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How power supplies work

The AC receptacle is the socket where you connect the power cable at, often accompanied by a simple on / off switch. The switch does nothing more than isolating the power supply from the power grid by interrupting the live wire.

On older power supplies you could find an AC voltage selection switch which had to be set to comply with the utility grid voltage (110V or 220V) depending on where you live at. This is no longer necessary because newer units automatically sense the grid voltage through the Active-PFC circuit. Since the installation of an Active-PFC circuit became obligatory by law in many of the world's countries, it is highly unlikely you will find a passive PFC unit (or a unit without a PFC circuit at all) being sold today.

AC receptacle and on/off switch


The first thing the power supply has to do is make sure that the AC power is as clean as possible. That includes both the AC power the power supply is receiving but also its effect on the AC line itself; switching power supplies generate harmonics which must be filtered before entering the AC power grid. This is done by implementing a transient filter circuit. The transient filter usually consists of capacitors and induction coils, but sometimes it also includes a metal-oxide varistor (commonly known as a MOV) and a simple glass fuse for extra protection. We say "usually" and "sometimes" because there are no restrictions concerning what manufacturers can and cannot use. High quality units try to meet and exceed the ATX specification guidelines. Very poor quality units will probably have no transient filter at all, so they will flood your household power grid with electrical noise and can be seriously damaged by even a very slight power surge.


Transient filter components at the back of the AC receptacle.


Transient filter components on the PSU PCB.



The most common transient filter components are:

Common capacitors: Y type capacitors (picture: small round disc) are used for line bypass, connected between Live or Neutral to Ground. They are used in applications where capacitor damage may lead to electric shock and they shunt current to Ground. X type capacitors (picture: white rectangle) are being used across the lines, connecting Live to Neutral. They are used where damage to the capacitor cannot induce electric shock and also operate as fire retardants.


Y-Type and X-Type Capacitors 


Induction common mode (filtering) coils: Common mode choke coils are used to suppress common mode noise (EMI/RFI filtering). This type of coil is produced by winding the signal or supply wires one ferrite core. Since magnetic flux flows inside the ferrite core, common mode choke coils work as an inductor against common mode current. Accordingly, using a common mode choke coil provides larger impedance against common mode current and is more effective for common mode noise suppression than using several normal inductors.


Common mode coil


Metal Oxide Varistor (MOV): MOVs are semiconductors that protect electronic components and systems from transient voltages. Their design allows current to flow in only one direction. MOVs have very high resistance at low voltage and very low resistance at high voltage, which is where their name comes from (Variable Resistor). Thus applying low to moderate voltage has little to no effect, however a high voltage triggers an "avalanche effect" and causes the diode to break down. In short, these very little components are your typical, reliable surge suppressor. MOVs tend to degrade if they are repeatedly exposed to transients.

Metal Oxide Varistor


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