above but using a capacitor for this. But this generates design issues beyond most users, and is not to be tried without addressing the issues in the design first. Failure to take heed of this is liable to cause a fire. (See safety section.)
  4. Reducing the filament feed capacitor sizes in electronic ballasts that derive filament preheat from the main discharge current.
  5. With some ballast / tube / mains voltage combinations it is possible to disconnect one of the filament wires at each tube end and let the tube start cold. (This applies most often to small tubes on 240v with magnetic ballasts, and the more aggressive very fast starting electronic ballasts.) This reduces tube life.

[edit] Light output

The increase in light output is not linear with the current draw. Efficacy may increase or fall, depending on tube type and level of overdrive, but in most cases it increases.


[edit] Safety Concerns

Overdriving can be unsafe if one is unfamiliar with electrical wiring practice, basic electronic design principles and local legal requirements.

Overdriving a fluorescent tube increases the amount of heat produced. Fixtures not designed to handle the additional heat can melt or sag onto a very hot tube end, or catch fire. Parallelling electronic ballasts also introduces additional fire risk factors.

The use of capacitors to limit filament current with magnetic ballasts should be treated with a little caution. Vector maths is required to calculate suitable capacitor values and voltage ratings, and failure to calculate correctly often introduces resonant conditions that produce fault currents, cause heavy overheating and insulation breakdown, creating an immediate fire risk and failure.

Use of higher powered ballasts up to 2x power is a relatively safe approach, with ballasts normally able to handle the load happily, as long as the remaining safety issues are addressed:

  • filament resistors are suitably rated (for power and voltage)
  • the filament resistors are mounted in a way able to safely handle the high temperatures produced.
  • filament current limiting capacitors have been properly calculated
  • filament current limiting capacitors are rated to sufficient voltage (which is well in excess of mains voltage)
  • filament current limiting capacitors use a self-healing dielectric (known as 'X rated' and a legal requirement in UK)
  • the enclosure is able to handle the extra heat generated
  • sufficient ventilation is ensured to prevent any part or wire from exceeeding its safe working temperature
  • high temperature insulation is used, eg rubber.
  • sufficient restraint of all parts (especially wiring) is used to avoid them coming into contact with hot parts
  • all parts, wiring and construction conforms to locally applicable legislation
  • Fluorescent tubes are mounted such that shattering can not cause injury.
  • Tubes and electrics are splashproofed

There may be additional safety issues or requirements. Exact requirements will vary according to local/national law.

Finally, working with ballasts some of which store power when disconnected, and some of which can deliver a lethal shock, requires the skill to handle such equipment safely.


[edit] Disadvantages

The main disadvantages are:

  • safety issues
  • the need to customise
  • In some circumstances, custom built equipment not meeting local requirements could possibly affect insurance policies, liability, etc.
  • Reduced tube life (though less tubes)
  • Reduced equipment reliability with some designs
  • The tube and often the ballast are operating out of spec, so overdriven performance is never guaranteed, and guarantees and supplier liability are mostly void.


[edit] Performance

Overdriving is able to increase efficiency. T12 tubes work especially well. Tests show light output roughly tripled for 2x power consumption with T12s.

T12 tubes are usually halophosphate, and significant output degradation occurs over time with halophosphate. Degradation is worse when overdriven. (The linked to performance data does not contradict this, it fails to report the phosphor types of any of the tubes used).

More T8 tubes will fit a given space than T12s.

Small diameter tubes have greater risk of failure and shattering (T5, T6). Small diameter tubes run hotter, and operating temp is elevated further by overdriving. Extra cooling, eg a small fan can reduce the resulting issues.

High colour temperature tubes (cool whites) (4000K and up) produce more brightness per watt than warmer whites (up to 3500K).

Triphosphor tubes produce a little more light per watt than halophosphate, and triphosphor output deteriorates less over time. However since T12s appear to respond better to overdriving, and T12s are normally halophosphate, (with T8s generally triphosphor), which performs better in the long run remains to be seen.

Electronic ballasts operate at high frequency, which produces slightly better tube efficiency than mains frequency magnetic ballasts.

Electronic ballasts are susceptible to working in hot environments, and hot working tends to produce high failure rates. For hot lighting enclosures, magnetic ballasts and externally located electronic ballasts are both suitable. Magnetic ballasts are not normally susceptible to heat, within reasonable limits.

Magnetic ballasts have much better reliability than electronic, with mean life of well over a century.

For a given total light output, use of less tubes close together creates higher contrast and more shadow in the viewed image.

Tube life is reduced significantly. Designs that don't correct filament overcurrent will see tube life reduced even further.


[edit] See also

Fluorescent lamp

[edit] External links