I'm trying to get a set of PC speakers to work off a cigarette lighter socket... I've got speakers that run off a 12 volt 1 amp dc adapter. So I figured it would be really damn easy. Just make up the right plug and plug it on in. But no... Blew a 15 amp fuse pretty quickly. So I tried plugging it into my jump started battery thingy. When I plugged it into the cigarette lighter hole the jump starter made ding ding noises and flashed lights at me. So I got brave and put the jump starter leads straight on the cigarette plug. Music played for about 10 seconds then smoke and sparks from the plug. I'd actually melted the plug.

So I figure the amplifier in the speakers are drawing as much current as they can get their greedy little hands on. So I'm trying to lower the amperage down to 1 amp as per the ac/dc adapter. The man in Jcar has given me a 10W2.2OhmJ ceramic resister which he says will reduce current to 1 amp.
I have looked it up on the interweb and found the good old formula I = V/R. Using this formula I reckon I need a 12 Ohm resister...
Any thoughts. I really don't want to blow the speakers up if I can avoid it...

Firstly why are you trying to get PC spreakers to work in your car?

Secondly, what are you using as a source of audio?

No offence but it reads to me like you've put a knife in a toaster and are wondering why the power went out.

Ifr you're speakers are designed for 240 volt get rid of them and get some proper car stuff and a head unit.

I'm trying to get PC speakers to work in my car cause There is no room in my car for a head unit and so I can use it for pc speakers. (This is my camper van not my mini but I figured I could still ask the question .)

My phone is the source as it has headphone out which is just an aux out.

Sometimes you need to put a knife in a toaster just to find out what happens. It may be that all your purpose in life is to serve as a warning to others...

Okay I get what you're upto now.

How good is the power adapter and the wiring to the 12v socket?
What is the power rating on the speakers?
(probably 10-20 watt I'm guessing).

I can give you the in depth technical answers to your problem, but try this simpler one first:
Resistor won't do what you want it to do. Forget it.
Fuses blew because of excess current - you have a short.
Plug melted most likely because this is where you problem is / was.
An adapter lead is easy to make up, but you will require soldering skills to do it properly. Take care with the polarity, there should be a little diagram on the speaker next to the D.C socket showing this.
All it takes is a couple of wire strands or a poorly / incorrectly assembled plug and it will do what you described.
Hope this helps

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How can it be rusty when it's covered in so much oil?

I think you've got a plain old fashioned dead short. Check your wiring in your plug (with a multimeter set to ohms) and also confirm your polarity. The point is supposed to be +ve, the outside is meant to be ground.
If it was a head unit you'd be forking over for a new one by now

But also, is it 12vDC or 12vAC supply? I know you reckon its 12vDC, but is this off the back of the power supply or an assumption?

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

I prefer the zap that my microwave gave me once while "fixing" it once over the toaster.
Made me all tingly inside.

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Chuck Norris is the only person alive who can kick you in the back of the face!!
mooo, I'm a pig!

Back of the adapter says 12 volt 1 amp. It is an AC/DC adapter so I guess its 12 volts. Centre of the plug is positive according to the diagram.



What is the technical answer? Why wont an inline resister do what I want? Are you suggesting the plug is shorted? It didn't seem to mind when I plug it in with nothing plugged into the other end. The plug is stuffed now but wasn't before I don't think.
Is it possible that these speakers will just draw as much current as they can cause they expect the adapter to limit the current? I have another plug with an inline fuse and another plug and I'll solder it up but I expect similar results. At least this time the 1amp fuse will blow first I suppose...

Its just a simple problem, PC speakers of that size will never draw 15 amps unless the supply side is shorted to ground.

The idea of a series resistor as stated above is very bad idea. Explaining it could take a long while, but consider the formula V=I/R and W=V*I. Everything relates to something else, so as the current draw went up and down with the noise of the system, the speaker input voltage would go up and down inverse to the current, and the amount of heat generated off a resistor big enough to handle a maximum of 12 volts and 1 amp would melt your upholstery.

But forget all that, you've got a short circuit. There's nothing more technical than that. Its in your plug, your cable, or in your PC speakers. A multimeter set to ohms will find it perhaps it is in your speakers now, but a wire sticking through a piece of heatshrink or electrical tape is enough to do the same.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Jesus Christ....what a recipe.

say what Mick.....what you eat for Breakfast, Encyclopedia Wagnell...??????

pfft...that's first year apprentice stuff davo, from way back when...

I reckon you could teach me a few rules as well...

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

I have never opened any computer speaker but i'd imagine they are an 8 ohm speaker.
If the power supply states 1 amp then the current draw should not exceed it at full volume.
I once ran a 6v dc (battery) baby rocker on a 6v ac transformer, the music stopped working but the rocker kept working.

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Dean Hutton's first car was a Mini.

Thanks for all your help guys... Turns out it was a short in the plug.(as was suggested by a few. Well done) I bought a new set of plugs for either end and some wire. Wired it all up and plugged it into my jump starter. No bing bing from the jump starter and music flowed from the speakers. No flashes or smoke. Just music.
Thanks Mick and minimanic. I'll keep the resistor for a rainy day. Not exactly sure what you do with a resistor on a rainy day but I'll find something to resist...

Now I get to have music for my holidays. Yay...

Yup, Mick's got it right.
Part of the Techinical answer is: the 'speaker' will have a impedance, which is unknown, so your circuit resistance is:
Current resistor (R) plus impeadance (Z) or formula using Ohms Law,
R=(V/I)-Z. The current draw through the speaker is constantly changing therefore the voltage and current across the series resistor would be changing also. The resistor disipates the energy across it in heat, so the series resitor in this case would be a variable electric heater, changing heat with the music! There are more problems it creates, I could go on.....
To limit the current effectivly, you would have to use an active current limit circuit... lets not even go there, you don't need it.

Just find and fix the short!

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How can it be rusty when it's covered in so much oil?

Sounds like you fixed it while I was typing!
Happy listening!

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How can it be rusty when it's covered in so much oil?