Halogen Lights on Long Cable

Low voltage halogens can be run on long cables if there is a good reason to do so, but long cables introduce significant voltage drop, and this issue must be addressed one way or another.

Ignoring the voltage drop issue results in bulbs running at well below intended light output, and well below their usual level of energy efficiency.

If voltage drop differs between bulbs it also results in uneven lighting appearance and differences in colour temperature. This does not make a lighting installation look good or perform well.

The options

• Avoid the Issue Entirely
• Minimise voltage drop
  • Thick cable
  • Parallel cables
  • Extreme cables
• Increase transformer output voltage
  • Oversize toroidal transformer
  • 15v transformer
  • Rewinding
  • Wave shaping
• Sidestep the issue
  • Series bulbs
  • 24v bulbs

Avoid the Issue Entirely

The voltage drop problem only occurs wth low voltage high power consumption lamps on long cable runs. Changing to mains voltage lighting, CFL, or fluorescent would all remove the problem. There are however many cases where 12v lighting is preferred, and when long cable runs are involved it is an issue that can be worked around in various possible ways.

Minimise Voltage Drop

Thick Cable

This is the option most often chosen to minimise V drop, and the simplest. The Cables article gives cable voltage drop for any given current.

However this approach has its limits, and can lead to the need for impractical cable sizes in some cases. When this occurs, there are other options.

Parallel Cables

Paralleling cable conductors can increase total copper conductor area without making the cable too large to handle.

T&E is a good candidate for parallelling:

• 2.5mm² T&E has a total of 2.5+2.5+1.5 = 6.5mm² of copper area.
• 4mm² T&E has a total of 4+4+1.5 = 9.5mm² of copper area.

Thus 2x T&Es can be used in place of single 6.5mm² or 9.5mm² cables.

Extreme Cables

Very large cables can effectively address voltage drop. The prime candidates are:

• Very large stranded cable as used by supply utilities
• Copper water pipe with soldered joints

When used, such megaconductors are often made a feature of. (With a little ingenuity a water valve in the pipe could even be made to turn the lights off.)

Increase Transformer Output Voltage

Increasing transformer output voltage to offset cable V_drop is another option if you're able to alter the transformer output voltage.

Oversize Toroidal Transformer

Toroidal transformers are unregulated, and loading at below rated power causes an increase in voltage output. We can thus increase voltage output by using a transformer rated at a higher power than the load it will run.

Measuring output voltage off-load will show how much V_out rises with no load. V_out_rise is inversely proportional to load, meaning for example half rated load will produce half the voltage rise.

V out |
      | .
      |   '.
      |      '.
      |         '.
rated |            '. 
      |             
      |             
      |          
      |             
      |          
      |             
      |
    0 |_______________
       0          rated    Load ->

    Voltage output vs load for 
    non-electronic transformers

This trick does not work with electronic transformers.

15v Transformer

15v rectangular transformers are available from electronic component suppliers. These are larger than the equivalent toroidals. These supply the necessary 12v plus a good 3v margin. The extra 3v can be lost in cable drop plus either of 2 options:

• removing turns from the transformer
• an added voltage dropper

Note that these transformers are designed to be built into equipment, and generally have no fusing, no overheat cutout, and bare live terminals. They should be bolted into a box (earthed if metal), with a suitable fuse on both mains and secondary sides.

These transformers exhibit the same deregulation as toroidals, so any voltage measurements should be taken with the same load as will be used permanently.

To ensure transformer current rating is sufficient, transformer rated power (in watts) should be 25% (or more) above total connected lamp load.

Remove Turns

Removing turns from the transformer secondary reduces voltage output. 4 turns per volt is typical for such mains transfomers. This is easier than adding turns to a toroidal.

Voltage Dropper

This option is included for the sake of completeness, but is not recommended.

A resistor makes a simple dropper. They must be rated for at least the power they will dissipate, ie 25% of the power rating of the lamp(s) they will supply.

Dropping 3v reduces energy efficiency by 25%. This makes for appreciable power waste, and this is why this option is not recommended.

Each lamp should have its own dropper resistor, otherwise normal lamp failures will cause overvoltage and hence more lamp failures.

Power resistors run very hot, and suitable care should be taken to ensure flammable materials can not come into contact.

Chokes (inductors) can also be used as droppers, with much better energy efficiency and thus less heat dissipation than resistors. However the calculations required to pick the right choke are significantly more complex.

Boost Winding

Toroidal Transformers are easier to tweak than electronic transformers. One only need add some more copper wire turns onto the transformer, and connect this extra winding in series with the existing output winding to give a voltage increase.

