How to connect a toroidal transformer

[giralfino]

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

 

[AndrewT]

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.

 

[giralfino]

without a regulator you will need ~9Vac for 12Vdc.

Can you provide me something to read about that?

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

 

[Rodeodave]

The "Power Supplies" section of DIY Audio Articles has always helped me with these kind of questions...

 

[AndrewT]

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.

 

[giralfino]

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

 

[AndrewT]

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.

 

[AndrewT]

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.

 

[SonnyOneness]

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)

 

[giralfino]

Folks,
thank you for all the useful link and information.

Build that bulb tester and use it on every mains powered project.

Can you give me a link?

Ralf

 

[AndrewT]

decibel dungeon and many posts in this Forum.

 

[giralfino]

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.

I was thinking about that, but I don't understand. If you interrupt only one line between Live and Neutral, in an attached transformer the current should not flow. Or I'm missing something?

Thanks, Ralf

PS. I've found references about the bulb tester. Thanks.

 

[jitter]

I was thinking about that, but I don't understand. If you interrupt only one line between Live and Neutral, in an attached transformer the current should not flow. Or I'm missing something?

Indeed, any interruption - whether it be in L or N - stops current from flowing.
AndrewT is talking about two pole switches, but despite having reversible plugs here on the mainland, the use of them is not mandatory, at least not in devices such as hifi or TV. All of my commercial hifi has single pole switching.

I think when it comes to mains electricity, the British have a few more rules and regulations. In The Netherlands, sockets are permanently powered (i.e. there's no switch), and plugs don't have a fuse.

 

[AndrewT]

Hi,
if one uses a single pole ON/OFF switch, it can only break one Line.

If it breaks the Live Line than all the appliance downstream of the switch is connected to neutral, i.e. 0V

If the single pole switch breaks the Neutral Line (because the plug has been inverted) then the whole appliance from the live incomer to the Neutral switch pole is Live, i.e. 220Vac on the mains cabling, 220Vac on the mains fuse (both ends), 220Vac on the transformer primaries (both Live and Neutral), 220Vac on the switch (neutral pole only). That is just waiting for an accident to happen.

In the UK we have many belt and braces rules. The two pole switch is just one of them and we can't invert our BS1363 plugs.

 

[jitter]

If the single pole switch breaks the Neutral Line (because the plug has been inverted) then the whole appliance from the live incomer to the Neutral switch pole is Live

Not the whole appliance, only the primary winding of the transformer or the primary side of a SMPS. But since this is inside the appliance, it doesn't really matter. I don't know if there's even a rule that says how to wire L and N on a Schuko socket, so I think it's a fair assumption that half the appliances are plugged in one way and the other half the other way.

Even floor or table lamps come with single pole switches over here and in this case the potential danger is a lot bigger because the contacts in the E14 or E27 fitting are exposed when no bulb is installed. When installing a bulb, I always take care not to come close to the threaded part with my fingers, in 50% of the cases this part carries mains voltage, even when switched off.

 

[giralfino]

I think I'll go with a 9V AC transformer. If I'm not wrong, this transformer with a rectifier will have an output of (9 x 1.4) - 1.4 = 11.2V DC (or better, between 9V and 13.5V considering fluctuations in the main 220V). These values are OK for my amp.

Now I'm asking advice for the bridge rectifier and smoothing capacitors:
I have this rectifier: D5SBA20 - is this OK for my amp?
What capacity do I need for smoothing? I have 2x4700uF caps but I suspect they are not big enough.
And what voltage rating do I need? Is 16V enough or is it better 25V?

Thanks, Ralf

 

[AndrewT]

I think I'll go with a 9V AC transformer.
And what voltage rating do I need? Is 16V enough or is it better 25V?

16V caps should be OK, but you should really check (measure) your worst case highest voltage from the transformer before getting this close on cap voltage rating.
Safer to go with 25V caps and those will allow you to use a 15Vac transformer for your next project.