Power Supplies

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Introduction

In older style 240Vac systems, each component (controllers & lights) are powered directly from mains power. While 240Vac systems are still popular, they can be more difficult and dangerous to deploy in larger DIY displays due to the lethal voltages used. This is not to say 240Vac systems don't have their place, more that there are safer alternatives available now at reasonable prices.

This is in contrast to more modern lighting, where low voltage power supplies are now almost always part of the equation. LED lighting has also made this more practical due to its reduced power demands. You would have seen power supplies many times before. Most of the OTS (off-the-shelf) light sets come with a plug-pack (wall-wart for USA) to convert the mains voltage to the lower AC or DC voltage used by the lights.

Until the introduction of LED light sets, most outdoor rated light sets used incandescent lamps. These sets were supplied with a plug-pack, usually with a 24Vac or 36Vac output. The change to LED has seen the need for power supplies remain, just with different voltages and now with DC options.

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AC Type

An AC power supply can be a simple plug-pack to power a small section, or a larger transformer that powers an entire display. Most 240Vac controllers should be able to be used with low voltage AC power supplies, but will likely need modification to function correctly.

An example of a toroidal power transformer:

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240V Connection

Fitting a 240Vac plug and cord to a toroidal power transformer is a bit tricky. As it is just a bare transformer, you must add a suitably rated fuse (check data sheet for value) inline with the Active wire. The Neutral and fused side of the Active wires go to the "primary" side of the transformer. The green/yellow wire of the mains lead is the Earth, and goes to the case of the metal box you mount the transformer into. After you are done, check with you multimeter (set to Ohms) to make sure the earth pin of the mains plug is indeed connected to the metal case. You should get a reading of less than 1 Ohm. Do this check before plugging in the completed unit.

Low Voltage Connection

If the transformer has more than one secondary (output) set of wires, check the data sheet for details on how to connect them to get the desired voltage. A mistake here can result in either no damage (but no output), a blown fuse or a destroyed transformer, so double check before powering up. With an AC output, the polarity (which way around the final two wires go) is not important.

Mounting

When mounting a toroidal transformer, it is important to use the supplied hardware (rubber pads, bolt, top disc and nut). When mounted in a metal case, it extremely important to not allow the top of the bolt (the nut end) to touch the case. If it does, the bolt forms a "shorted turn" and can cause sparks (where the bolt tip touches the case), heat rise and failure of the transformer.

DC Type

A DC power supply can also be a simple plug-pack. With the advent of SMPS (Switched Mode Power Supply) units however, high power (large wattage) power supplies are now available at very reasonable prices.

An example of an SMPS unit:

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Voltage Adjustment

Most of the SMPS units like the one pictured above are adjustable in voltage, usually by a few percent. This can be handy when you need a non standard output voltage. For example, 24Vdc version of the popular SD-350 SMPS can be adjusted from 23Vdc up to 30Vdc via a small screwdriver control on the end.

Max Output Current / Wattage

In theory, the maximum current a SMPS unit can supply is simply the rated wattage divided by the output voltage. eg. 350W / 24V = 14.6 Amps. In reality, you need to check the data sheet for the specific unit you have. It should state what the actual maximum current is. For the SP-350-24, it is indeed 350W or 14.6 Amps, but the 5V version (SP-350-5) has a maximum output current of only 57 Amps, making a total of only 285W, not 350W. Tricky huh! That's why it pays to check the data sheet for the exact brand and model you are using.

Here's an extract from the Mean Well SP-320 specifications as an example:

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Regulation

See this page on PSU Regulation for an explanation.

240V Connection

Fitting a 240Vac plug and cord to an SMPS unit is not difficult. There are three terminals to connect. The brown wire of the mains lead is the Active, and goes to the "L" (or "A") terminal. The blue wire of the mains lead is the Neutral, and goes to the "N" terminal. The green/yellow wire of the mains lead is the Earth, and goes to the "E" (or "FG" or "Earth") terminal. After you are done, check with you multimeter (set to Ohms) to make sure the earth pin of the mains plug is indeed connected to the metal case of the SMPS. You should get a reading of less than 1 Ohm. Do this check before using the SMPS. It is also important to install a non conductive barrier (plastic) over the exposed 240V terminal area to prevent accidental electrocution.

