Basic understanding type of PSU.

The power supply unit (PSU) is one of the few items in an electrical device that will affect the performance and reliability of your entire system. It's often the most unvalued, unappreciated component within any device, yet one of the first layer to cause failure.

This article I write focuses on the factors to look for when purchasing a PSU for radio shack.

Determine the load you will be applied to that PSU. Don't purchase a PSU just above your requirements unless you plan not to add any additional device connect to that PSU. Do remember, PSUs age, losing power over time. Purchase a PSU that will take you through your next few upgrades or add up, over a multiple-year period.

Look for PSUs with high-efficiency ratings and Regulation. Once total loads determined. Looks for PSUs that well 20% more above the loads applied. Mean the loads that applied to the PSU with good working efficiencies are at 80% from the PSU’s total capacities. Say you have a 40amp MAX PSU. Then 80% of 40amp is about 32amp continuous operation without failed provided sufficient ventilations. Other than capacities, output voltage regulation also a factor that a PSU need to maintain a constant voltage when heavy loads applied or high ampere drawn from the PSU.

Ventilation efficiencies. Ventilation is the last but important role in preventing an overheating PSU when working under intense load or high ampere loads.

Types of PSU Switched-mode and Transformer type.
A Switched mode PSU (SMPS) is a type of PSU that not required a transformer to step down the incoming AC to a desired voltage. The switched PSU use switching regulator to provide the require output voltage. An SMPS is actually a power converter or transformer that transmits power from a source to a load with ideally no losses. The function of the converter is to provide a reliable output voltage often at a different level than the input voltage.

A transformer PSU is a PSU that transfers electrical energy from one circuit to another through inductively coupled coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary (NS) to the number of turns in the primary (NP) as follows:

Numbers of turns at the secondary winding (NS) determine the value of output voltage.
Choosing a PSU. Both type of PSU worth choosing from cause both works fines in comparison.

Article by 9M6WST
(Base on years of experience in Communication industries and HAM)