How to get 12V-3 amp from 18V-3 amp transformer ?

Thread Starter

venkee

Joined Nov 12, 2010
4
Hi,

I have a 18V-3 amp transformer and I want to get 12 V - 3 amp for an audio amp circuit from this. Is there a simple way to do it or is it better to get a new transformer ( in case the conversion is complicated) ?
[ I am a newbie in electronics and I have been doing only simple circuits, including connecting such circuits to a microcontroller board... ]
Thanks in advance.
Venkee.
 

Audioguru

Joined Dec 20, 2007
11,248
A stereo car amplifier using bridged amplifier ICs and 4 ohm speakers produces 14W per channel at clipping when the car battery is 13.6V. Each amplifier produces about 8W of heat so the total power from the battery is about 44W. Then the DC current is 44/13.6= 3.2A when the amplifier is playing as loud as it goes when just barely clipping.
When the power supply is only 12VDC then the power from the amplifiers is less, about 10W per channel at clipping.
 

t06afre

Joined May 11, 2009
5,934
What are you connecting to your power supply. Does the output need to be regulated? Provide more deatils and we may help better and faster
 

nomurphy

Joined Aug 8, 2005
567
Here's something coincidental: if the xfmr has a center-tap, then the 18VAC can be split into two 9VAC lines by grounding the center-tap. Using a full-wave bridge rectifier and appropriate sized filter caps, you will get an output of approximately +12VDC and -12VDC @ 3A total. Now you have a split supply with which to make a decent 10W/Ch stereo amp using 8 ohm speakers.
 

Audioguru

Joined Dec 20, 2007
11,248
Here's something coincidental: if the xfmr has a center-tap, then the 18VAC can be split into two 9VAC lines by grounding the center-tap. Using a full-wave bridge rectifier and appropriate sized filter caps, you will get an output of approximately +12VDC and -12VDC @ 3A total. Now you have a split supply with which to make a decent 10W/Ch stereo amp using 8 ohm speakers.
No.
10W into 8 ohms is 25.3V p-p. An LM1875 amplifier (and many others) will need plus and minus 15V supplies to produce 10W into 8 ohms at clipping.

Your 9V-0-9V transformer will produce a supply that is only plus and minus 11.7V then the LM1875 amplifier produces only 6W into 8 ohms at clipping.
 

Thread Starter

venkee

Joined Nov 12, 2010
4
WOW ! So many knowledgeable folks responding to a newbie question !!
Thanks a lot to all of you.
---------------------------------

As per the meagre info available with the (readymade amplifier ) board,
it requires 12V - 3 amp DC.

Looks like, as per @t06afre, if it is not a wise thing to do, I'll get a new trasnsformer ( 12V, 3amp).

Venkee.
 
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