I have been repairing a lot of power supplies lately and I have noticed that there are a few that have capacitors on the input side of the step down transformer. Any ideas why this is? Stability, transient protection?

Thanks,
the boxer

 

Originally posted by Screamtruth@Apr 20 2006, 06:37 PM
I have been repairing a lot of power supplies lately and I have noticed that there are a few that have capacitors on the input side of the step down transformer.  Any ideas why this is?  Stability, transient protection? 

Thanks,
the boxer

[post=16375]Quoted post[/post]​

I have a strong suspicion that the cap to which you are referring is placed on the primary side as a "power factor" correction measure. I have noticed that many new power supplies are beginning to include power factor correction as a feature.

The idea is to make the power supplies net input impedance appear resistive. The capacitive reactance is placed in parallel with the inductive reactance to pull this off. The power generating companies are beginning to look unfavorably on large power consumers that do not take measures to maintain there connection to the power grid to be primarily resistive. In the past, power consumers have connected to the power grid with highly reactive loads resulting in making their power consumption appear to be lower than it was in reality. This means that they end up paying for only a portion of the power they consume. You can see that there would be strong economic reasons for the power generators to start levying fines on power consumers that try to exploit this phenomenon to their own advantage.

You should be able to check the power supply manufacturer's datasheet and confirm my suspicion.

AAC has as good introduction to the topic of power factor calculation in VOL 2:Chapter 11:Sectioni 3. (see link below) The following section (Sect. 4) presents the topic of power factor correction.

Power Factor Info

Enjoy,
hgmjr