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Old 14th September 2014, 06:49 PM   #1
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Default Help me solve my buzz, please!

Hi, I have a Raspberry Pi with Open Elec installed, which seems to make me a happy media player. So I after being happy with my install, decide to build me a linear supply! Acquired a transformer v-0v-v and with a couple of 5 volt regulators I have built a supply using 1 rail plus centre tap of the transformer and created a supply. I did originally parallel the 2 yellow wires on the transformer to double the current, black being 0 volts, but the transformer started cooking, oops! So I just used one wire and the 0v tap and I got 5 volts for the Pi! Well getting carried away, I built another 5 volt supply for my USB DAC! Great I thought, eager I wired it up, BUZZZZZZZ! It only goes away, if, I unplug the feed to the Pi! I used star earth, and just realised I don't have any caps across the 5 volt rails after the Regs, only before! Would having no caps after cause the buzz, or is it because I'm sharing the Centre Tap!
Thanks, Mike
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Old 14th September 2014, 07:50 PM   #2
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My psu, added caps after regulator, still buzzing!!
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Last edited by Deafuser; 14th September 2014 at 08:14 PM. Reason: `update
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Old 14th September 2014, 08:07 PM   #3
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You need to have a common ground for both outputs.
You don't and thats the problem.

Normally the center tap is the ground and this is how I would do it.
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Old 14th September 2014, 08:16 PM   #4
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Hi payloadde, thanks for replying. Why no Bridge rectifier?
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Old 14th September 2014, 08:25 PM   #5
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Default Point a

Is the output of the lower diode connected to the upper diode? If so, why 2 reg's? Thanks
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Old 14th September 2014, 08:32 PM   #6
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With CT xfmr 2 diodes make a perfect full-wave rectifier, no need for a bridge.
The two yellow ends of a CT xfrmr are of opposite phase with respect to the CT, so either the upper or the lower diode conducts.
Center tap - Wikipedia, the free encyclopedia
If you don't have 2 separate diodes at hand, it can be done with a bridge, but only one half of it will be active.
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Old 14th September 2014, 08:44 PM   #7
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Quote:
Originally Posted by Deafuser View Post
Is the output of the lower diode connected to the upper diode? If so, why 2 reg's? Thanks
I thought you needed 2 regs because of power consumption and because you wanted to separate the two outputs so that noise / power spikes from one device would not affect the other ... if that's not your concern, just use one, but if you already have them, why not use them.
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Old 14th September 2014, 08:50 PM   #8
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Quote:
Originally Posted by payloadde View Post
The two yellow ends of a CT xfrmr are of opposite phase with respect to the CT
and that's also the reason why your xfmr cooks when you connect the 2 yellows, thats equivalent to a short.
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Old 14th September 2014, 09:39 PM   #9
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Default Thanks payloadde

Many thanks for helping, will rebuild and post back with results, soon! Thanks again.

Mike
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