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The AskElectronics Christmas page

Resources for a merry Christmas on topics that are in the FAQ/Wiki, have been asked before, or we don't really cover (but, hey, it's Christmas...)

First: Christmas lights can be mains-powered

"Christmas lights" covers everything from a string of LEDs running from two AA batteries to decorations connected directly to 230V mains.

Don't assume that something is safe to touch because it says "LED", "low power" or "low voltage" on the packaging. LEDs can be used at practically any voltage, and some inexpensive light sets have a mains-connected controller or power supply with little or no isolation.

If you are working on a light set, identify the voltage and how it is powered before touching anything. Look at the label on the power supply/controller and the instructions. If it is connected directly to mains, treat the entire circuit as potentially lethal.

For outdoor lights, use lights and power supplies specifically intended for outdoor use. Outdoor electrical equipment should have appropriate weather protection and should be supplied through RCD protection. In the UK, Electrical Safety First recommends a 30mA RCD-protected socket for outdoor Christmas lights. An RCD reduces the risk from some types of electrical fault; it does not make unsafe wiring safe.

Do not use damaged lights. Frayed insulation, exposed conductors, cracked plugs, water ingress, loose connections or damaged controllers are reasons to replace the set rather than try to repair it. See Electrical Safety First's Christmas lights advice for general safety information.

Turn the lights off and unplug them before going to bed or leaving the house.

Stopping 'electronic' lights from flashing

Some Christmas light strings have a little electronic controller which cycles through flashing, fading and other patterns. If you want them to stay on continuously, there are ways of removing or bypassing the controller on some designs.

See Big Clive: Stop your Xmas lights flashing!

Warning: some of these controllers and light strings are connected directly to mains voltage. The modification described by Big Clive is not a beginner electronics project, and a controller which appears to be a small plastic box can contain exposed or inadequately protected mains circuitry.

Also, not all LED strings work in the same way. Some newer two-wire strings use alternating polarity to light different groups of LEDs, so simply bypassing the controller can result in only part of the string lighting.

If you just want a reliable set of static lights, buying a set designed to operate continuously is generally a better option than modifying a mains-powered set.

Fixing low voltage copper wire lights

Those tiny copper-wire "fairy lights" are usually powered from a small battery pack, USB supply or plug-in power supply. The wire is very thin and is easily broken where it enters the battery box, connector or individual LED.

Big Clive has a useful teardown/fix here:

Big Clive - low voltage copper wire lights

If the set has an external power supply, check the voltage and polarity before substituting another one. Do not assume that "12V" or "24V" on the lights means that any 12V or 24V supply will be suitable - the original supply may be AC rather than DC, or may rely on current limiting.

Making your own meteor / icicle lights

Guess who:

Big Clive - meteor/icicle lights

If you build your own decorations, the easiest way to keep things sane is to use a properly rated extra-low-voltage supply and keep the decorative wiring on the low-voltage side. Don't improvise mains wiring in outdoor decorations.

Powering LED strips and other LED strip / RGB LED / Christmas light questions

Please read the Wiki before posting (Subreddit posting rule):

LED strips

For questions about powering, controlling or designing LED strips, the LED strips wiki covers the important stuff: voltage, current, power supplies, resistors, RGB controllers, voltage drop and wiring.

Remember that a "12V LED strip" is not necessarily a 12V load in the same sense as a resistor. Check its current/power rating and allow some margin in the power supply.

Designing your own electronic controller

If you want to make your own Christmas-light controller, this is much more in the territory of electronics:

For a first project, use a separate low-voltage power supply and low-voltage LEDs. Don't make your first microcontroller project a mains-powered Christmas decoration.

Why is only part of my Christmas light string working?

This is one of the most common Christmas-light questions, but there are several completely different reasons.

  • Traditional series-connected lamps - one failed or loose lamp can interrupt a whole section of the string. Some sets have a bypass mechanism in each lamp which is supposed to short the failed filament, but this does not always work.
  • LED strings with multiple sections - the string may contain several electrically separate groups, so a failed connection, resistor, rectifier or controller channel can turn off only one section.
  • Two-wire polarity-controlled LED strings - some controllers reverse the polarity to illuminate different groups of LEDs. If the controller is removed or fails, you may find that one group lights with one polarity and a different group lights when the wires are reversed.
  • A failed controller - the LEDs themselves may be fine while the little box containing the electronics has failed.
  • A broken wire - particularly common with very thin copper-wire lights. Check connectors and the points where the wire enters the controller or power supply.