Enamelled copper wire of the same (or larger) size as the output winding already uses should be used to maintain the transformer temp rating and avoid significantly reducing heat dissipation ability.

When the extra winding is connected it will either reduce or increase the V output. If its reduced, swap the connections of the extra winding round.

4 turns per volt is typical for mains transformers. When measuring V_out, bear in mind that V_out varies significantly depending on load for wound transformers, so the intended bulbs should be used as a load when measuring V_out.

Many toroidal transformers are partly potted to reduce the chance of buzz. These can't be rewound, you need to use a non-potted transformer. These have 2 metal discs, one on top and one underneath to bolt the transformer in place. When doing up the central fixing bolt afterwards, go easy with the pressure. Overtightening is a cause of toroidal failure, and the extra winding will reduce the amount of force the toroidal structure can withstand.

It is not stictly necessary to use any central bolt fixing, the transformer can just be laid in place if it will never be moved, if preferred. When doing this, best sit it on rubber feet to avoid the possibility of hum or buzz.

Wave Shaping

Another way to compensate for V drop is to use a rectifier and capacitor to increase the rms output.

The transformer output is fed through a bridge rectifier, to a reservoir capacitor, and then to the light bulbs. The reservoir capacitor increases rms output by widening the half wave shape of the waveform.

This technique should only be applied to non-electronic transformers, ie toroidals and rectangular iron core transformers.

The bridge rectifier's current rating needs to be well above the rms lamp current to deal with peaks and starting surge. Starting current is limited by transformer resistance, and is typically in the region of 4x run current.

You'll need a true rms reading voltmeter to set this up accurately. It is also possible to set it up approximately by comparing by eye one lamp on a very short cable next to another identical lamp on the full length of cable you will use fed via the wave shaper.

The larger the capacitor, the more rms voltage boost occurs. Transformer power rating should be 50% higher than the lamps it will run. The size of capacitor used is much smaller than those used as PSU reservoirs, as the aim is just to extend the peak a little rather than provide near peak voltage for a whole half cycle.

Schottky diodes need to be used rather than cheaper silicon diodes to keep voltage drop down. Don't be tempted by cheaper non-schottkies, you would pay the difference in power use many times over each year.

This method introduces about a total 0.8v drop in the 2 conducting schottky diodes, resulting in an energy efficiency reduction of apx 6-7%.

Tweaking an Electronic Transformer

These require some electronics knowledge to modify, so are outside the skill set of most DIYers. For those with the necessary skills, a diode in the voltage regulation feedback path can add 0.6v to the output, 2 diodes can add 1.2v.

Sidestep the Issue

Series Bulbs

Running 2x 12v bulbs in series on 24v halves the current draw of the pair, reducing voltage drop and making longer cable runs practical. The downsides are:

• When one bulb blows, both will go out.
• The 2 bulbs must always be the same wattage as each other.

Extinguishing of bulbs in pairs can be a problem when only 2 bulbs are used, but much less so where more than a pair of bulbs are in use.

Dead Bulb Indicator

A simple dead bulb indicator can be wired across each light to show which needs replacement. This is made from a bicolour LED and a 2.2k resistor in series. A cheaper single colour LED will not work, though 2 of these can be wired in reversed parallel to replace the bicolor if wished.

o----/\/\-----+----|>|-----+-----o
              |            |
     2.2k     +----|<|-----+

24v Bulbs

24b bulbs use half as much current for a given power, and the same absolute amount of voltage drop only makes half as much difference as with 12v bulbs. Thus 24v bulbs improve the voltage drop situation by a factor of 4.

The downside is that using 24v bulbs will limit you to using truck lightbulbs and other 24v bulbs, making this a far from ideal choice in most cases.

Note that nominally 24v truck bulbs are in reality 27v rated, and will need a 27v rms supply to give full light output.

More Information

Avoid Hf Transformers

Some lower cost electronic transformers use a very high switching frequency, causing significant losses on even quite short cables. Such transformers are unusable for long cable runs.

Balancing Voltage Drop

Voltage drop should be equalised for each bulb. The simplest way to do this is usually to use equal cable lengths and equal loads on each cable.

Where unequal lengths are needed or cable loads vary, the cross sectional area of each cable may be chosen to give matching voltage drops for each lamp. Thus short runs would use thinner cable than longer runs.

It is also possible to run nearby bulbs on a standard transformer with no mods, and the further away ones on the custom setup. It is recommended to ensure all bulbs see the same voltage.

See Also

• Halogen Lighting
• Lighting Glossary
• Lighting category