We have had questions from our customers about the
differences in Linear and Switching power supplies. What are they and which do
I want to use.
The basic function of an AC to DC power supply is to convert
the incoming AC power supplied by the power company into DC output. This is called rectifying. It also transforms the voltage to the desired
output voltage level. In order to
achieve usable current, it also must regulate and filter the output so that it
remains constant under load and consistent regardless of the conditions at the
input.
There are two technology approaches for achieving this
–linear or switching technology.
Switching power supplies are the most common and popular
power supplies in the access control realm.
They achieve the voltage step down by switching on and off very rapidly.
They use a low-current, high voltage bridge rectifier to transfer power from
the input side to the output side. This
allows the power supply to operate with less heat and to be made of lighter materials.
As a result they are less expensive and weigh less.
However, the switching technology has some drawbacks. These can create interference, referred to as
noise and ripple, in several ways.
One way is by operating in a frequency similar to a
contactless card reader, which can impact the readers’ ability of the reader to
interact with the card. In another, the
magnetic field created from the on/off switching of a poorly designed power
supply may induce voltage onto nearby wires through inductive coupling. The electromagnetic field itself may cause
electromagnetic interference (EMI) or radio frequency interference (RFI), which
can impact the performance of audio equipment or technology that relies on RF
communication.
This will show up at the reader as reduced read range
(having to scrub the card on the reader), rejected reads or “phantom” access
attempts.
A well-designed switching power supply with good regulation
and filtering can greatly reduce the interference. The power supply may cost a little more, but
the result will be better when used with RF and proximity devices
Linear power supplies achieve the desired output voltage by
getting rid of the excess power through resistors. They transfer energy from the input side to
the output side via circuitry called a linear bridge.
Because they eliminate excess power through resistance, they
can generate a lot of heat and are less efficient. Because of the larger heat sink and heavier
core materials needed to support the energy transfer, linear power supplies
usually weigh more and usually cost more because of the types of material
required.
But because they the always “on” there are no fluctuating
magnetic fields and the voltage is steady, so no EMI or RFI. This is why manufacturers of certain
equipment require or recommend linear power supplies for use with their
devices.
