Bench-top power supplies are used by technicians to provide a reliable source of direct current voltage. As they are often used to troubleshoot defective devices such as car radios, amplifiers, and other automotive or 12VDC devices, being internally protected against short circuit and overload is a must. A well-designed overload protection circuit can shut down the output of the supply in a matter of milliseconds, preventing damage to the supply and possibly (further) damaging the device connected to it.
The current rating on the power supplies determines how much power the supply can deliver to a connected load. Power is equal to voltage times current. Some have an adjustable voltage, though 12V and 13.8V are the most common. Such power supplies are also regulated. This means that the supply always delivers the rated voltage regardless of variations in the load (current draw). This is a very important feature, as the actual voltage output of a non-regulated power supply is a function of the current draw, or how much of a load the connected device represents. This will cause great changes in the actual voltage output, and is commonly seen in wall adaptor type power supplies.
Current capability is directly related to price on power supplies. It is important to use a supply with enough current capability to meet your needs. For example, if a device such as a car amplifier can draw up to 10 amps at 12V (120 watts), and assuming you needed to test it near full power, a 15 to 20 amp supply would be best. In this situation, a 10A supply would have no "headroom", would work at the extreme of its capability, and may shut down.
Most bench-top supplies have binding posts as their output connections, to which bared wire is connected. Most of the larger capacity units will have thermally-controlled cooling fans