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B Source of Heat in CD

  1. Jul 26, 2016 #1
    While changing a CD in my vehicle CD player I noticed the CD is quiet a few degrees above ambient after it has been playing for awhile, not measured just felt very warm to touch.

    the question is what is the source of the heat?

    I considered the LED but that makes no sense.

    Is it the motor that rotates the CD, seems unlikely due to the air gap between the CD surface and the rest of the system???
     
  2. jcsd
  3. Jul 26, 2016 #2

    Drakkith

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    I would assume it is a combination of the laser/LED and the heat of the electronics. I think the laser is only about 5 mW in power, but I don't know how much power the rest of the electronics uses.
     
  4. Jul 26, 2016 #3
    5mW seems like a lot of power?
    I was unaware they were that strong or needed to be, why would that much power be required to read off a CD at such short range.


    the system playing radio or other formats eg mp3 does not generate the same heat as when playing the CD.

    interested if Blu-ray generates more or less surface on the disc heat being a colder frequency than red on the CD.
     
  5. Jul 26, 2016 #4

    Drakkith

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    MP3 files are compressed and take up MUCH less room than regular CD audio. It's possible that the disk drive doesn't leave the laser continually turned on to read the compressed data as it does with uncompressed audio. That's just a guess though.

    A Blu-Ray DVD requires light around 400 nm in wavelength, which is on the blue/purple end of the spectrum. I know blue and purple are considered to be "cool" and red to be "warm" by many, but those concepts have nothing to do with heat.
     
  6. Jul 26, 2016 #5
    "A Blu-Ray DVD requires light around 400 nm in wavelength, which is on the blue/purple end of the spectrum. I know blue and purple are considered to be "cool" and red to be "warm" by many, but those concepts have nothing to do with heat."


    but why not, they have everything to do with "heating" as per eg; incubators are red not blue, warmth from fire.....etc, etc, etc.
     
  7. Jul 26, 2016 #6

    Drakkith

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    Incubators and fires emit light primarily in the IR range, which is where most of their heating comes from. This is because a hot object emits EM radiation along a spectrum, whose emitted power vs wavelength depends almost entirely on the temperature of the object. Very cold objects emit barely any radiation and the majority of it is at very long wavelengths. For example, the CMB spectrum is essentially identical to that emitted by an object just under 3 kelvin and sits primarily within the microwave band. You and I are much warmer and the radiation emitted by our bodies falls primarily in the long-wavelength part of the infrared band. Incandescent light bulbs are around 2,000 - 3,300 kelvin and while they emit lots of visible light, the majority of the radiation falls within the near-IR band.

    Fire and incubator lights are cooler than a standard tungsten incandescent light bulb used for household lighting and thus they emit barely any visible light in the blue or green areas of the visible band, making them look red, orange, or yellow.

    See this article: https://en.wikipedia.org/wiki/Black-body_radiation
     
  8. Jul 26, 2016 #7

    CWatters

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    Most in car systems have a CD player, radio and audio amplifier all in one unit. At sensible volume levels I think they draw a bit under 1A @ 12V. So potentially you might have several watts of heat being dissipated in a box that's not exactly well cooled.
     
  9. Jul 26, 2016 #8

    Vanadium 50

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    It is almost certainly the amplifier for the CD player. That has between a thousand and 100,000x more power than the laser.
     
  10. Jul 26, 2016 #9

    davenn

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    and that would be on the extremely low side

    consider a system with 4 speakers ( not uncommon in cars) and even a decent system at low power putting out say 30W per channel
    there 120W of music power, power drawn from the 12V system will be around double that.
    Lets err on the conservative side and say 200W from the 12V supply = 16.6 Amps

    the power dissipation within the stereo unit is going to cause LOTS of equip heating
    any heating effects of a 5mW laser wouldn't even be noticeable



    Dave
     
  11. Jul 26, 2016 #10

    Drakkith

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    Makes sense. My old handheld CD player never noticeably heated up the CD. I'm sure it only used a few watts since it didn't have to drive speakers larger than headphones.
     
  12. Jul 26, 2016 #11
    thanks for replies and thoughts. just seemed like an interesting question to me.
     
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