Power Supplies

HOW TO CHOOSE THE CORRECT POWER SUPPLY FOR YOUR APPLICATION:

The following basic principal of electricity is to be followed from beginning to end of your installation. – The equation below can be associated to any LED installation and must be conformed to no matter what, as it dictates the standard of components to be used in the installation. Ohm’s Law is used to either work out Current, Voltage or Resistance using two known variables out of the three. – Eg. You are using 200 x LEDECO3S-W Modules in your installation, which each consume 0.72W of power. You would multiply the amount of modules being used in your installation, by the power consumption of each module to get the amount of power consumed by the installation.

As a rule of thumb, add 30% to that total power to equate for any inefficiency or change in resistance in the system. This addition allows the power supply to run under its maximum capacity and ensure the lifespan of the system.

Completing the equation we get 144W + 30% giving you a total of 187.2W. This figure is the one to be used to decide which power supply or power supplies are to be used in your application.

Note: Never use a power supply that is not capable of delivering your total draw +30%. This will decrease the lifespan of your power supply in many ways as the amount of inefficiency cannot be calculated without a constantly monitoring the amount of power vs. resistance in the system. It is better to have too much power, in a constant voltage system, than have too little and strain all components involved.

HOW TO STORE YOUR POWER SUPPLY CORRECTLY

Depending on what type of Power Supply Unit you are using in your application, there are different ways to store them, but all boiling down to one critical point: Heat

For example, let’s use a 12V, 100W power supply loaded to 70% capacity, in two examples:

  1. Non-Waterproof PSU – This unit needs to be stored in a cool, moisture free area. The unit will heat up, depending on the load applied, and needs to cool itself down by convection (Cycling air) whether or not it has a built in fan. When the power supply cools down, it heats up the air around itself. If that air is stagnant or cannot disperse the heat away from the unit, it will not cool down, only increase in temperature until it shuts down via its thermal overload switch (a ashing effect will be seen with anything connected to the PSU in this case). Heat also increases resistance in the circuit, and more resistance requires more power from the power supply, in turn making the power supply work harder to deliver the required need. This in turn makes it heat up faster and the cycle continues until failure occurs. Because the PSU is not completely sealed, air ow over the components is easy unless otherwise obstructed by dust, wrongful placement, or if the unit has been sealed inside another unit, eg. Small wooden cabinet.
  2. Waterproof PSU – This unit needs to be stored in an even cooler environment. The power supply is waterproof to an extent and can be installed outdoors in harsher environments but runs a greater risk of overheating than the non-waterproof version. The reason being for this is that this unit cannot ‘breathe’ as easily. The waterproof units are encapsulated in a thick layer of resin that keeps water out. The problem is that they keep heat ‘in’ a lot more than their counterparts and this is the part overlooked by many. These units are often installed inside waterproof electrical boxes, where space and air flow are not present most of the time. As I have mentioned above, this does not let the power supply cool down, and temperatures of over 70°C have been recorded at many sites in the early hours of the cold mornings. As you can imagine, this is not ideal, as 90% of all PSU units have a maximum ambient temperature value of 50°C. At this point the power supply cannot provide the power it says it can and decreases by 80% over the next 25°C on average. So your installation that is drawing 70W, has a power supply that can only provide 20W, even though it says 100W, at these temperatures. This dramatically decreases the lifespan of the power supply and will lead to failure.

