Back To Basics: Selecting The Right Power Supply

A basic door control system may be comprised of a power source, wiring which connects the system elements and a locking device. It could also be several locks and controls. Or the system power supply may have multiple outputs, supplying locks and...


It is estimated that the life cycle of electronic equipment can be increased up to 30 percent with the use of surge protection measures, longer with the proper protection. A surge can be induced onto ANY metallic conductor.

Induction is caused by changing the magnetic flux inside a loop of wire and is a phenomenon commonly used and found in electrical and electronic technology. Transformers use induction. Extraneous voltages can be as easily induced as the intended ones.

Isolation

Isolation is the electrical and sometimes magnetic separation between two circuits in close proximity to each other.

Isolation is accomplished by devices such as transformers, fuses, relays and opto-isolators.

Isolation is used:

  • To protect system elements from overload
  • For safety: Transformers used within power supplies function as a safety isolation transformer.
  • For Voltage Level Shifting: Isolation is often required to achieve the potential level shifting
  • To Obtain Multiple Outputs: The isolating characteristics of a transformer also allow the design of power supplies with multiple outputs by adding windings to the transformer along with rectifier and filter components. A very common example of this type of power supply is the desktop computer supply with +12, +5, and +3.3 volt outputs.
  • To prevent Grounds Loops which occur when two or more circuits share a common return path.

Underwriters Laboratories (UL) tests the quality of Power Supply Isolation as part of its Listing and recertification of power supplies by performing a “Hi-Pot” test. This test applies either a DC or an AC voltage across two isolated circuits. The resulting leakage current is measured. Another method is to measure the resistance between two isolated circuits.

All insulator material used in isolation applications exhibits capacitance resulting where two circuits may be DC isolated but not AC isolated. If the circuits are physically close enough to each other, and the frequency components are high enough, one circuit will be able to disrupt the operation of the other circuit. This is known as “crosstalk.”

Insulation is a closely related to isolation. An electrical insulator is a material which does not conduct an electric current, under the influence of an electric field. A perfect insulator does not exist, but some materials such as glass and paper, which have high resistivity, are very good electrical insulators. Electrical insulation is the absence of electrical conduction.

Hard Wired vs Plug-In

If the old power supply was a hard-wired type, then it will be easiest to simply replace the circuit board or step down transformer inside the power supply’s metal enclosure so you do not have to get involved with electricians to rewire and the removal and reinstallation of the sheet metal enclosure.

Hard-wired is a term which refers to when the power supply line voltage is permanently attached to the power supply using flexible metal sheathing or rigid conduit. The transformer is typically inside the enclosure and steps the voltage down from line voltage (120 VAC) to whatever the operating voltage range is of the equipment being used. Typically this voltage is 24 Volts. The transformer transforms the 120 down to 24 volts AC, and the AC voltage is input into the power supply circuit board where it can be rectified, filtered and regulated and further mangled.

Plug-in transformers (also referred to as wall warts) step down the line voltage as described above, but they plug directly into a line voltage receptacle. They are not necessarily inside a metal enclosure.

There are drawbacks to using plug-in transformers:

  • If you do not verify the branch circuit is non-switched, the power may be turned off inadvertently from a wall switch.
  • If the receptacle you use is on a branch circuit shared by other receptacles on the same circuit breaker, you need to be sure your equipment does not exceed the load capacity for the circuit.
  • If the receptacle you select is convenient for you, it may also be convenient for others. Callbacks resulting from plug in transformers being removed so other appliances can be connected (like vacuum cleaners) or kicked loose are pretty common where step down transformers are used.

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