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Old 12th September 2006, 09:51 PM   #1
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Default monica PSU

Problem from a newbie...

I wanted to build a simple power supply for my monica dac and house it all together in one box.

I bought a traffo with 12VAC output, 250mA, bridge rectifier and a 7812 fixed voltage regulator.

I hook it up and I get 12VAC from the traffo secondary. I connect up to the rectifier and I get about 10VDC from the rectifier, and then about 8VDC from the regulator. If I put a MM across the rectifer + and - outputs, I get the above mentioned 10VDC, but if I switch the MM to measure AC, I get about 4 to 5VAC steady on the MM. If I hook it all up to the DAC, the DAC will lock on, give me music but also a whopping great hum, which I think must be from the AC.

So I thought it must be the rectifier, I replaced this with a new one, but still the same. Same with the regulator.

I recently built a similar PSU for a phono stage (only with + and - VDC output) and it worked out fine. What am I doing wrong? Seems like the transformer is the only thing different. On the phono stage, the VDC after rectifier is up at 17VDC from a 12V secondary. Is the traffo too small?

Oh, I know the DAC is OK, because I tried it out with a different PSU and it works AOK.

thanks for any help,
Fran
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Old 12th September 2006, 10:01 PM   #2
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what size is the cap after the rectifier?
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Old 13th September 2006, 12:12 AM   #3
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God this is going to sound stupid, but I don't have a cap in there yet.....



Thats whats wrong, isn't it? The caps are on a separate board on the phono stage.

God I feel stupid now!

OK, so that would be a helping of humble pie, with cream please!

When you have stopped laughing, what size caps would you reccomend? I have some 3300uF to hand, also 470uf and 100uf - all polar electrolytics.


red with embarrasment (though I did say I was a beginner!),
Fran
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Old 13th September 2006, 01:09 AM   #4
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use whatever you want-start with 3300UF
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Old 14th September 2006, 07:58 PM   #5
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I know I mentioned I had 3300uF caps, but what would you guys think would be a good size for input and output caps here?



tia,
Fran
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Old 14th September 2006, 08:16 PM   #6
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Quote:
Originally posted by woodturner-fran
I know I mentioned I had 3300uF caps, but what would you guys think would be a good size for input and output caps here?



tia,
Fran

use 3300 before 78xx
then again 3300 after

put some protection diodes if you want

look for pdf data of 78xx , there you'll find mucho info


in your boots,I'll use LM317 instead
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