Transformer setup

Arzhur

Member
2013-03-04 11:27 am
I am wiring a transformer for an amplifier and just want to make sure that I'm not making a mistake.

Here is a copy of the diagram on the transformer.
aiQ3ilP.png

It can be either setup in parallel or series.

On the right I have connected the red and orange to the earth of the amp. Should I also connect them to the mains earth? The black and yellow are connected to one of the AC IN of the amp (which has the smoothing caps integrated in it) each.

For the left side of the transformer the brown goes to live and blue to neutral. As for the gray and violet they are connected together but I'm not sure if they should be connected to ground or not.

I am in Ireland so have 240V mains. The tordial transformer is 90VA. Is a 5A slow blow fuse sufficient on the live wire?
 
You need to check what AC input the amp requires. Most likely is three wires (black, red+orange, yellow), but it is just possible that it requires four (black, red to one pair; orange, yellow to the other). Check the datasheet.

The amp datasheet should include grounding details. It is likely that there is some connection to mains earth, but this almost certainly will not require you to directly ground the red/orange wires. Get the grounding wrong and you get hum/buzz. Get the grounding seriously wrong and you get danger.

Build and use a lamp limiter. I always use this when messing with mains PSUs, and I only have nearly 50 years experience of electronics.
 

Arzhur

Member
2013-03-04 11:27 am
Than you all for the replies.

When I have finished the wiring I will test it with a bulb tester.

I am recycling the transformer form an old circuit (I didn't make that circuit) so I don't have the data sheet. All that I have is a sticker with the image that I included in my first post (which is a copy which I made using Power Point as a photo would not have been very clear). The GRY and VIO wires were connected together when I took the transformer out of the old circuit so I left them like that for now (and that way it will take an input of 240 VAC).

The amplifier is a TDA7294 which I bought on AliExpress. The bridge rectifier and smoothing caps are included so I just have to connect a transformer to it. The inputs are 2 AC INs and 1 GND.

Looking at this project the GND should be connected to the mains Earth. I believe that floating ground is illegal in Ireland so I have left it to be sure.
 
If you have no enclosure during the build/testing phase, then there is nowhere to connect that PE wire.

Once you put the amplifier into an enclosure, that becomes the time you connect that enclosure to the Mains PE wire.

But long before that you have a transformer. Test it alone to check your primary wiring.
Add on the rectifier and test again. Then measure the output voltage at the +ve and -ve of the rectifier. This confirms you have wired the secondaries correctly.
Then add the smoothing capacitors. Test again, measure the voltages. This checks you have wired the whole PSU correctly.

Only AFTER you have proved the PSU, do you start connecting one amplifier.
 
The case must be grounded, to the incoming mains ground conductor.

The circuit may be grounded (to the case) and in most cases should be grounded, but exactly where you do this will affect hum, noise etc. As I said, the secondary CT is about the worst possible place to do this. At the input or the output is usually better.
 
I have a small loose lead version that I can stick in a bag to take on site visits. I use this portable one as the heater for making joghurt.
My semi-permanent version is installed in the same box that holds the Variac. The box is an old speaker cabinet.
You need a mains plug top, a bulb holder, a socket outlet and some 3core double insulated cable/cord.
 

Arzhur

Member
2013-03-04 11:27 am
Thanks. I have a spare kettle lead so I was planning of just cutting it in two and connecting the tester in-between both parts. That way I could avoid getting the mains plug, cable (I have spare insulated copper wire for the internal connections) and IEC connector.
 
use the wires in the old iec lead.

Two go straight through, the PE wire and the Neutral wire.
The Live wire goes into the bulb holder and the other half of the Live lead comes out of the bulb holder.

If you can strip off about 100mm of the outer insulation without damaging the inner insulation you don't need to cut the N+E. Just cut the Live. Prepare the two ends to fit the bulb holder.