rectifier/filter before transformer PSU? lower noise?

[darron]

as the topic says - would that give lower noise? any reason why not to do that other than the need for massive caps?

thank in advance. hope it's an easy question (:

[George Giblet]

You means  Rectifier -> Filter - Transformer  ?

That just can't work the output of the rectifier and filter is DC, you cannot transform DC with a transformer.   You will put DC through the transformer and it will overheat and fry.

You need the large caps because the transformer current is effectively pulsing on and off at the mains frequency.  During the off phase the transformer cannot supply any power, something has to hold the energy and that's why the caps are there.   The ripple is basically the drop in capacitor voltage that occurs while you are drawing energy from it in the off phase.  Larger caps produce less ripple simply because they can hold more charge.

One way to get low ripple with smaller caps is to follow the filter with a regulator.  The DC rail on the filter caps has ripple with peaks and troughs.  The troughs must be a high enough voltage so the regulator can still regulate.  If you want a large ripple you have to have a high voltage trough voltage and hence a high peak voltage.  If your large caps produced say 1V ripple then using 1/5th cap size would produce 5V ripple and you would need to up you input voltage by 5V.    With a large input ripple you are relying on the regulator's ripple rejection to drop it down.  It will usually be better than trying to get low noise with large caps.  The penalty is the regulator is less efficient and will dissipate power.  The best solution is often to use the largest caps you can then see how things pan out from there.

[darron]

some of my 'buttons' don't worc - so j can't type properly!! haha.. annoyjng.

thancs heaps George... exactly the answer j was loocjng for. j thought you mjght not be able to transform dc - but j wasn't sure. even wjth the regulator jn place some people stjll complajn.

yep yep... just what j needed! j had my thoughts that was the case

thancs!

[pjwhite]

George is right.

You can reduce noise and spikes from the line (mains) voltage with a common mode choke on the input side, however.

See this page for more info:

http://www.butlerwinding.com/inductors/common-mode2.html

But usually, a good filter capacitor and regulator after the rectifier is enough to get rid of any noise you'll typically encounter.

[darron]

George is right.

You can reduce noise and spikes from the line (mains) voltage with a common mode choke on the input side, however.

See this page for more info:

http://www.butlerwinding.com/inductors/common-mode2.html

But usually, a good filter capacitor and regulator after the rectifier is enough to get rid of any noise you'll typically encounter.

ahh!! 'll defntely have to add that to my favourtes... but t's a bt much to read RGHT now! lol

sorry some ceys not worcng stjll lol.

[Paul Perry (Frostwave)]

sorry some ceys not worcng stjll lol.

J mjght have a spare ceyboard here jn Carlton for you, Darron!

[George Giblet]

You can enter ASCII codes for keys:

http://www.robelle.com/smugbook/ascii.html

For example to get an 'i':

Hold down the alt key then type the decimal ascii code on the numeric keypad (i=105 decimal) then release the alt key, out pops an 'i'.  (maybe you don't want your i's to pop out)

[darron]

sorry some ceys not worcng stjll lol.

J mjght have a spare ceyboard here jn Carlton for you, Darron!

hey! you wouldn't seriously have a spare macbook pro one would you? i might just get one on ebay or something but don't feel like coughing up the money right now. it's working RIGHT now, though it will stop soon. it's a pain to plug into an external board plus i left it at the factory



George, good call on the ASCII tip. i used to use that to get to the accented letters. not sure that it works on mac, plus i'd have to turn numlock on to use the keypad probably. plus my 8 key is one of the ones that doesn't always work, though you say i don't need it to make an 'i'..

haha.. i had a good crack up at the 'i's popping out joke. that was so bad that it was good.