# 12V DC to 12V AC?

I have a speaker with an embeded amplifier that uses 12VAC ~ 2.9A and want to connect to a 12VDC in my car, a simple bridge rectifier will do the trick converting the corrent?

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5 months ago

It might be the case, for this particular thing, your speaker with amplifier, that you can just power it with 12 volts DC, through the same terminals where the 12 volts AC would normally go.

The reason I am thinking this might work, is because I am imagining the amplifier actually does run on DC, and it has some kind of simple rectifier circuit, probably a full wave bridge,

https://en.wikipedia.org/wiki/Rectifier#Full-wave_...

for to change 12 VAC, into DC that it uses internally. There might be a three-terminal linear-type voltage regulator in there too, like the 7812, or similar.

https://en.wikipedia.org/wiki/78xx

Anyway, this is just a hypothesis, but, you know, the way you can prove or disprove this hypothesis is by taking the cover off your amplifier, and discovering what is inside. This involves, looking at what parts are inside, which wires go where, reading the funny the hieroglyphics printed on the parts, making a hand-drawn circuit diagram of what you find.

I mean if the wires from the power jack connect to four diodes and an electrolytic capacitor, just like in the picture of the full wave rectifier I linked to at Wikipedia above, well that is kind of a big clue.

Salazar2 (author)  Jack A Lopez5 months ago

i was looking at the board and you might be right, the power port leads to a 4 diodes and capacitors and then to a fuse..

i was going to mesure the current on the board to see if when power on enters by the port is 12VAC and then turns out 12VDC, should i do it? or can leed to any harm?

5 months ago

I am guessing that when you say, "measure current on the board", you mean, measure voltage on the board. Moreover I am guessing that the tool you are using a voltmeter, or more likely a multimeter set to measure DC voltage.

Regarding your question of, "...can this lead to any harm?", well, the voltages present are not that high, so there is not too much danger of shocking yourself.

Although, in any case, the kind of tricky thing about measuring voltages on a circuit board while it is energized... I mean the tricky thing about touching the voltmeter probes to random traces on the circuit board, is that this requires steady hands.

Also it gets more tricky if when the traces on the board are very close together. Especially when the spacing gets to be about the same size as the width of one of the voltmeters probes, typically around 1 to 2 mm. In other words, there is maybe danger of the voltmeter probe touching two traces at the same time, and shorting them together, if you are trying to touch the probe to the board in a place where the traces are that close together.

However, it turns out there is a smarter way to figure out what is happening on your circuit board, and that way is to look for ICs (integrated circuits), also called "chips", that you can identify.

The ICs are these little, or big, black plastic boxes with numbers printed on them. The number tells you the name of the IC, and then you can look up the data sheet for that IC, e.g. by using a site like:

http://www.alldatasheet.com/

and the data sheet will tell you the pinouts; i.e. which pin does what.

In particular, some of these pins will be pins to supply power to the IC, and you can use that info to figure out which traces on the board are the power supply rails; i.e. ground (defined to be 0 volts), a positive rail ( like +5 volts, with respect to ground), or a negative rail (like -5 volts, with respect to ground)...

Wait. I just thought of something. The rectifier stage on your board might be making a single sided supply (e.g. ground=0 and +10 volts) or it might be making a double sided supply (e.g. ground=0 and +10 volts and -10 volts).

Previously, when I answered this question, I was kind of naively thinking your amplifier's rectifier stage was making a single sided supply, and it might be. Or it might be double sided, since that is kind of common for audio amplifiers.

Would it be possible for you to take a picture of this circuit board? And also attach that picture to a new question, saying that you want to know what the rectifier stage (AC to DC) on this board is doing, is it making a single sided supply?, or a double sided supply? And the goal in mind is that you want to power this board from a 12 volt battery (which is a single sided supply) , if that is possible?

Also if there are ICs (aka chips) on this board, with numbers printed on them, it would be helpful, for the people looking at this picture, if we could tell what IC is what.

If the picture is super sharp, then people looking at it will be able to just directly read the numbers printed on the ICs.

But often the numbers printed on ICs are super tiny, so if this writing is fuzzy, you might need to tell us, in text, what the ICs are, assuming you can read those numbers with your eyes.; e.g. you see some voltage regulator ICs, a 78xx and a 79xx, and an audio amplifier IC XYZ1234, or whatever it is.

Salazar2 (author)  Jack A Lopez5 months ago

after testing with the multimetre, the rectifier circuit transformes the 12VAC to 17VDC, can a normal car battery whith 12VDC and taking into account that when the engine is running and the alternator is charging the voltage will rise to around 14.2 volts, run the amplifier without problems?

5 months ago

Yeah, maybe. Usually electronic things are not hurt by running with a little less voltage. So it is probably safe to try it, and see what it does. Or rather, to listen, and hear what it does.

You know, for an audio amplifier, the proof that it is working, or performing in a certain way, will come through listening to it.

Salazar2 (author)  Jack A Lopez5 months ago

Could in the long run damage the amplifier since the combination of the battery + amplifier would go around the values between 12 and 14v.

Another question that has arisen is, can I normally connect the plug where the 12VAC normally enteres?

5 months ago

Look, I still do not have a complete mental picture of what your amplifier looks like, and as consequence of this all the answers I give you are vague, like, "yeah... sure... I dunno. I guess so..."

So I am not going to try to make prognostications about, "long run damage"

Regarding the question of connecting a DC voltage to the inputs of a bridge rectifier, (i.e. "connect the plug where the 12VAC normally enteres") I can almost picture that.

Two of the diodes in the bridge will be forward biased, conducting, all the time, and the other two diodes will be reverse biased, blocking, all the time.

Contrast this to what the diodes would be doing with AC voltage input. In that case, the diodes, any or all of them, are conducting half of the time, and blocking half the time.

There is a small voltage drop across a forward biased diode, about 0.6 volts. Because the bridge has two diodes, forward biased, at almost any given time, the difference between the input voltage to the bridge, and the output voltage, is about two diode drops, or 2*0.6 V = 1.2 V.

As an example, if the input voltage is 13.5 V, the output voltage will be

13.5 - 1.2 = 12.3 V

Or if you wanted to rewire things a little bit, I guess you could solder some wires to the capacitor on the output side of the bridge, and input your DC voltage through those wires.

But it might be more convenient to not rewire things, and just input your DC voltage through the bridge rectifier, where the AC used to connect.

As I already explained, the downside to doing that is you lose a small amount of DC voltage, i.e. 1.2 V, and there are two diodes forward biased all the time, and wasting a small amount of power.

I am not really worried about the diodes being on all the time, even if they are dissipating approximately twice the (small amount of) power they were in the AC case. The reason I am not worried: diodes are tough. Hard to break.

I guess if you are worried about the diodes, the answer to that is rewire things, so the input current goes to that capacitor at the output of the bridge, and the diodes in the bridge never get forward biased, always reversed.

Or you could unsolder the diodes and remove them from the board completely. Maybe use them for something else?

5 months ago

As long as you don't short any thing out you should be able to measure the current by pulling the fuse and measuring the current there.

It might not have the same power on a 12 volt DC source.

12 volts AC converted to DC is around 17 volts DC.

The big thing I would look out for is hum from the ignition.

iceng5 months ago

A simple inverter just for you..

5 months ago

Click the pic to see the whole image !

deluges5 months ago

Buy a small inverter that produces AC from 12V DC at your country's standard mains voltage (110 or 220VAC) and step it down to 12 volts with a small transformer.

It might seem overkill but it's probably the cheapest, easiest solution.

Salazar2 (author)  deluges5 months ago

thank you for the answer, i realize that what i mean is 12VDC to 12VAC because the car baterie is 12VDC and the amplifier works with 12VAC