The key points to note when choosing a PSU are:
- 1. Is it AC or DC output?
- 2. If DC, is it regulated or unregulated?
- 3. What is the output connector type (and polarity if DC - usually centre is positive).
- 4. What is the maximum current it can supply?*
- 5. What is the output voltage?
Obviously point 5 is most important. The voltage has to be correct. If it's too low, the equipment won't work correctly. If it's too high, the equipment will be damaged.
The current rating should be equal to at least the maximum that the equipment will require. It does not matter if it is rated higher, except that you will probably be paying a little more for capability that you don't need. (Think of it as buying a bucket that is larger than you will ever need.)
Power is often labelled as a WATT or VA rating. Watts = Volts x Amps so, for example, a 12 volt power supply unit with a rating of 24 Watts (or labelled "24VA") will be able to supply 24/12 = 2 Amps of current.
Some people refer to a PSU (Power Supply Unit) as "a transformer". Nowadays this is seldom correct, although the unit might contain a transformer as part of its circuit. A transformer passes only AC.
Modern Power Supply Units are of a type called "Switch-Mode". They contain a bridge-rectifier (to convert AC to high-ripple DC) and a capacitor (to smooth out the ripple). This "raw DC" is then fed to the electronic switching circuit that "chops" the voltage at high frequency at around 350 volts. The resultant spikes are fed into a tiny transformer that changes the voltage to one or more lower voltages (which are still high frequency pulses). Then a rectifier and capacitor smoothes out the pulse chain into a DC voltage. Finally, if a steady (regulated) voltage is required, the current is passed through a voltage regulator circuit that removes any fluctuations.
The reason for using a "Switch-Mode" PSU is that it is more efficient (wastes less power as heat) and physically smaller and lighter than a conventional PSU that used a transformer capable of running only at 50 Hz or 60 Hz.
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