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Low Voltage Connection

Your average DC power supply has a 240Vac input and a low voltage output with positive (+) and negative (-) terminals. It is important to connect these to your controller or lights the right way around, lest you release the magic smoke from the system!

A lot of SMPS units have more than one pair of + and - terminals (often 3). Internally, each of the + terminals are connected together, the same goes for the - terminals. The multiple sets of terminals are provided to make connection of more than one cable easier. If you are only connecting one output cable, it makes no difference which ones you use, as long as you use one + and one - terminal.

EMI

Some cheaper brand SMPS units have been known to cause EMI (Electro Magnetic Interference), with local TV reception and modem operation being affected. Units with "dirty outputs" (lots of EMI) can radiate that interference via the output cables. With a typical Christmas display, these cables can be quite long and act as very good aerials. If you do want to buy cheaper (off-brand) SMPS units, it would be wise to check one first for potential problems before committing to a whole batch. There are clones of reputable brands that have quite poor performance compared to the genuine units.

An example of TV interference. You want "The Hoff" to look good don't you?

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Computer PSUs

Another power supply sometimes used for displays is the ATX computer type. These can be a good source if you need 5V and/or 12V. A typical 600W ATX PSU can supply up to 5V @ 35 Amps and 12V @ 18 Amps. There are other outputs (3.3V, -12V, -5V), but these are not generally useful for Christmas lights.

To use an ATX power supply outside of a PC, you'll need to short the PS_ON# pin to 0V (COM). You may also need to add a resistor (try 4.7 Ohms / 10 Watt) between the +5V line and 0V (COM) to prevent the PSU shutting down due to insufficient load. (remember that the resistor will get hot!) There are plenty of articles on the web that discuss this further.

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Circuit Protection

All power supplies need a fuse on the 240Vac side. Most SMPS units have this built in, but as noted above, bare transformers will need a fuse adding inline with the Active wire. The purpose of the mains fuse is to protect the power cable and plug from excessive current if a fault in the power supply occurs.

When using small power supplies (plug-packs under, say 15W), no further protection is usually required. Large transformers and SMPS units can however supply a lot of current on their outputs. This is a potential problem as I'll show:

Imagine you have a 24V, 350W SMPS. That's a maximum output current of over 14 Amps. You may have many small cables running of to your light sets, each one only drawing a few hundred milliamps. If one light set develops a short (bad connector, pinched wire, etc), that thin wire (that is only rated for 1 or 2 Amps) is suddenly subjected to 10 times its current rating. Without a low value fuse between the SMPS output and each thin cable, you may soon see that cable turn into a molten, flaming mess!

Using a fuse block (row of fuses) on the output of your high current power supply is one solution. Choose fuse ratings that are appropriate for the connected load - a little higher than what the load normally draws. Then a short on any fused output will simply blow that fuse, instead of bringing a halt to your whole display. It also makes troubleshooting easier as you'll know which cable has the short on it, without waiting for the smoke to appear!

Some controller boards have a fuse on each output. The E16 from Stellascapes for example, has 16 outputs to drive pixel strings. The power pin on each output has its own fuse, therefore limiting any short circuit faults to just that output. Conversely, most controllers use just one or two fuses (on the input side) and need to use much higher fuse values, as it needs to be able to pass the total current of all channels. The result is that a short on any output will (hopefully) blow the input fuse, but that whole bank of channels will be disabled. If the input fuse does not blow, the channel wiring or switching device (MOSFET or TRIAC) may be damaged.

Safety

All 240Vac wiring should be carried out by a suitably qualified person. (Electrician or similar required by law in Australia - check local regulations)

Only a few specialised 240Vac power supplies are rated for external use. These are usually fully encapsulated types. Care still needs to be taken with the wiring or course.

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