Filtering a Laptop Power Supply

[mth5044]

Based on some old advice in a few other threads by RG and other online institutions, I've doodled a theoretical way to help filter noise from laptop power supplies or switched mode supplies, although I'm not sure about some component values. Mainly the common mode choke and the inductor.



If the input is from a 12V 5A supply, and the purpose to regulate down the voltage as little as possible, would this be an appropriate common mode choke?

http://www.mouser.com/ProductDetail/Schaffner/RN242-6-02/?qs=sGAEpiMZZMsVJzu5wKIZCVSrvfzdSugYrB3QpM9udx8%3d

I'm not sure if there is an amp safety net that is needed, but that is rated at 6A.

The second item, the inductor, I unsurprisingly know just as little about. I'm not sure what the inductance value will do to the filtering. I'd imagine one would want the lowest coil resistance to reduce the voltage loss. There seems to be overlap in inductors and chokes will browsing through the various online retailers.

If there are any other filtering components that would be beneficial, please advise! Thanks.

[armdnrdy]

Here's a LC filter calculator:

http://www.ampbooks.com/home/amplifier-calculators/LC-ripple-filter/

The size of the inductor depends on the amount of ripple that is deemed acceptable.

[mth5044]

Thanks for the link. I tried using the calculator based on inductors at Mouser and there doesn't seem to be any that are rated +5A and higher than a couple mH inductance, not to mention DC resistance causing voltage drops of 6V to 55V. THe caculator doesn't seem to want to use anything less than 1H.

What is an acceptable ripple? I'd assume it varies depending on what you're using it for, but in this context - audio purposes. Stompboxes, amps and whatnots. So I guess it would be low relative to the audio signal. So if a guitar signal is 100mV... 10mV acceptable? 1mV? a uV?

[armdnrdy]

You can weed through the information here:

https://www.google.com/search?sourceid=navclient&ie=UTF-8&rlz=1T4GGHP_enUS450US450&q=acceptable+audio+power+supply+ripple

It really depends on your application. If you are powering fuzz circuits, a larger ripple is probably acceptable.

[mth5044]

I had attempted to go through google links, but 9/10 say it depends and the other 10% were way above my pay grade.

I tried to extrapolate the value of the reduction based on the calculator, as it only goes down to 1H. Around 10mH, it starts to give an increase in ripple attenuation. At 270uH it boosts the Hz to +37db. Maybe a flaw in the calculations at that low of Henry's. On one forum, this type of inductor was recommended. I found one that was rated above the 5A. Is 270uH too low?

http://www.mouser.com/ProductDetail/Bourns/2317-V-RC/?qs=ybAy/pS4FePYU4X4qdj4Uw==

As for powering a fuzz, I was thinking more along the lines of a general purpose power supply. Not for any practical reason besides something like breadboard or testing.

[GibsonGM]

One way to see what might be acceptable would be to do it "empirically"....make a transistor gain stage or two, and power them with this!   See what it sounds like. 

If you were to try this with a PS with a lot of ripple, oh - you'll KNOW it!  And as you add filtering, you'll get to a point where you don't get increasing gains from adding more filter sections.

Just an idea....it's how I might approach this in my half-baked, easier to set it up than calculate it way!    I've actually done this with good results, using only R-C pi filters, but YMMV depending on what kind of power supply you have/how well it's already filtered, how close it is to your circuit....