Dumb question about transformers

[ormo]

I've tried Google to find my answer, but nothing seems to answer this properly.

Assume I have a mains transformer that outputs 15-0-15V AC. The output connections are A, B and C with B being the centre tap. How would I need to connect A and C to get 30V AC output? Could this output then go through a full-wave rectifier and a big capacitor to give me a near-DC power supply?

The transformer in question is: **broken link removed**

Thanks,

 

[ericgibbs]

I've tried Google to find my answer, but nothing seems to answer this properly.

Assume I have a mains transformer that outputs 15-0-15V AC. The output connections are A, B and C with B being the centre tap. How would I need to connect A and C to get 30V AC output? Could this output then go through a full-wave rectifier and a big capacitor to give me a near-DC power supply?

The transformer in question is: **broken link removed**

Thanks,

Connect the A & C leads to the FWB, then add smoothing caps after the FWB

You will get approx [off load] 1.4 * 30 = 42Vdc.

BTW: whats the psu for.??

 

[ormo]

Thanks for the info!

The supply is going to be used for a 2 x LM3886 based audio amplifier. Just out of curiosity, where does the 1.4 come from to take 30v to 42v?

 

[ericgibbs]

Thanks for the info!

The supply is going to be used for a 2 x LM3886 based audio amplifier. Just out of curiosity, where does the 1.4 come from to take 30v to 42v?

The voltage of 15Vac is the RMS value [ root mean square].
As you know 'ac' is a sine wave, the RMS voltage is the equivalent voltage of 'dc' voltage.

The sine wave peak of the 'ac' is about 1.4 time greater than the RMS value thats quoted on the transformer. So the Vpeak is 30 *1.4 = 42Vpk, this is the voltage to which the capacitor will charge upto when offload.

So you have two 15Vac windings, so the final voltage [ignoring the 0.7V drop across the diodes] is about 42Vdc.
As you load the capacitor the Vdc will fall to about 32Vdc.

OK.?

EDIT: more info
**broken link removed**

 

[Hero999]

This little 15V transformer is only rated for 250mA, much too low for a 68W audio amplifier.

 

[ormo]

Yeah, the part I showed was just to serve an example, the LM3886 requires a dual supply anyway