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

Emulating car battery output through home power?

  1. Jul 20, 2015 #1
    I'm setting up an amplifier which is intended for use in a car, and I'm not using it in one. I'm trying to power it through a typical DC 250 volt wall outlet. What kind of transformer should I use? (I'm also trying to power a car stereo with it as well)
     
  2. jcsd
  3. Jul 20, 2015 #2

    NascentOxygen

    User Avatar

    Staff: Mentor

    You mean 250v AC, I assume?

    How many pins does your wall outlet accommodate?
     
  4. Jul 20, 2015 #3
    3 pins. 2 regular and the optional ground.
     
  5. Jul 20, 2015 #4

    berkeman

    User Avatar

    Staff: Mentor

    You buy a power supply module that is already safety approved for the 240 (not 250) Vrms wall AC Mains output, and provides you with the output voltage required by the stereo or other load. What is the input power for the stereo specified for?
     
  6. Jul 20, 2015 #5

    NascentOxygen

    User Avatar

    Staff: Mentor

    You need a lot more than a transformer. You need a heavy duty 12v DC regulated power supply; this is a box containing not only a transformer but also transistors, ICs, large capacitors, and more.

    If you indicate how many watts your amplfier can put out, I'm sure someone here will be able to say what amp rating the regulated supply will need. If you plan to simultaneously power a second power-hungry device, its power needs must also be included in your calculations before deciding exactly what you need to buy.
     
  7. Jul 20, 2015 #6
    On the page in which I'm buying the stereo from, it doesn't specify what the input power is. It just takes whatever the car battery gives it, hence why I'm wondering to just make a wall output act like a battery.
     
  8. Jul 20, 2015 #7

    berkeman

    User Avatar

    Staff: Mentor

    So the input voltage is 12V. What is the power output specification of the amp?
     
  9. Jul 20, 2015 #8
    The amp is 1,500 watts.
     
  10. Jul 20, 2015 #9

    berkeman

    User Avatar

    Staff: Mentor

    Holy crap that's a lot of power. I = P/V, so that gives you the specification for the output current from an equivalent DC power supply.
     
  11. Jul 20, 2015 #10

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    Be careful with amp manufacturers specs. There are plenty of ways they can inflate the spec to more than it actually is. If it truly is able to drive 1500 watts into a load then it is unlikely you will be able to find a 12 volt power supply capable of running this amp. If the amp were 100% efficient (it's not) you would need a power source capable of supplying 125 amps.
     
  12. Jul 21, 2015 #11

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Last edited by a moderator: May 7, 2017
  13. Jul 21, 2015 #12

    anorlunda

    Staff: Mentor

    A 12v auto battery charger may also provide what you need, and they are relatively inexpensive.

    I agree that 1500 watts sounds excessive. Your car would have a hard time supplying so much current.
     
  14. Jul 21, 2015 #13

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    The transformer type charger is not "filtered" so he'd want to add a "supercapacitor" at charger's output. That's available in any car radio shop, even Walmart.
     
  15. Jul 21, 2015 #14

    anorlunda

    Staff: Mentor

    I would try it first without the filter Jim. The auto 12v environment is very noisy so the amp may have lots of it's own filtering. If it doesn't work well, the capacitor could be added.
     
  16. Jul 21, 2015 #15

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Indeed.
    Transformer type chargers hum, too.
    Inverter type is quieter both electrically and acoustically
     
  17. Jul 21, 2015 #16
    P = E^2/R, to deliver 1500W RMS using a 12V supply voltage, the resistance (impedance) of the connected speakers would have to be about 0.1 Ohm. With a more practical 2 Ohm speaker impedance, you would need more than 55V to deliver 1500W.
     
  18. Jul 21, 2015 #17

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    This really isn't relevant. Car audio amplifiers in this range have built in switch-mode power supplies to step the voltage up to appropriately drive 2 and 4 ohm speakers. I suspect the amp in question is a single channel class D designed to drive a subwoofer.
     
  19. Jul 21, 2015 #18
    Ah - The OP is "setting up" an amplifier. Thanks for pointing this out.
     
  20. Jul 21, 2015 #19
    Well as a low cost solution why not connect a 60 amp battery charger to a car battery and use that . The battery should provide enough filtering.
     
