Electrical theory 1.8 - Switching power supplies.

In this section we'll discuss switching power supplies (SPS) which are a computer derived product.  As stated before, low voltage lights are safer than mains supply lights (110V+).

Switching power supplies provide electrical safety by isolating and reducing the voltage from 110 … 240Vac to regulated 12 or 24Vdc (other voltages are available). Regulated means:

• the input voltage can vary between 110 to 240V and the output voltage stays constant;
• the output current can vary between zero to full load and the output voltage stays constant. This makes it easier to design light displays;
• usually you can adjust the output voltage by a small potentiometer - refer to manual;
• the DC voltage is constant so the correct type of electronic switch must be used. We'll cover this later.

SPS have a power rating which must exceed the lights' diversity load (see next paragraph) otherwise it overheats and burns out. Usually high power units are fan cooled so it's important that cool air surrounds the unit and expelled hot air can escape the enclosure. Failure to do this can cause the unit to fail or cause internal overload protection to operate >> LIGHTS OUT!!

 

Usually you'll buy a SPS unit to power up all your lights operating on a common voltage say 12Vdc. An important factor is whether light strings are flashing or light strings are being switched on/ off as required. So to use the case of all lights on in a load calculation can be an expensive mistake. A reasonable load diversity factor (ratio actual load/ theoretical maximum load) is 80%. Here’s the equations:

• LED loading: no. LEDs * power per LED (V*I) * diversity
•  
• eg 660*3.0*.02= 39.6W * .8 = 31.7W
• So buy a 36W power supply.

END