# Capacitor-selection

Discussion in 'Electronic Basics' started by Rikard Bosnjakovic, Mar 31, 2007.

1. ### Rikard BosnjakovicGuest

For my next project I've just built a PSU using a small 9V-transformer.
Hooking it up to a Graetz-bridge for rectification, I measure 9.5 volts
over Vcc/GND (i.e. over "+" and "-" on the bridge) using my DMM.

Now I'm going to add filter caps. What I'm unsure of here is if
10v-ratings will work, or if 0.5 volts is a way too small safety margin. I
have big (~4000uF) 10v-caps in my junkbox, and since the junkbox serve a
purpose (using older but working components) I feel that I want to pick

So, will 10v-ratings work or will it be a too narrow margin?

2. ### ChrisGuest

more than 10V. The DMM is reading the average DC, not peak. Once you
add a cap you'll see close to the peak voltage, which will go up
substantially.

It looks like you'll have to spring for the higher voltage caps.
Sorry.

Good luck
Chris

3. ### John PopelishGuest

A 9 volt AC sine wave swings from +1.414*9 to -1.414*9 or
+-12.7 volts. A 9 volt transformer under no load may
produce something like 110% of 9 volts (the rated voltage is
produced at full rated current load), so those peaks might
by more than a volt higher than that, approaching 14 volts.
And sometimes the line voltage will be a bit higher than
normal. The rectifier will waste somewhere between a volt
and 2 volts of the transformer output, and the capacitor
will tend to charge up to the peak of the rectified output,
under light load. I would go with at least a 15 or 16 volt
capacitor (little cost or size penalty for using a 25 volt
unit), at about 4000 uF per ampere of expected output
current. Expect to get a DC output current of only about 50
to 60% of the AC current rating of the transformer secondary.