12 VAC power supply for audio amp

[AndrewT]

post5

you need either a centre tapped transformer 230:12-0-12vac of about 5VA or higher.
Or you can use a dual secondary 230:0-12, 0-12Vac again >=5VA - And convert this to centre tapped.

 

[DIM44]

I got this one:
Do I connect the two 12VAC outputs to the outer connections on the board and combine the 0 centre outputs to the connection incorrectly marked GRND on the board?

Specifications:
Primary Voltage Rating 230V ac
Secondary Voltage Rating12V ac
Power Rating 6VA
Number of Outputs 2
Minimum Operating Frequency50Hz
Maximum Operating Frequency60Hz

The two outputs are 3VA respectively. Is that sufficient?

Sorry for all the newbie questions.

Nico

 

[AndrewT]

First !
use a continuity meter to identify the separate windings of the transformer.
Does it have a single 230Vac primary? or dual 115Vac primaries?
Does it have 4 different colours or four taps for the two secondary windings?
Find which colour/taps connect to which other.

Power rating.
I gave the method to calculate the maximum DC current rating after passing through a capacitor input filter, but I'll go through it again.

max continuous Iac = VA / Vac = 3VA / 12Vac = 250mAac
De-rate to find the maximum continuous DC current after the filter by dividing by ~2 this gives the max current as 250/2 = 125mAdc The manufacturer should have details for that De-rating Factor, usually about 1.8, but it varies (2 is easy to remember).

Now reduce your loading to give cooler operation, aim for <50%. (this step removes the need for precision in applying the De-rating Factor).
i.e. your cool running continuous DC current is ~63mAdc

Notice that the cool running DC current ends up at one quarter of the rated AC current ! Again an easy number to remember.

 

[DIM44]

Thanks Andrew.
I hope to receive the transformer by Wednesday. Then I will take a picture or at least have the visible descriptions of the outputs.

For starters it has a single 230VAC primary and two 12VAC (four taps) secondaries each with 3VA power rating. Each secondary has two terminals, 0 and 12VAC.

Nico

 

[AndrewT]

Thanks Andrew.
I hope to receive the transformer by Wednesday. Then I will take a picture or at least have the visible descriptions of the outputs.

For starters it has a single 230VAC primary and two 12VAC (four taps) secondaries each with 3VA power rating. Each secondary has two terminals, 0 and 12VAC.

Nico

Find which taps are connected together.

Then power ON via the Mains Bulb tester.
Measure the voltage across each secondary winding.
Power OFF.
short one tap on sec1 to one tap on sec2.
Power ON.
Measure the voltages across each winding and measure the voltage across the two open taps.
You should get about 16Vac for each winding and for the last measurement you will either get near zero volts AC, or ~ 32Vac

report back after safely insulating all the primary connections.

 

[DIM44]

I received the transformer today but the system complains about the picture that is too large to post.

It has two input and four output terminals. 0-12 and 0-12VAC
I will try your suggestion and take the measurements on the output terminals. If you see a huge spark you will know I made a boo-boo....

 

[DF96]

No. A huge spark means you didn't make and use a lamp limiter.

 

[DIM44]

I was just kidding about the spark. I have the mains bulb tester which I made on your recommendation and propose to use it always.

When I get to shorting one tap on one side of the secondary to another on the other side ("... short one tap on sec1 to one tap on sec2") (post #25) I presume it should be the same, i.e. 0 to 0 in order to establish the output VAC?

You can probably tell I am a complete newbie - thanks for being patient.
Nico

 

[AndrewT]

I was just kidding about the spark. I have the mains bulb tester which I made on your recommendation and propose to use it always.

When I get to shorting one tap on one side of the secondary to another on the other side ("... short one tap on sec1 to one tap on sec2") (post #25) I presume it should be the same, i.e. 0 to 0 in order to establish the output VAC?

You can probably tell I am a complete newbie - thanks for being patient.
Nico

with 4 output taps, (label them a, b, c, d) there are two different ways to connect them in series (or two ways to connect them in parallel).
and two more ways to short a single turn.
winding 1 is a & b.
winding 2 is c & d.
Connect a to b and the bulb will light up.
Connect c to d and the bulb will light up.
Connect b to c and the bulb will not light up (yet). measure the voltage a to b and c to d and a to d. This last can be either near zero Vac or roughly double the a to b.
Connect b to d and the bulb will not light up (yet). Measure the voltage a to b and c to d and a to c. This last can be either near zero Vac, or roughly double the a to b.

You want the bulb to not light up AND for the measurement across the two windings to be DOUBLE the single winding voltage.

BTW,
a similar complication applies to wiring up a dual primary. It either shorts the mains and blows the fuse/MCB or you do it more safely using a Mains Bulb Tester. The BIG danger is that experimenting on the Primary side is VERY dangerous even through the MBT. Do not get electrocuted while trying to identify the correct wiring on the Primary side.

 

[DIM44]

Got it thanks.

I propose to permanently connect the mains input (two terminals, neutral and live) and insulate them properly with heat shrink or similar, and add the ground (earth) to the metal shroud of the transformer in case of a leak. In taht way I will kind of minimize the risk on the primary side. However, I do have some experience with electrical mains connections and already know where the "sharp" end of the danger lies. Having said that it is easy to make mistakes - like the professional electrician who connected the power to my newly built workshop some years ago. He reversed the proper connections of the neutral and ground terminals producing an exciting flash and bang when he tried to snip the wires with the mains switch off...