Wiring Information

MAINS WIRING UP TO THE POWER SUPPLY:

  1. Choosing wire
    When wiring mains, care must be taken as there is the risk of fire and shock. Use good quality new wire of the correct size. Avoid using red or black as you may confuse these with the DC output. If at all possible use conventional brown, blue and green wiring for mains. If multiple PSU’s are wired in parallel you must allow for the total current requirement.
  2. Choosing wire size for mains wiring
    Add up the total power requirement, assume full load case. Ie if you are wiring 2x 300W supplies in parallel, assume 600W. Divide this by the mains voltage eg 600/230 = 2.6A
    Note: Wire size is usually given in the cross section area of the wire, not the diameter e.g. A 1mm diameter wire has an area of:

    There are many factors to be taken into account when working with mains wiring, the current, temperature, %volt drop, open/conduit runs, grouping and there are some nice calculators on the internet (see references at the end) but for LED PSUs at normal temperatures, allowing a 3% volt drop and a maximum length of 20m the table below gives an idea

    Wire size for mains (230V) wiring 3% volt drop
     Power W  Amps  Wire Diameter (mm)  Area mm2
     700 or less  3  1.3  1.5
     1200  5  1.8  2.5
     2400  10  2.5  5
  3. If you have to take a wire through the side of a box or cabinet made of metal always use either a cable gland or grommets.
  4. Earthing
    Always earth the PSU and metal parts of the sign assembly in case they make inadvertent contact with any 230V wiring. Also, in certain situations e.g. signage on high buildings, proper earthing may save the system in the event of nearby lightning. Always use an adequate size wire (at least the same size as the mains wire you used) and the shortest possible route to a good, tested earth point.
  5. Remember that although the DC side may only be 5 or 12V there is a large current flowing. Any very small resistance in the connector or joint is going to cause a voltage drop resulting in heat build-up and of course dimming of the LEDs.

Installation Information

DC WIRING FROM THE POWER SUPPLY TO THE LED’S

  1. Choosing wire
    Note, that the power coming out of the PSU although at a lower voltage is at a considerably higher current: eg for a 300W 230V input 5V output supply, the mains input current is 300/230 = 1.3A, but the output DC current is 300/5 = 60A. So this of course makes a huge difference to the choice of wire size – you cannot use the same type
    of wire to wire up both sides to the PSU. As we saw earlier, Ohm’s law will produce an ever- increasing loss of voltage in the wiring as the current and the length increases, this in turn will cause dimming of the LED brightness, and heating of the wiring. The table below shows wire CROSS SECTIONAL AREA – (not diameter) for a
    3% drop in voltage. Use the next highest value to match.

    Wire cross section area for 12V wiring with 3% Volt drop max
    Length
    (Meters)    		  Current in Amps
     5 10 15 20 25 30 40 50
     5  1.5 3 5 5 8 8 13 13
    6 2 3 5 8 8 13 13 21
    8 2 5 8 8 13 13 21 21
    9 3 5 8 13 13 21 21 34
    12 5 8 13 13 21 21 21 34
    15 5 8 13 21 21 34 34 42
    18 5 13 13 21 34 34 42 42
  2. Avoid using long wires with large currents; it is better to locate the power supply closer to the center or to use additional power supplies. Long wires are prone to voltage drops (dimming) and res.
  3. Feeding long strips:
    Inside the LED strip there is not much space for heavy wire, the result is that the thin copper will cause a noticeable volt drop (and dimming). The thinner the LED strip, the worse the volt drop becomes. If the length exceeds about 8m for standard width strip, or 3m for 4.5mm or narrower strip, the strips should be fed from both sides or broken into smaller sections and fed from separate supplies or transformers with multiple outputs, as shown in g 15 below.
  4. Connections and joints
    Always cut strips where the maker indicates. To extend them, you can overlap and solder, or use connecting strips. Always protect joints against the weather with heat-shrink tubing or insulation. NEVER use silicone sealant on top of or close to LEDs, as it penetrates the protective coating of the strip and will kill the LEDs  underneath it.
  5. Remember that although the DC side may only be 5 or 12V there is a large current owing. Any very small resistance in the connector or joint is going to cause a voltage drop resulting in heat build-up and of course dimming of the LEDs.
  6. Make sure all joints and connections that are exposed to the outside air are properly protected from the effects of the weather, the slightest bit of damp on a joint that has voltage on it will cause a galvanic build-up of corrosion.