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Old 6th March 2010, 05:02 PM   #1
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Default Noob transformer question

Hello, I am planning on building a small amp using a TDA2009, I have all the parts but I am not sure how to wire the transformer, it was salvaged from a set of powered speakers and is a 20v non center tapped transformer, so it would be 28vdc after rectification.

my question is does it matter which side of the mains lines is wired to the neutral side of the plug at the wall? both wires are black.

how do I tell which side on the secondaries is neutral/ground? both wires are blue and are not marked.


I have tried looking for answers on line but was not able to find information that made it clear enough for me to attempt, better safe then sorry so I thought I would ask the experts.


Thanks for any help.
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Old 6th March 2010, 05:37 PM   #2
AndrewT is offline AndrewT  Scotland
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look up mains light bulb tester.
Build one.
Use it.
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regards Andrew T.
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Old 6th March 2010, 06:27 PM   #3
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Quote:
Originally Posted by AndrewT View Post
look up mains light bulb tester.
Build one.
Use it.
HI, does that help me identify which wire is which? After the light bulb you still have a wall socket right? so how do I know which is which?

Sorry if this is a dumb question, but the amp circuit I want to build needs the + voltage and a ground, But I don't know how to tell which of the secondaries is ground, the other small transformers I have are marked as to which is neutral.
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Old 6th March 2010, 08:16 PM   #4
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You can connect the primary either way. You don't connect the secondary directly to the ground. You connect it to the bridge rectifier, doesn't matter which way. Then the negative terminal of the rectifier goes to ground.
See this picture.
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Old 6th March 2010, 08:20 PM   #5
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Quote:
Originally Posted by Corben View Post
You can connect the primary either way. You don't connect the secondary directly to the ground. You connect it to the bridge rectifier, doesn't matter which way. Then the negative terminal of the rectifier goes to ground.
See this picture.
thank you very much, I thought it might be the rectifier that gave me the +/- but was not sure and didn't want any accidents.

I'm new to actually building electronics, so better safe then sorry.

Thanks again.
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Old 7th March 2010, 05:11 AM   #6
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well I got the power supply assembled, 27vdc.

I was not aware the smoothing cap affected the voltage, I thought I did something wrong with the bridge rectifier, I was only getting 16vdc from the 20v secondaries after the bridge, but I did some reading and now all is clear.

Thanks again.
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Old 14th March 2010, 10:50 AM   #7
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Quote:
Originally Posted by AltecDarpa View Post
well I got the power supply assembled, 27vdc.

I was not aware the smoothing cap affected the voltage
It does not. You don't have a true RMS meter and the high ripple was messing with your measurement. But of course, a DC supply can only be considered DC after the output capacitor. It's more of a series of pulses before it (or an absolute sine when you're on sinewave AC like is the case here).
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