Understanding power suppliesUpdated a day ago
Electrical note: Power (watts; W) in a DC system is simply voltage (volts; V) multiplied by current (amps; A). So if you have a 12V power supply providing 5A, there is 12x5 = 60W of power involved; or put another way, a 300W 12V supply can provide 300/12 = 25A of current.
Power and efficiency
LEDs are DC (direct current) devices – you can't run them off the mains, which is a high voltage AC (alternating current) supply. To convert and condition this mains power you need a PSU (power supply unit), also known as a driver. This is commonly called a 'transformer' in the industry, but that is quite incorrect.
A PSU takes the AC mains supply and converts it into a regulated DC supply (the DC output does not fluctuate, hence it is regulated). The power supply may support a quite a large range of input voltages (perhaps 90 to 260V), but the DC output remains at a constant voltage. Common DC voltages are 5V, 12V, and 24V, but others are also available (such as 48V).
Most PSU's are described as being either 'switch-mode' or linear regulators. Linear supplies are not efficient and require lots of heat sinks and heavy transformers, whereas switch mode supplies are now the industry standard and are very lightweight and efficient.
Typical efficiencies for switch mode supplies are usually over 90%. What this means is that for a 300W output supply, it would only need about 330W mains input. Let's use this example to calculate a mains input requirement: Assuming a 120V AC mains supply, the current required would be 330/120 = 2.75A. This is not much at first glance, but on start up there is an inrush current which the local circuit- breaker must handle without tripping; the normal 15A circuit-breaker is more than adequate for 1 PSU.
How much current can this PSU give us at the DC end? We said it was a 300W supply and if it was a 12V system, it would be 300/12 = 25A. Should we run it with a 25A load? No, you have to allow some head-room for the supply to allow for heating, current surges, etc. A usual value for maximum loading is 80% of the rated output. This means we can load this supply up to 80% of 300W; i.e., 240W; i.e., 240/12 = 20A max.
Mounting and cooling
We said earlier that switch-mode power supplies are very efficient; however, that little bit of inefficiency results in heat and has to be taken notice of. In our earlier example of a 300W 90% efficient supply, there is 30W of heat. In a closed box with no ventilation this is going to build up, and without any way for that heat to dissipate will lead to the supply failing. This is especially relevant if the box is exposed to sunlight, as that will additional heat.
Obviously, the supply should be protected from water, dust, and vibration and should have the correct IP rating.
Constant current vs Constant voltage
A constant voltage supply attempts to maintain a fixed output voltage regardless of the load current. They will try to supply as much current as needed, until they either burn out or shut down (for models with overload protection).
A constant current supply is basically a constant voltage supply with built-in current limiting. It acts similarly to a constant voltage supply, but will decrease the output voltage as necessary (but within a certain range) to prevent more than the set current from being drawn, thus maintaining a "constant" current.
Which you use depends on what you need to power. Almost all LED strips are "constant voltage", which means they must be supplied with a fixed voltage; these require a constant voltage supply. Other LED fixtures, such as downlights, spotlights, and many rigid LED bars and similar, usually require a regulated input current; these require a constant current supply.