Question Convert AC device that uses transformer with 3 wire output to 12v DC

[DIYDanger]

Hi….i have a project to convert my old surround sound system to work in my car.

It runs on 240v AC. There is a transformer that converts this to 12v AC……on the coil there is a label that shows;
“SEC:11.5v — 0v — 11.5v - 1.7A” and directly below that (and in line) “YEL — BLK — YEL”. Obviously there are 2xYellow and 1xBlack wires going to the circuit board.

I’ve found the (rectifiers ?) KA7812 and KA7912 for the +ve and -ve 12v.

So how best to wire in 12v DC from a car battery instead of the transformer coil?

From other posts I ‘think’ I understand but these are my options as I see them;
(1) use something like a A1212S-2W.. that can create the -ve, ground and +ve 12v DC from a 12v DC input. The amps are low though and I suspect 250mA max load won’t be enough.
(2) Connecting one yellow and the black to the 12V battery brings up the ‘Power On’ LED but there is no sound from the amp/speakers. Should I swap the black for the other yellow when connecting to the battery?
(3) get a step up DC transformer 12V—>24V (2 input and 2 output wires)? Here I think the wiring gets a little scary! Am I right in connecting as follows….Battery +/-ve connect to the step up input. Battery +ve connects to circuit black. Step up -ve/Grd connects to the Yellow -12v circuit (as identified by use of IC KA7912) and the step up +ve (24v) connects to the Yellow +12v circuit (as identified by use of IC KA7812).

I think option 3 is likely the right way to go, gives me greater amps on the power in but I’m nervous about what connects to what to make it power up fully without the need for fire extinguishers, burns unit appointment etc

I’m sure it’s simple to do but being an amateur tinkerer, I’m defo in the “a little knowledge makes you dangerous” camp!

Any help gratefully received. Thanks!

 

[bill001g]

I am not so sure I would be connecting a battery directly to any so called 12 volt device. It all depends on how sensitive the circuit is. A car battery many times is in excess of 13 volts and it is being charged by a the alternator which is not very clean power.

There are massive amounts of dc/dc power converters that can produce different voltages and amperage. I don't think any use transformers to accomplish this any more. Many are very tiny boards. Mounting them to some kind of heat sink is likely recommended but you would have to read the specs.

 

[TerryLaze]

Hi….i have a project to convert my old surround sound system to work in my car.

It runs on 240v AC. There is a transformer that converts this to 12v AC……on the coil there is a label that shows;

If the sound system really uses 12V AC internally then you have to give it AC and not DC, it expects the positive and the negative to alternate which is why it has two 12V wires and one ground, it will alternate between -12 and +12.

If it has some full bridge rectifier, 4 diodes all right next to each other, then it turns that into Direct Current at that point, probably 24V which is the difference between -12 and +12.

I’ve found the (rectifiers ?) KA7812 and KA7912 for the +ve and -ve 12v.

You would need an oscilloscope to see what these do, they could keep the voltage AC as they are just voltage regulators making sure that voltages don't go out of range.