Transient Filtering

The first stage of a PC power supply is the transient filtering. In Figure 8, you can see the schematics of the recommended transient filter for the PC power supply.

Transient FilteringFigure 8: Transient filter.

We say “recommended” because many power supplies – specially the cheap ones – won’t have all the components shown in Figure 8. So a good way to check whether your power supply is a good one or not is by checking if its transient filtering stage has all recommended components or not.

Its main component is called MOV (Metal Oxide Varistor) or varistor, labeled RV1 on our schematics, which is responsible for cutting voltage spikes (transients) found on the power line. This is the exact same component found on surge suppressors. The problem, though, is that cheap power supplies don’t carry this component in order to save costs. On power supplies with a MOV, surge suppressors are useless, since they have already a surge suppressor inside them.
L1 and L2 are ferrite coils. C1 and C2 are disc capacitors, normally blue. These capacitors are also called “Y capacitors”. C3 is a metalized polyester capacitor, normally with values like 100 nF, 470 nF or 680 nF. This capacitor is also called “X capacitor”. Some power supplies have a second X capacitor, installed in parallel with the main power line, where RV1 is in Figure 8.

X capacitor is any capacitor that has its terminals connected in parallel to the main power line. Y capacitors come in pairs, they need to be connected together in serial with the connection point between them grounded, i.e., connected to the power supply chassis. Then they are connected in parallel to the main power line.

The transient filter not only filters the transients coming from the power line, but also prevents the noise generated by the switching transistors to go back to the power line, which would cause interference on other electronic equipments.

Let’s see some real-world examples. Pay attention to Figure 9. Do you see something strange here? This power supply simply doesn’t have a transient filter! This power supply is a cheap “generic” unit. If you pay attention you can see the markings on the power supply printed circuit board where the filtering components should be installed.

No Transient FilteringFigure 9: This cheap “generic” power supply doesn’t even have a transient filtering stage.

In Figure 10, you can see the transient filtering of a cheap power supply. As you can see, the MOV is missing and this power supply has only one coil (L2 is missing). On the other hand it has one extra X capacitor (placed where RV1 is in Figure 8).

Transient FilteringFigure 10: Transient filtering on a cheap power supply.

On some power supplies the transient filter can be broke down into two separated stages, one soldered to the input power connector and the other on the power supply printed circuit board, as you can see on the power supply shown on Figures 11 and 12.

On this power supply you can find a X capacitor (replacing RV1 in Figure 8) and the first ferrite coil (L1) soldered on a small printed circuit board that is connected to the main AC power connector.

Transient FilteringFigure 11: Transient filter first stage.

On the power supply printed circuit board you can find the other components. As you can see this power supply has a MOV, even though it is placed on an unusual position, after the second coil. If you pay attention, this power supply has more than the recommended number of components, as it has all components shown in Figure 8 plus an extra X capacitor.

Transient FilteringFigure 12: Transient filter second stage.

This power supply MOV is yellow, however the most common color is dark blue.

You should also find a fuse near the transient filter (F1 in Figure 8, see also Figures 9, 10 and 12). If this fuse is blown, beware. Fuses don’t blow by themselves and a blown fuse usually indicates that one or more components are defective. If you replace the fuse, the new one will probably blow right after you turn on your PC.

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.