Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Power supply and creative inspire 2.1 2500 question

  1. Jan 4, 2007 #1
    urgent power supply and creative inspire 2.1 2500 question

    hello every one

    this is urgent....

    i am trying to fit computer speakers in a car
    the computer speakers need 12V@ 2.9 A DC
    the car battery is of 12V DC but of a lot more amperes

    the problem is i want to make a filtered power supply for the speakers that puts out exactly 12V DC@ 2.9A not more and not less

    i know from experience that if more current is applied speakers tend to pump a lot which leads to damaged speaker cones

    can you guys help me make this power supply
    the input is 12V from car battery
    and car batteries are mostly rated at something abt 50 AMPERES

    also does any body here own a creative inspire 2.1 speaker system model 2500

    can that person tell me if it is possible to run those speakers of 12V

    thanks in advance
  2. jcsd
  3. Jan 4, 2007 #2
    The current drawn is the current demanded not the current supplied.
    Your Car Battery will easily supply 3 times the rated current but that current isn't going to go into your speakers.
    Check your basic electronic principles.
  4. Jan 5, 2007 #3
    i forgot to mention
    2.9A is the current supplied

    the circuit will take more than 2.9 amperes the circuit will not be damaged
    but the excess current will go to the speakers and cause them to pump more
    that i know from experience

    but how would you make the filtered circuit just to make sure not more than 2.9 amperes is supplied
  5. Jan 5, 2007 #4


    User Avatar
    Science Advisor
    Gold Member

    " i am trying to fit computer speakers in a car
    the computer speakers need 12V@ 2.9 A DC"

    This sounds like you are trying to power a set of computer speaker that includes its own amplifier. If so, just use your car battery for the supply. The 2.9 amps is the current that the amplifier will "take" from your battery.

  6. Jan 5, 2007 #5


    User Avatar
    Science Advisor
    Gold Member

    With car audio it always pays to take the feed directly from the battery, but I doubt it'll really make much difference with just 3A or so.

    And saad, the replies are correct, the 2.9A is the (rated) current the transformer will draw. If you want to ensure that no more than this is ever drawn, you could use a fuse...
  7. Jan 6, 2007 #6


    User Avatar
    Science Advisor
    Homework Helper

    The problem is that a car is NOT 12v.
    The car will provide about 13.6v when the engine is running.
    This is why your computer speakers will draw more current.
    It has nothing to do with the fact that a car battery can source more amperes.

    Probably the cleanest solution to "Filtering" in this case is to use a DC-DC converter.
  8. Jan 15, 2007 #7
    "also does any body here own a creative inspire 2.1 speaker system model 2500"

    i was thinking of doing the same with my 2.1, 2400 speakers. it needs 12V, 1.5A. will it effect the system if voltage goes up to 13.6V?

    i agree with NoTime, using a DC-DC converter. but i would use a DC-AC converter so u can easily plug ur speaker into the converter and dont have to worry about over voltage or current.
  9. Jan 15, 2007 #8


    User Avatar
    Science Advisor
    Homework Helper

    In some cases the square wave output of common DC-AC converter can cause problems.
    Many things will work just fine on the square wave output.
    Just a thought in case you end up with issues.
    An option here would be a sine wave inverter (more expensive).

    Note that the generic wall wart 12v supply provides no voltage or current regulation.
  10. Jan 15, 2007 #9
    tell me if it'll work. assuming the battery while engine running, gives 13.6V suppling 50A. connecting a resistor in parallel to the source taking up 48.5A. A fuse of 1.6A connected in series between the resistor and the load. ill choose the resistance of the fuse so supply would reduce from 13.6 to 12V.
    so i can get 12V and 1.5A through the load, i.e. speakers. the fuse should protect the speakers if current exceeds.

    NoTime, what did you mean by generic wall wart?
  11. Jan 16, 2007 #10


    User Avatar
    Science Advisor
    Homework Helper

    No - It won't work.
    The amp determines how much current it will draw not the battery.
    Also the current is NOT constant and will vary with the volume.
    With the volume set to zero the current will be only be about 0.02A.
    Very much less than the max current of 1.5A.

    The potential for a problem here is that 13.6v input can increase the ability of the amp to produce power.
    Power is voltage times current not either one by itself.
    If the amp produces more power than it was designed for it will burn out.

    The little black box you plug in the wall that they give you to power your amp.
  12. Jan 16, 2007 #11

    I think you may be getting confused, or else your initial post was missing something. You said you needed 12V 2.9A DC and now you are talking about a DC-AC converter.

    If your AMP wants to draw more than 2.9A and 2.9A is all there is available then the volts will drop in the AMP. You could use a DC-DC converter but a simple 12V regulator or even a simple zener diode regulator will give you a smooth enough 12V signal for your AMP.

    You obviously want to use something not compatible with the CAR stereo e.g. IPOD.
    Have you got a Stereo in the car already?
    If so you can get tape convertors that you put into the cassette slot and then can plug in any device with a headphone socket and it plays through the Cars speakers.
  13. Jan 16, 2007 #12


    User Avatar
    Science Advisor
    Homework Helper

    Most 12v regulators require about 14v to operate properly.

    A zener, or the equivalent in a circuit since the normal zener internal resistance is too high, needs to dissipate about 20w in this case.
    Thats quite a bit of heat to get rid of.
  14. Jan 16, 2007 #13
    guys your right, i totally forgot volume depends on current.
  15. Jan 16, 2007 #14
    hi again guys

    the computer speakers are being used to replace the car speakers
    the car speakers are very old and its cheaper to get computer speakers than car speakers for providing good sound

    i first used a 7812 regulator but that did not work properly because its not capable of providing 2.9A. then i looked up an NTE1912 but was not sure wether it would work with a supply of 12V
    then i got tired of it and used a 5A car fuse (i could not find a 3A car fuse) i will replace that with a 3A fuse later on.
    that took care of the power supply and the speakers worked properly.
    then i hit another snag. that was the input to the speakers from the car tape. the car tape is a very old one and does not have any line out options for using with amplifiers. i first directly connected the speakers to the input.
    but the power to the speakers was about 30Watt. hence the amplifier of the speakers worked well at very low volumes. any higher and the distortion was too great. then i thought that since the sound signal to the speakers is AC so i thought "why not use a step down transformer" so i took 2X 2A 220V to 12V transformers. connected the 220V side to the car speakers and used the 12V side as input for the amplifier.
    the sound was ok but it was a bit too low. the amplifier when used in it original setting provides awesome sound but it did not go to that level in the car. but still the sound quality is pretty good.

    but i want to improve this as the way of taking the input is not exactly proper
    i just had the materials to hand and improvised.
    please tell me if you guys have any suggestions, or if there is a proper circuit for providing a proper input can any body send me the circuit diagram
    my email id is guess_whosak@hotmail.com
  16. Jan 16, 2007 #15
    Wire an op-amp to the signal from your tape deck. Something like an LM386 should work great (last I can recall). If you use a power transformer, you're going to have a lot of problems with getting the sound frequencies through. It'll probably sound really "tinny". Try finding an audio transformer if you want to go that route. Jenson is known to make some nice ones.
  17. Jan 17, 2007 #16
    thanks triden
    i am thinking too about the audio transformer
    i asked some electronic shop owners in my area thet were also saying that there is some sort of transformer for my application but no one could point out exactly what it was
    do you have any pics or links about the transformer
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook