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Old 2nd December 2010, 10:47 AM   #1
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Default How to connect a toroidal transformer

Hi,
I would like to build an AC to DC transformer for my amp.
The needs are 230V AC input and 12V DC 4A output.

I know that I need first an AC transformer, then a rectifier+smoothing+regulator parts.

My first question is: in order to have 12V DC do I need a 12V AC at the output of the AC to AC transformer?

The second question is related to a toroidal transformer like this one: Toroidal transformer (RS-Components)
It has 2 x 12V output, I think they are rated at 2.083A each, how I can have only one output rated at 4A? Do I simply connect the orange and black wires together and also the yellow and red ones?

Thanks in advance,
Ralf
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Old 2nd December 2010, 10:52 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
without a regulator you will need ~9Vac for 12Vdc.
With a regulator you will need between 12Vac and 18Vac depending on the regulator drop out voltage and the ripple at the input to the regulator at high current demand.

To check your wiring to the mains and of the secondary/ies, use a mains bulb tester for every power up until you have completed all modifications.

What is your expected maximum continuous DC current demand?
The AC current rating of the transformer MUST be at least double your DC continuous demand.
I prefer to aim for 4times to allow the transformer to run cool.
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Last edited by AndrewT; 2nd December 2010 at 10:54 AM.
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Old 2nd December 2010, 01:22 PM   #3
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Quote:
without a regulator you will need ~9Vac for 12Vdc.
Can you provide me something to read about that?

Quote:
What is your expected maximum continuous DC current demand?
I think that I need a peak of maximum 4A, sustained will be less.
I build an amp around a TDA7240 chip, and used a regulated switching power supply rated at 12V 1.3A.
Connecting a speaker to the amp, at low level is all fine, whereas at medium-high levels with solid bass content the sound changes to muddy. I suspect the chip simply needs more current it this case.
I can buy another switching power supply rated at 4-5A or even more, but was thinking about a toroidal transformer. It is a good idea?

Ralf
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Old 2nd December 2010, 01:37 PM   #4
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The "Power Supplies" section of DIY Audio Articles has always helped me with these kind of questions...
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Old 2nd December 2010, 01:44 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
the signal going to your speaker is the Power Supply that is modulated by the amplifier.
You are listening to the Power Supply.
1.3A regulator has no hope of powering room speakers. Ear speakers (headphones) come into that current category for PSU peak capability.

Forget SMPS. Forget regulated. Wait until you know how to design an integrated pair of regulator and amplifier that will be capable of driving your reactive load and stay stable.

Till then, use a transformer + rectifier + smoothing. Far simpler to get right.

The (contaminated) sinewave coming from your mains is transformed to a contaminated lower voltage sinewave that is isolated from the mains.

A 9Vac sinewave has a peak voltage of sqrt(2) * 9Vac = 12.7Vpk
Pass this through a bridge rectifier causing a voltage drop of ~1.4V leaves you with ~11.3Vdc at the smoothing capacitors.
A real transformer does not give exactly 9Vac all the time, in fact it rarely gives 9Vac. you will more likely measure 10 to 12Vac from a 9Vac transformer and an even wider range if the mains supply moves to the extreme end of the tolerance range specified by your electricity supplier.

9Vac will probably give you 12Vdc to 14Vdc that varies throughout the day/night and varies with the power you draw from the PSU.

To run at 12Vdc your circuit must remain undamaged when supplied with a considerably higher voltage. I would expect it to be safe with at least 15Vdc and maybe fully operational at this voltage as well.
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Old 2nd December 2010, 03:45 PM   #6
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The TDA7240 chip can work with far more than 12VDC so I think it will be safe with a 9VAC transformer.

But I need to know the answer for the parallel connection of the output section of the transformer: looking at the link I posted does the parallel connection gives 1 output at a double VA than the 2 single outputs? That transformer is rated for either parallel and series connection.

The last question is as follows:
here in Italy you don't know what cable are you wiring (blue or brown) because the plug can be inserted either way. I think (but I could be wrong!) that if you connect in the wrong way the 2 input wires of the transformer, also the output wires are inverted. Referring to the link I posted, if you invert the brown and blue cables, does the red one carry the 0V and the black the 12V output?
If that is correct, does that bring a problem for the rectifier part?

Thanks so far,
Ralf
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Old 2nd December 2010, 03:53 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by giralfino View Post
But I need to know the answer for the parallel connection of the output section of the transformer: looking at the link I posted does the parallel connection gives 1 output at a double VA than the 2 single outputs? That transformer is rated for either parallel and series connection.
use the mains bulb tester.
If you get the wiring of the primary and/or the secondary wrong the bulb will light and protect the transformer and you and the mains fuse from blowing.

Build that bulb tester and use it on every mains powered project.
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Old 2nd December 2010, 03:59 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by giralfino View Post
here in Italy you don't know what cable are you wiring (blue or brown) because the plug can be inserted either way. I think (but I could be wrong!) that if you connect in the wrong way the 2 input wires of the transformer, also the output wires are inverted. Referring to the link I posted, if you invert the brown and blue cables, does the red one carry the 0V and the black the 12V output?
If that is correct, does that bring a problem for the rectifier part?
the mains side is isolated from the secondary side.
The mains side is AC it does not have Positive nor Negative.
It has Flow = Hot = Live and Return = Cold = Neutral. Swapping these around can have a very small effect on the way the parasitic capacitances leak RF into the isolated side. This is not a safety issue so forget the RF & parasitics for now.

The reversible mains plug requires that your mains switch must break both Live and Neutral. Never only one. In the UK, where we cannot reverse the plug, we have to use 2pole switching, just in case some eejit wires up the socket or the plug top wrongly.
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Old 2nd December 2010, 04:01 PM   #9
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Hi giralfino.

I'm not sure how much the TDA7240 chip can handle. chose any transformer that is center tapped or have dual winding (most trannies today have dual windings) use 2 diodes and make the center tap grounded.

Not sure what you mean with the the primary, but no need to be thinking about phase or polarity here, just connected either wire to LIVE and NEUTRAL (might be good to use a fuse )

You could ground the secondary center tap (ground) to wall socket ground if you want to.

the output voltage would be secondary voltage (12V) x1,41=16,92 - diode drop (some 0.6-1.1v)
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Old 2nd December 2010, 04:38 PM   #10
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Folks,
thank you for all the useful link and information.

Quote:
Build that bulb tester and use it on every mains powered project.
Can you give me a link?

Ralf
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