Linear Power Supply Caps

[Rochey]

I'm designing a PCB to put a 'generic' power supply on. I never get single pcb's, always a eurocard full, so it's handy to be able to design a card that i can make a power supply for all voltages on.

Assuming I make a bog standard linear power supply, that is:

transformer -> bridge rectifier - > "storage" capacitors (linked to earth) -> fixed voltage regulator -> Capacitor (linked to earth) -> Output

what I'm trying to understand is what capacitors i should use before and after the regulators.
I understand that they need to be able to smooth out the voltage before the regulator, and that they should be specified 2x the RMS value of the AC. What about before their capacitance? Does the capacitance need to change if i change the voltage output?

Any resources on this topic would be appreciated.

cheers

R

 

[gyraf]

Psu capacitor values are selected according to the maximun current drawn, and the maximum ripple voltage allowed at that current. By rule-of-thumb for fixed regulators, I use ca. 2000uF for each Ampere of supply current.

The capacitor AFTER the regulator should be as specified by the regulator manufacturer - it's purpose is to maintain regulation stability against HF oscillation stuff. As a rule-of-thumb I use a 10uF electrolytic and a 100nF ceramic or polyester in parallel..

AN-556: Application Note 556 Introduction to Power Supplies:
http://www.national.com/an/AN/AN-556.pdf

AN-1148: Application Note 1148 Linear Regulators: Theory of Operation and Compensation
http://www.national.com/an/AN/AN-1148.pdf

Jakob E.

 

[clintrubber]

[quote author="Rochey"]I'm designing a PCB to put a 'generic' power supply on. I never get single pcb's, always a eurocard full, so it's handy to be able to design a card that i can make a power supply for all voltages on.
[/quote]

Thinking of something like this ?

(Fabio's board)

..or do you want to mount the power-TX on the PCB as well ?

 

[Rochey]

looks about right