  21. Jul 21, 2015 #20

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    That would be the cheapest solution by far and, as long as you use fat enough supply leads, it would be truly representative of how that amp is going to work in the car.
    Does the amp manufacturer have any recommendations about how to supply the amp when mounted in the car? If they don't then their quoted Peak Power value is pretty suspect.
     
  22. Jul 21, 2015 #21

    anorlunda

    Staff: Mentor

    60 amps times 14 volts is only 840 watts.

    1500 watts is still hard to believe. I looked at the owners manual for a 2500 watt car amplifier for a sanity check. It says 2500 watts peak power. It also says "current draw 1.4 amps". It also says that under seat mounting is fine. I believe that the continuous power load is only 17 watts, not 1500 watts. It should be possible to power that amp with only a trickle charger, instead of a welders supply.

    http://audiobahn.com/phocadownload/Manuals/Amps/ClassD.pdf [Broken]

    Notwithstanding the above, I have seen cars driving past with the sound so loud that the metal of the roof visibly bounces with each drumbeat. How on earth can someone sit inside without ruining their hearing in seconds? My own hearing was ruined by a factory job where the noise level was much lower than the music in those cars.
     
    Last edited by a moderator: May 7, 2017
  23. Jul 21, 2015 #22

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Marketeers have no qualms about outright lying.

    To engineers, power rating of an amplifier is its capability at reasonable distortion without overheating.
    That would be continuous RMS watts.
    Peak power, calculated using the peak instead of RMS voltage and current would be twice that. That's a modest marketing lie.

    To marketeers it is permissible to unabashedly lie by advertising the "Peak Instantaneous Power", or "Music Power" .
    That is the absolute maximum volts X amps the amplifier can deliver during a transient lasting only an instant , disregarding distortion and duration of the transient.
    It'll be probably the sum of all the channels of a multichannel amplifier because that makes the number bigger yet
    Some crum-bums will even give a theoretical value that's calculated assuming power supply startup transient voltage !


    This paper by Texas Instruments explains the "dirty tricks" .
    http://www.ti.com/lit/an/slea047a/slea047a.pdf
    and gives actual test results.
    .
    Just looking at one of the devices in that paper
    • TAS5142 – 4 half-bridges (2 BTL channels or 4 single-ended [SE] channels)
    let's use the BTL (bridged) rnumbers

    That IC can be rated
    87 watts RMS continuous , no clipping (not noticeably distorted )
    108.5 watts RMS continuous at 10% distortion
    174 watts peak power (twice continuous as mentioned)
    318 watts peak instantaneous power using the maximum rated voltage for the device. There being two BTL channels in the device, it could be marketed as 636 watts, maybe even more in some countries.

    So - TI's VERY respectable 87 watt per channel bridged digital amplifier should be powered by about a two hundred watt supply, ~15 amps at 14 volts
    even if it was bought as a "636 watt" amplifier
    That's why i suggested OP look at the fuse.

    and it's why i posted that preposterous supply . (well, also partly because i found one last week at my metal recycle yard , cost me 30 cents a pound not quite twelve bucks)

    I would be surprised to find OP's "1500 watt" amplifier actually capable of 150 honest watts at 1% distortion.

    Check out that TI paper , it's an eye opener.

    old jim
     
    Last edited: Jul 21, 2015
  24. Jul 21, 2015 #23

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Interesting
    i just looked at the link anorlunda posted
    http://audiobahn.com/phocadownload/Manuals/Amps/ClassD.pdf [Broken]

    that manufacturer publishes RMS watts.

    upload_2015-7-21_18-29-3.png


    1500 watts into 1 ohm is 38.7 amps
    My preposterous supply would be about right for their AT12001DJ

    Audiobahn looks like a name to respect.
     
    Last edited by a moderator: May 7, 2017
  25. Jul 21, 2015 #24

    anorlunda

    Staff: Mentor

    Yes Jim, but I think it can supply that much power only in short bursts, like a drum beat. Averaged over time, the current draw is 1.4 amps at 12 volts.
     
  26. Jul 21, 2015 #25

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Agree with your seconds last post, Jim

    NONE ... I repeat NONE of these hi power car amps can supply the stated wattage
    its all sales hype PMPO Peak Music Power Out blah blah blah
    its been a sales con job on customers who know no better for years, like the OFC (oxygen free copper) speaker cables garbage :rolleyes:

    in all seriousness you are probably lucky if it produces 700W peak
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook