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.

 

[DIYDanger]

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.


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.

Thanks….yeah there are diodes (4 big looking ones) that appear to do the conversion to dc, having looked at how to wire up that sort of setup from components.

So if it was running +ve and -ve 12v before the diodes, you think I should be picking up 24v on my multimeter immediately after them. So in theory I could just tap in to the circuit there with 24v supply?

What alternatives are there? What could I use to replace the old 240vac transformer….taking 12v dc in and putting out +12v, Gnd, -12v? What is the ‘thing’ I’m looking for to do this?

 

[TerryLaze]

Thanks….yeah there are diodes (4 big looking ones) that appear to do the conversion to dc, having looked at how to wire up that sort of setup from components.

So if it was running +ve and -ve 12v before the diodes, you think I should be picking up 24v on my multimeter immediately after them. So in theory I could just tap in to the circuit there with 24v supply?

It could be possible, it could also still use other AC voltages in different parts coming from before that point.

What alternatives are there? What could I use to replace the old 240vac transformer….taking 12v dc in and putting out +12v, Gnd, -12v? What is the ‘thing’ I’m looking for to do this?

They are called inverters but I don't know if there are any that only output 12V usually they are for 110-220V

 

[kanewolf]

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!

First thing I will say is that your transformer puts out 24VAC (give or take) since is has a center tap and two 11.5VAC legs. Assuming this 24VAC goes into a full bridge rectifier (your "4 big diodes") then your DC voltage is closer to 30VDC.

 

[ex_bubblehead]

Hope you have the fire department on speed dial. This will not have a happy ending.

 

[DIYDanger]

First thing I will say is that your transformer puts out 24VAC (give or take) since is has a center tap and two 11.5VAC legs. Assuming this 24VAC goes into a full bridge rectifier (your "4 big diodes") then your DC voltage is closer to 30VDC.

So I need a 12vdc to 24vac inverter with centre tap?

Thanks again

@ex_bubblehead thanks for the faith! Cleverly I’ve asked before doing! They said I’ll wait until about 2am in the morning when fire brigade and hospitals are less busy…just in case! What can go wrong😬?

 

[stonecarver]

Just a suggestion and less conversions.

I used to think these mini amps were a joke we now have three of them plus I bought a tube pre amp + amp mini as well.

There 12v and or with a power brick can be used at home.

Just one thing don't even believe the watts they say it puts out total BS but can and will do a great job for what it can do. Just a thought if it makes your outcome easier.

All mine were bought off amazon but I now see them off Temu.com at least for now a great price.


https://www.temu.com/amplifier-400w...mHKN6ewEflqw&_x_ns_targetid=pla-2306216047902

https://www.temu.com/80w-amplifier-...mHKN6ewEflqw&_x_ns_targetid=pla-2306216047902

 

[Paperdoc]

The 4-diode bridge circuit to convert AC to DC is so very widely used now. It's simple because it uses 4 diodes to produce full-wave rectified DC with a LOT of ripple that can be run through a filter to remove the ripple for pretty smooth DC. A typical filter would use two large capacitors apparently in parallel between the + and - lines, but with an induction coil in series between the two capacitors' + ends. This is a simple Pi filter. The DC voltage of its output is approximately the Voltage of the AC input, so a transformer is required only if the output Voltage must be different from the 120 VAC commonly used as input. In OP's case this has been done so that the DC output is 12 VDC.

In the "old days" of high-impedance high-Voltage vacuum tube components a different full-wave rectifier circuit was used with ALWAYS a transformer with a centre tap secondary and only TWO vacuum-tube diodes. Simpler and less costly than four diodes. EACH diode tube is connected between one transformer secondary END and the Centre tap. The common line from the junction of diode tubes to the Centre tap is then the output of the rectifier circuit. That is, the DC supply to the filter and thence to the load was in series between the diode tube output junction and the transformer secondary winding Centre tap. Of course, one could do exactly the same circuit design today using TWO solid-state diodes rather than vacuum tubes.

I believe this is the design OP has encountered in his amp's power supply circuit. I AM surprised a bit by the statement that it involves FOUR large diodes, so that needs some checking.


To understand OP's circuit, look closely at the two diodes connected to the transformer secondary coil ends, and at where the secondary's Centre Tap is connected. I expect the JUNCTION of those diodes that are facing in opposite directions is the Rectifier output with high ripple, and that goes to the filter circuit for ripple removal. The filtered output then is the actual smoothed power supply at +12 VDC. The Centre Tap connection will be the (-) Ground. The actual amplifier circuit does NOT use both +12 VDC and -12VDC separate power supply lines. In fact those two outputs from the ends of the transformer secondary are just fed into the two-diode rectifier, and the final result after filtering is ONE DC supply at 12 VDC, with + and - output lines, very likely with negative connected to Ground.

If that is what OP has, then the + and - 12 VDC lines from a car battery should simply bypass all that power supply circuitry and be attached to where the final filter circuit output is fed to the actual amplifier load. Obviously a FUSE should be in that power feed to limit max current through the amp to prevent its being damaged by overload. HOWEVER, one important item MUST be verified. I assume that the NEGATIVE side of the existing amp power supply circuit is connected to its circuit GROUND, and thus possibly to its chassis. IF that is the case, then connecting that amp's chassis to the car's NEGATIVE battery post (that is, the car's chassis) is OK. BUT if the amp is wired with its chassis as the POSITIVE side of its power supply, connecting that to the car gets more tricky!

 

[DIYDanger]

Wow, great detail PaperDoc! Thanks….. that’s kind of what I was thinking I could do…trace through to find the standard 12v dc and connect in there.
However, as already stated and supported by others referring me to the fire brigade, I am coming to the belief this is beyond me without destroying the kitchen table and taking my marriage with it! 🤣

So thanks everyone. I’ve learnt somethings and think I’m better off buying the proper kit instead of recycling what I have, for fun.

 

[Paperdoc]

I applaud your decision to consider your knowledge and skills and NOT blunder ahead beyond what you KNOW you can do safely and without damaging the original device.