building a small power supply... question

[mr_fx]

ok so I am building a small variable voltage power supply,

I was wondering which configuration would be most useful

1.) transformer > rectifier > large capacitor > voltage regulator > out to device

2.) transformer > rectifier > voltage regulator > large capacitor > out to device

or should I run a large capacitor before AND after the voltage regulator ???

 

[KE5MC]

Check your data sheet for the regulator as they usually have fairly complete application notes addressing filtering for ripple (low freq) and stability (high freq).

Mike
P.S.
Item 3. Large cap before reg and larger cap after reg with .1 or .01 caps both sides.

 

[mr_fx]

I am more referring to a cap for storing and delivering power, not filtering it, a large electrolytic

 

[mr_fx]

with the large cap after the regulator any load on the power supply makes the voltage drop pretty fast... seems to work fine before the regulator though

 

[KE5MC]

Data sheet for a National Semiconductor LM317 does not agree.

Copy and paste page 1 lower left corner.

"†Optional—improves transient response. Output capacitors in the range
of 1μF to 1000μF of aluminum or tantalum electrolytic are commonly used
to provide improved output impedance and rejection of transients."

It does work without it, but works better with.

Mike

with the large cap after the regulator any load on the power supply makes the voltage drop pretty fast... seems to work fine before the regulator though

 

[mr_fx]

That explains a few things... I am using something in the ball park of 12,500uf

 

[KE5MC]

As many things in life to much of a good thing turns bad.

Designs are about compromise and shades of grey.

Good Luck with your project!

Mike

 

[Captain Kilowatt]

A large cap after the regulator should not affect regulation during large current draws. I replaced the 68,000uF filter cap in my Tripplite supply with a 100,000uF which is located after the regulator. regulation is great up to 40 amps but the supply has feedback to ensure good regulation regardless of an voltage drop.Standard convention is to place it AFTER the regulator and the PRIMARY purpose is to filter not provide peak current.

 

[mr_fx]

I will try to write up a schematic of what I built and maybe that will explain the problem

 

[Robb]

Power Supplies Galore

 

[Rb30]

Transformer-rectifier-10uf elecrtolytic cap-LM317-larger cap.

It is amazing how a small 10uf cap before the regulator will stabilize input to the reg and give less ripple on the output