Choosing a suitable power supplyUpdated a year ago
- Remember, power (expressed in watts) in a DC system, is simply volts x amps. So if you have a 12V power supply providing 5A, there is 12x5 = 60W of power involved, or put another way, a 300W, 5V supply can provide 300/5 = 60A of current.
- Power and efficiency:
LED’s 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 230V mains you need a power supply unit or PSU, but this is commonly called a ‘transformer’ in the industry which is quite incorrect, so I shall call them PSUs from here on.
Most PSU’s are described as being either ‘switch- mode’ or linear regulators. What this means is that the PSU takes the 230V AC mains supply and converts it into a regulated DC supply (the DC output does not fluctuate, hence it is regulated). The mains voltage may vary over quite a large range (perhaps 90 to 260V) while the DC output remains constant at 12V,say. Other available regulated voltages may be 5, 12, 24 or 50V). Linear supplies are not efficient and require lots of heat sinks and heavy transformers, whereas switch mode supplies are now the industry standard and very lightweight and efficient. Typical efficiencies for switch mode supplies are usually over 90%. What this means is that for a 300W DC out supply, it would only need about 330W mains input. Let’s use this example to calculate a mains input requirement: Assuming a 230V ac mains supply, the current required would be 330/230 = 1.4A. 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 and a usual value is 80%. This means we can load this supply up to 80% of 300 ie 240W ie 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 with any heat coming from the outside from the sun, will lead to the supply failing.
- Obviously, the supply should be protected from water, dust and vibration and should have the correct IP rating.
- 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 with any heat coming from the outside from the sun, will lead to the supply failing.
- Constant current vs Constant voltage:
- These are terms that are quite incorrectly used in the LED industry. What they are generally assumed to mean is that for a constant voltage supply, the output remains at a constant voltage regardless of the load current. A so-called constant current supply also does just that, but when the load reaches the maximum limit, instead of dropping the voltage it regulates the current to a constant specified value. The advantage is one of protection against faults or overloading.