Lets talk fisheseses (sp)

[jronald]

OK, safety department is handled...gfi, double insulation, water sensors.

A half a lifetime or so ago I built a little device that took 120 volts AC and swapped it into 120 volts DC. Best I recalled in the output stage I used a 220uf Cap for filtering. Nothing fancy, it did the job for what I needed it to do.

Fast forward to today, Im looking at a set up and its using a 100up cap as the filter in the output stage.
Without getting over technical, whats the difference? Im kind of assuming that the higher cap will result in a fractionally higher output voltage because of the smoother ripple, but other than that, is there additionally other benefits?

Thanks
Ron

[RealBlackStuff]

A general rule of thumb is for 1V of ripple the capacitor needs to be 10,000µF for every amp of loading, if you want to halve the ripple then double the capacitor size or halve the capacitor size if you don't mind double the ripple.

Here's the simple formula:

C = I/(2F*V).

C is the capacitance in Farads.
I is the current.
F is the mains frequency, normally 50Hz or 60Hz.
V is the maximum ripple.

There's a very good explanation here: http://www.electronics-tutorials.ws/diode/diode_6.html
Scroll down to: The Smoothing Capacitor

[jronald]

Im working on a nominal 120 volt ac motor, so Im not that worried about the ripple, on the dc side. As this is a junk box build (left over parts) My available caps run the scale from around 100uF to 450uF (200volt to 600 volt). As I have in the past vaporized a 12 inch piece of solder via a Cap (dont ask), would it really make any difference in the Cap I grab? Im not looking for any real precision, as long as Im in the ball park.

Ron

[RealBlackStuff]

I would suggest the cap with the highest (600) voltage
I have no electronics knowledge other than building several amplifiers when I was a kid and later on messing with car electronics. But all that is more than 40 years ago.
Nowadays I go by the seat of my pants.
Recently my 5 year old flatscreen television conked out, i.e. all of a sudden picture was disappearing at a very fast rate, like within 30 minutes, from full screen to no screen.
I opened up the back panel, and noticed some bulging capacitors.
Having nothing to lose, I unsoldered one, and found the matching one online.
Going through their (www.digikey.com) catalog, I noticed the caps had life expectancies between ~800 and ~10'000 hours. The longest lasting cost only $1.04 each, and I needed 2.
Put them in, switched it on, and voila, I had full screen picture again, at a cost of a handful of dollars and 10 minutes assembly work, rather than buying a new telly!
And my best friend here is a 20-year old tiny multimeter, that only needs a new battery every 4-5 years or so.
I can replace fuses in a laptop, and even did a brain transplant (read BIOS-chip).
And I can also follow examples from others (see the SATA-mod in my signature).
But that's experience and a steady hand, rather than electronics knowledge.

[jronald]

We are in the same boat. I have recapped at least a dozen desktop boards, but that was a like for like swap, for the most part. Im a total novice on what the change in caps as far as filters will do, can do.

Ron

[RealBlackStuff]

If you read the link I gave you, they answer that Q there.

[jronald]

Yep it does, but that is considerably over my head.....
The inference is: the higher the cap, the less ripple. The less ripple tends to increase voltage.
That said, Im not sure it will make a hill of beans difference in my application. I have 470uf's now, Im using them!

Ron

[poshgeordie]

Not done Capacitor stuff for decades, as a general rule use the highest voltage rated caps and connect them in parallell to give the greatest smoothing.