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Old 18th November 2005, 10:35 AM   #1
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Default is this PSU correct?

Click the image to open in full size. look at the output of the transformer.

I dont have separate secondary wirings, my trafo is like this:

Click the image to open in full size.

can I use 2 bridges?

yesterday I blow 2 bridges, maybe I connected wrong something, but I'm pretty sure I didnt.

now I have 2 new briges.
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Old 18th November 2005, 11:01 AM   #2
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in such case u should use only 1 bridge and find schematics of a dual supply using single bridge. there are tonnes available on the net and ever here on the forum.
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Old 18th November 2005, 11:06 AM   #3
Nordic is offline Nordic  South Africa
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Make sure you connect the AC wires from the transformer to the pins marked ~ on the bridges... the + and - are the DC output pins.

I have used the circuit below, you may want to modify, adding the stuff before the transformer from your circuit... The diodes are 2 oversized jobbies, I think I used 2 8A ones, it can be low voltage units.

Click the image to open in full size.

The ground symbols marked with lines, is connected to the PSU case, and the mains earth wire also connects to the case

R1 use 5W resistor
Use plain ceramic for the 100nf cap as a short circuit to radio frequency signals, effectively grounding them.


Above circuit desigend by Rod Elliot.
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Old 18th November 2005, 11:10 AM   #4
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thanks for your answers.

i will be using this PSU for 1 bridged dual lm3886.

for now i will use only 1 bridge.
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Old 18th November 2005, 11:14 AM   #5
Nordic is offline Nordic  South Africa
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Zaf, just check quickly as I edited the post a bit, with more info.
The circuit I showed should work with your transformer without issues. Remember the fuse has to be a slow-blow or T-fuse
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Old 18th November 2005, 11:27 AM   #6
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thanks nordic.


I have another question....

my trafo is 18 0 18, and makes 26.5DC with the firsts caps & 2k2 ohms resistor (the ones who discharges the capacitors after psu shutdown).

i have 2 caps rated 17000uf 25v, they're smaller than the other ones that i have (20000uf 75v).

the biggers dont fit in my case, so I was wandering what will happens if I use the smallers (25v) with 26.5v.

what do you think? 1.5V is a risk?
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Old 18th November 2005, 11:42 AM   #7
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I wouldnt use the 25volt caps. It might work at first but it will fail. You want caps with working voltage of at least 1.4 times secondary voltage.
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Old 18th November 2005, 12:38 PM   #8
Nordic is offline Nordic  South Africa
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I aggree with Jaudio, you will either have to find a biger case, or look for lower profile capacitors.
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Old 18th November 2005, 03:29 PM   #9
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Quote:
Originally posted by jaudio
I wouldnt use the 25volt caps. It might work at first but it will fail. You want caps with working voltage of at least 1.4 times secondary voltage.

1.4 times !!!

It means one wouldn't use some 40V caps on 34.5V (no load) rails ?!

Caps lifetime is guaranted for the nominal voltage, and mainly depends of ripple current, according to datasheets.

Is there a reason for this 1.4 factor ?
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Old 18th November 2005, 04:01 PM   #10
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Quote:
Originally posted by youyoung21147





It means one wouldn't use some 40V caps on 34.5V (no load) rails ?!
I use 35volt caps on a 29volt(unloaded) rails without problem but I think it kinda close.

Quote:
Originally posted by youyoung21147

Is there a reason for this 1.4 factor ?
Nominal voltage plus peak.
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