quick question re: Power supply caps

[Rodgre]

Is there a rule when it comes to filtering caps in power supply circuits? Is there any reason to use higher voltage caps, or are small, 16v electrolytics fine for a LM317-based 9vdc power supply?

Is there any rule when it comes to values of filtering caps? More to the point, is there any benefit to using higher capacity caps (say, a 2200uf in place of a 1000uf)?

Thanks!

Roger

[Minion]

A Higher Cap Value (In uf) will provide better ripple rejection and therefor create a Less noisy PSU but will also take more current to charge the caps which could cause a small delay when the pedal is first turned on before it will opperate....For low current operation(for say a Pedal board PSU) you probably don"t need a Lot of Filtering to make a relitively quiet PSU, Maybe 1000uF before the regulator and say 100uF after the regulator, Pluss most Pedal curcuits have a bit of Onboard Filtering closer to the IC/Transistor pins which helps if the Regulator board is Far away from the Pedals....


Cheers

[Rodgre]

Thanks.

I really need to dig out my old Basic Electronics textbooks to teach myself why certain values are called for in certain positions in certain circuits...

Can anyone briefly explain why 1000uf before the regulator and 100uf after are good values? Is 1000uf after the regulator just as well, if not overkill, or is there a detriment in using a larger value?

Roger

[Minion]

You can Put more Filtering after the regulator if you want but most poeple put the bigest filtering before the regs.....If you are useing LM78XX/79XX or LM317/337 regulators it is advised that you put a Protection diode on the Regulator if the Output cap is over 100uF (see Datasheet) to protect it in case of accidental reverse connections and Transient Voltages..... Also if useing a LM317/337 type regulator it is good to put a 10uF-22uF cap from the Adjust pin to ground which will improove noise rejection....


Cheers

[Rodgre]

Thanks Minion. I keep forgetting that manufacturer's data sheets can often give a wealth of knowledge as to how a device behaves and how some of the component values in everyday circuits are decided upon.

Roger

[stephanovitch]

For best ripple rejection use low ESR electrolitic caps or had a little polyester cap or ceramic with the electrolitic cap.
For adjusting filter cap, you can apply this simple rule :
2200µF cap for 1A current after filter.
You can easily calculate multiples (1000µF for 0.5A ect...)