Capacitor-selection

- R
- Rikard Bosnjakovic
  
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posted
17 years ago

Sat, Mar 31, 2007 8:49 AM

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 those instead of new 16v-rated caps instead.

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

--
Rikard.

- C
- Chris
  
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posted
17 years ago

Sat, Mar 31, 2007 9:41 AM

Hi, Rikard. You know what? Your peak rectified voltage is already 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

- J
- John Popelish
  
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posted
17 years ago

Sat, Mar 31, 2007 4:41 PM

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.