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Having just indicated that components can fail when being "chipped away at" substantiates the requirement for suitable filtering on power supplies. Although we cover the design of the filters found in modern day power supplies, I may as well get the controversial statement out of the way being... These filters are not designed, in any way, to protect the equipment. They are placed there to stop the rubbish the switching action of the circuitry makes from getting out! But, as passive electronics are involved, they also do have a fair amount of protective ability against short-term disturbances from making their way into the sensitive electronics of the power supply. It is the foolish designer, however, that relies on this fact, especially if the power supply is a ready-built module that is merely incorporated into the design. When incorporating a ready-built module, the grounding to the power supply filter is usually found to be insufficient or non-existent. All the everyday whacks that appear on the mains now find themselves dissipating in the very electronics the designer would rather leave unharmed. Instead of placing decent filtering within the design, the developers would rather hope for (and usually demand) a "clean supply". This is a tad unrealistic and most times land up doing what they should have done in the first place - put in a brute force line filter. Effectively, this is pretty much the same sort of filter that is on the input to the now common switch-mode power supply, except it is wired up the other way round i.e. the two filters are therefore 'back-to-back'. The filter on the SMPS is stopping the 'junk' from the SMPS getting out, and the brute force filter is stops any mains borne muck from getting in! Unsuitable In-rush Protection >>
© 05.10.01 |
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