Bypass cap - why is big not better?

[Bill Layer]

Hi All,

(Bill chiming in)

>Sooo... Mark got me a bit worried when he said:
>
>Mark Amundson wrote:
>
>> 3. Use 10uf to 100uf tantalum or electrolytic caps for local supply sag
>> issues. Do not use these caps if you have regulated supplies and good
>> beefy wiring as they may cause more power supply hum than their curative
>> effects.
>
>Exactly how do these "larger" caps on the power supply inputs to the module
>cause power supply hum?

Large power supply caps create a saturation issue for the transformer, as
well as putting more pressure on the rectifier (which creates diode noise
and EMF).

Imagine a powersupply filter cap hooked to a rectifer, but under no load.
The voltage across the cap rises to (1.4)(Vrms) and stays there. As a load
is applied, the voltage across the cap begins to sag to (1.2)(Vrms). Now as
the power waveform approaches the crest, it suddenly overcomes the resting
voltage of the cap. At this precise moment, the rectifier switches on very
hard, and the cap now looks like an dead short to that small difference in
voltage. Dead short or not, the power waveform is now cresting and trying
vainly to raise the cap voltage.

Unfortunately, our theoretical dead short now draws (theoretical) INFINITE
current... Since our transformer cannot supply infinite current, it
saturates and quickly corrupts it's power waveform. This creates harmonic
distortion and noise, as well as harms the power factor for the whole AC
side. Meanwhile, the saturation generates heat, which may also in turn
damage the transformer.

Nice, eh?



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