Caps x10

[hjl4]

15 years ago, during my study as an EE, I was taught that, good filtering aft of the bridge rectifier, could be accomplished by using a 100uf cap, a resistor, or better yet an inductor, and then a 20uf cap.

That was following the 5x rules.(back then).I have always built my psu's with that train of thought.

Yesterday, I took apart a relatively new psu, two years old, and was amazed to find that it has a 100uf cap(electrolytic), inductor, and a 10uf(electrolytic) secondary cap.
Is'nt this a little over kill??
The PSU, came out of a motion control board???

Am I mistaken here?

Maybe I should go back in my books and check, I guess.
If anybody knows why such an increase in differential, please let me know.

 

[Nigel Goodwin]

It depends entirely on the load it's intended to feed, it's very uncommon to have either an inductor or resistor in series (and has been since the advent of transistors!), and the 100uF and 20uF you mentioned are far too small for any practical purposes. Was the guy who taught you that particularly old?, and was he refering to valve equipment?.

The modern supply, with small capacitors and an inductor is most probably a switch-mode supply?, where you only require small capacitors because of the much higher frequency of operation, and can use a low inductance coil to great effect as well. Generally (for a non-switchmode supply) you would have MUCH larger capacitors, and no series resistor/inductor (due to the high losses through it, and the high cost of a suitable inductor).

 

[hjl4]

You're right Nigel,

Yes he was of age, but the values I gave were approximations, only to make calculations simpler.And yes, it is a switchmode psu.

But I checked my books and it does confirm what you're saying.The psu I was describing, had the caps in parralel.
I bet if I had a working scope, I would see alot of ripple from the output, which could explain the slight humming I could hear when the motors were runnning.
I guess it was good enough though for the job it was doing.(fan control-0.15ma)

Thanks Nigel.