Power supply is one the most important equipment for electronics engineers/hobbyists. The choice for power supply depends on the range of power requirement for your applications. The kind of power supplies available include dry and wet batteries, AC to DC, DC to DC, AC to AC, multi-channel, fixed and variable power supplies.
The main generic parameters to check in AC to DC low-voltage power supplies are minimum and maximum voltage and current. Other important parameters include line and load regulation, temperature stability, and short-circuit protection. Note that the voltage requirement for most hobby projects range from 3.3V to 15V DC with current rating from 1mA to 3A.
AC to DC power supply is used for everyday consumer devices such as cell phone chargers, electronics hobbyist and industrial devices. It converts AC power from the grid into DC power and changes the voltage of the power to suit the needs of the devices. Depending on the technologies there are two types: linear and switching power supplies.
Irrespective of the type of power supplies, some common characteristics to look out for in power supplies are:

Input and output voltage range, value, and accuracy.
Most conventional power supplies operate with fixed voltage and current ratings. But a variable power supply offers greater flexibility to give a wide range of outputs.
Required current, and presence or absence of transients
Load regulation. Typical well-regulated power supplies have load regulations of less than 1%.
Ripple and noise. Most linear power supplies should have less than 3mV RMS ripple and less than 50mV peak for switching supplies.
Efficiency, which is the output power divided by the input power, will always be less than 100%.
Temperature. Modern lab-quality power supplies should have temperature coefficients of under 0.05% per °C. This is usually specified over the operating temperature range, which often is 0 to 40°C.
Mounting area and height (form factor).
Whether under-voltage, over-voltage, and over-heating protection required.
Special environmental/operating conditions. On-board vehicle, space, communications, RF, and other factors.
Cost of a power supply is also important.

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