AC Filtering

There are two kinds of filtering that a surge suppressor needs to deal with: electromagnetic interference (a.k.a. EMI) and spikes coming from the power grid. Both can be caused by several means, the first being typically produced by motor-based appliances, like blenders, washers, microwave ovens and even your printer. Spikes are short moments where the voltage peaks to values way above the normal, which can literally burn your equipment.

The main problem with surge suppressors is that the majority of surge suppressors available on the market don’t have all filtering components necessary for the good EMI and spike filtering. Some cheap “suppressors” don’t have any filtering component at all, so they work just like a regular cord extension with multiple outlets. These “suppressors” are now being labeled as “power strips” to let you know that they don’t do any filtering at all. Don’t buy or use this kind of “suppressor” as they add no filtering at all to your equipment (even though they provide a circuit breaker and helps grounding all your equipments). The “suppressor” we have shown before on Figures 2 and 3 is one of these no filtering devices. We show it completely opened in Figure 6 so you can see by yourself what we are talking about.

Surge SuppressorFigure 6: This “surge suppressor” has no filtering components at all.

Typical components found on real surge suppressors include:

  • MOV (Metal Oxide Varistor): This is the heart of a surge suppressor, in charge of holding voltage peaks coming from the power grid. All surge suppressors must have at least one MOV, connected between hot and neutral wires from the power grid. Good models must have at least three MOV’s, one between hot and neutral, one between hot and ground and one between neutral and ground. MOV’s don’t last forever. After some years they stop working correctly (the exact life expectancy depends on some factor like, for example, how many times the MOV entered in action – a typical value is 1,000 surge protections), and some surge suppressors have a LED to indicate that their MOV’s aren’t working right. On surge protectors without this LED you will think that your surge protector is still protecting your equipment when their MOV’s have gone bad, since there still voltage on the AC outlets, while in fact your surge suppressor is working just like a power strip – i.e., cord extension. Every MOV is rated with a maximum absorption energy level, given in a unit called joule. If the amount of energy delivered to the MOV’s is higher than their ratings, they will burn – protecting your equipment, at least. It is important to know that MOV’s only holds overvoltage peaks. If a voltage or current higher than the maximum the MOV can handle is applied during a sustained period of time, the MOV will not only burn, but may pass overvoltage to your equipment, creating a risk of fire. Physically MOV’s are disc-shaped components with two terminals, usually blue in color.
  • X Capacitor: In charge of EMI filtering, this capacitor has two terminals and is usually squared and yellow in color.
  • Y Capacitors: In charge of EMI filtering, these capacitors always come in pairs. They have two terminals each, one capacitor is connected between ground and “hot” wires coming from the power grid and the other is connected between ground and neutral wires. Since these capacitors are connected to ground if you don’t have correct grounding this filtering won’t work. Physically they are disc-shaped and usually light blue, being smaller than MOV’s. When opening surge suppressors some people confuse Y capacitors with MOV’s. When you are doubt whether the blue disc in front of you is a MOV or a Y capacitor, keep in mind that Y capacitors always come in pairs and one of their terminals are always connected to ground.
  • Ferrite coil: Another component in charge of EMI filtering, it is a coil made from copper wire with a ferrite bar or ring in the middle.

Cheap surge suppressors don’t have the EMI filtering components, even though they may have at least one MOV. Good suppressors will have both MOV and EMI filtering components.

On Figures 7 and 8 you can see the surge suppressor from Figure 4 opened (a unit from APC that costs around USD 30.00, by the way). The components found in the middle of the unit in Figure 7 are from the phone and cable/satellite filter, which we will discuss in the next page, while on the left-hand side you can see the printed circuit board from the AC filter.

Surge SuppressorFigure 7: Inside a good surge suppressor.

In Figure 8, you can see the AC filter from this unit in details. As you can see, this unit doesn’t have one MOV but seven! This unit does not have Y capacitors, but it has two thermal fuses, that act just like a circuit breaker but without a reset switch – i.e., it shut downs the circuit if its internal temperature reaches a certain “trigger” value, meaning that the circuit is overloaded or there is a short-circuit. The smaller components at the bottom are used to control the two available LEDs.

Surge SuppressorFigure 8: Inside a good surge suppressor.

High-end surge suppressors have filtering also on the load side, preventing your equipments from generating noise to your power line.

Now let’s talk about the importance of the phone and cable/satellite filtering and how this filtering should be done.


Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.