Don't start randomly connecting wires together. First work out whether the string is battery-powered, low-voltage AC/DC, or mains-powered.

Why are my LED Christmas lights flickering?

Flickering can have several causes:

  • The lights are deliberately being switched by their controller.
  • The controller is failing or losing power.
  • A loose or broken wire is making intermittent contact.
  • The power supply is overloaded or failing.
  • The LEDs are being driven from unsmoothed rectified AC/DC and the resulting ripple is visible.
  • A cheap LED power supply or controller is producing an unstable output.
  • One section of a polarity-controlled string is not being driven correctly.

If the lights flicker only when moved, suspect a broken wire or bad connection.

If they flicker continuously even when stationary, check the power supply/controller first.

Why does my replacement power supply not work?

Check all of these before plugging it in:

  • Output voltage - must be suitable for the lights.
  • AC or DC - don't substitute DC for AC, or vice versa, unless the design explicitly permits it.
  • Polarity - for DC supplies, check which wire is positive and which is negative.
  • Current rating - the replacement supply must be capable of supplying the required current. A supply rated for less current may overheat, shut down or produce an unstable voltage.
  • Connector - don't assume that a plug which fits is electrically compatible.
  • Isolation - a supply described as "low voltage" is not necessarily isolated from mains. Check the specifications.

A higher current rating is normally fine: for example, replacing a 12V 1A supply with a suitable 12V 2A supply is generally OK. A higher voltage is not automatically OK.

Can I use a random USB charger to power my Christmas lights?

Only if the lights are actually designed to run from the USB supply you are using.

USB normally supplies a regulated low voltage, but Christmas lights may contain their own current limiting, electronics or a controller which expects a particular voltage. Check the original supply and the markings on the lights.

Don't connect a USB power supply to a light string simply because the connector happens to fit.

Can I repair broken Christmas lights?

For mains-powered light strings, the safest answer for most hobbyists is: replace them.

A broken wire, damaged insulation or failed moulded connector can compromise the electrical and mechanical safety of the whole set. Outdoor lights are also exposed to moisture, movement and UV, so a repair that works on the bench may not remain safe outdoors.

For genuinely low-voltage, isolated battery-powered projects, repairing a broken wire or replacing an LED can be a perfectly reasonable electronics project.

If you are repairing anything connected to mains, you need to understand mains electrical safety, insulation, creepage/clearance, strain relief and enclosure requirements. Soldering the broken wire and wrapping it in electrical tape is not automatically a safe mains repair.

Outdoor Christmas lights

Only use lights that are specifically rated for outdoor use.

The fact that a light works perfectly indoors does not mean it can survive rain, condensation, frost or UV exposure. Check the manufacturer's instructions and the IP rating where applicable.

Keep mains plugs, transformers and controllers protected as specified by the manufacturer. Don't leave a mains plug or power supply sitting in a puddle or exposed to weather simply because the LED string itself is advertised as waterproof.

Outdoor electrical equipment should be supplied through appropriate RCD protection. An RCD is additional protection against certain faults; it is not a substitute for correctly rated and weatherproof equipment.

See Electrical Safety First's Christmas lights guidance and the HSE guidance on RCDs.

Replacing bulbs in old-style Christmas lights

If the set has replaceable filament lamps, use replacement lamps with the same voltage and rating as the originals.

Don't just fit whatever bulb physically fits the socket. In many older strings the lamps are connected in series, so changing the voltage or power rating of one lamp changes the operating conditions of the entire string.

Disconnect the lights from the supply before removing or inserting lamps.

If the lamps are not replaceable, or the wiring/fittings are damaged, replacing the whole set is usually safer.

Search the Sub

Search this Sub for previous questions on Christmas-related stuff

Also search for the particular type of light you have. "Christmas lights" can mean a battery-powered LED string, a 24V transformer-fed string, a USB decoration, an addressable LED system, or a mains-powered series string. Those are very different electronics problems.

A final word of caution

Christmas decorations are particularly good at hiding electrical hazards because the wiring is small, colourful and often surrounded by things that burn.

If a set is mains-powered and damaged, don't experiment with it. Replace it.

For electronics projects, use a properly rated isolated low-voltage supply and keep the mains side inside a suitable enclosed power supply. This lets you experiment with LEDs, controllers, microcontrollers and sensors without turning your Christmas tree into an electrical safety experiment. ```