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Fan-cooled small box questionable design ?.

Discussion in 'Electronic Repair' started by N Cook, Jun 23, 2007.

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  1. N Cook

    N Cook Guest

    A commercial mixer amp in a tiny box. By gut-feeling I don't like the
    relative absence of air cooling and would like some justification to
    increase the , at least output vent size - there is space for much larger
    holes, but not necessarily aesthetic.
    2 switch mode power supplies plus 2 100W rms D class amps and an 8 channel
    mixer in a box only 10x6x2.5 inches. The 2 SMPS are "cooled " by a 12V 40mm
    fan blowing the internal air, not positioned near any inlets, over the
    heatsinks.
    Inlet vents , if all 1/4 inch sockets should have tight fitting plugs in all
    of them, are 30 x 2mm diameter holes.
    The other fan is 12V, 50mm fan that pulls air over the 2 amps and exhausts
    directly to the outside but via 8 small slots .44 inch long and .08 inch
    wide that seriously restricts the flow out. The owner reports the exhaust as
    being hot rather than warm and i've taken some measurements with thermometer
    etc with relatively low continuous sine to external dummy load. I intend
    giving the owner a thermometer to take some readings in normal use over an
    hour of guitar performance.
    I fixed, with sealing tape, a large diameter tube around the exhaust area to
    direct into a very thin plastic rubbish sack, with no perforations, to time
    and measure the air flow until about 2/3 full and holding the floppy bag up
    as well so I'm assuming relatively little back pressure or whatever the term
    is.
    I did not think, while inside , to note any amp rating as well as 12V, nor
    make and model, but generally 50 mm fans would be about 8 to 18 cfm cubic
    feet per minute.
    But from my calculation over 3 minutes then the flow rate was only 0.78 cfm.
    This amp is designed so it can be transported and used , recessed into one
    of the speakers, so not much heat lost through the walls of the box I would
    have thought, I'm advising, interim, for the owner to use the amp removed
    from the speaker.
    Any comments on the adequacy of venting?
     
  2. Eeyore

    Eeyore Guest

    Name names !

    There's plenty of bad design around especially if it's from Asia. Proper thermal
    management is a discipline that's rarely taught to electronics engineers. I had
    to learn it by researching for myself.

    Graham
     
  3. N Cook

    N Cook Guest

    an afterthought.
    How much added strain is there on an axial fan by stopping down the free
    flow rate by more than 90 percent , leading to premature failure even
    without being in a hot airstream.
    I'm thinking of kitchen extractor fans which are relatively small motors for
    their power , relying on the passing airstream of human scale temperature
    range passing over them.
     
  4. Meat Plow

    Meat Plow Guest

    Justify to who?
     
  5. N Cook

    N Cook Guest

    Using vol of air per min = watts / (air density*specific heat*temp diff*60)
    in metric terms
    60 seconds in minute , taking air density = 1.205, Cp= 1.05 and conversion
    factor of 35.3 for cu m /min to cf/m and measured outflow of 0.78 cf/m ,
    however I plug these measured results in,scaling for higher demand, does not
    look good for temp diff at higher output and fan duress apart from the
    electronics which could be in circulation dead spots.

    400Hz source sine fed into one amp only, delivering 12.5 watts continuously
    into an external dummy load. After 40 minutes the exhaust temperature was
    still increasing but very slowly, so cancelled then. So heat from 2 switch
    mode power supplies and one idle amp with no load and the other one active
    plus contributions from the voltage regulators and mixer circuitry etc.
    Temperature from bulb of thermometer laying directly over the exhaust vent
    rose to 42 degrees C from a room temp of 24 degrees C.
     
  6. James Sweet

    James Sweet Guest


    I avoid the use of fans, they draw a lot of crud into the device,
    particularly problematic if the mixer is used in a venue where smoking is
    allowed. If the thing is not failing in short order, the heat is probably
    not an issue.
     
  7. Eeyore

    Eeyore Guest

    You get what ?

    You won't get anything like a fan's rated cfm figure when the airflow's
    restricted btw.

    So, is it overheating ? Put a thermocouple on the heatsink and see what temp it
    is.

    Graham
     
  8. N Cook

    N Cook Guest

    The other figure is 90W consumption printed on the box at the mains inlet,
    presumably 90W plus 2x100W of audio, ie 90 Watts having to be vented from
    the box at full audio output.
    Plugging 90W in the formula and with 0.78 cf/m then temp diff would be 54
    above ambient or 78 degree C with presumably temperature hot spots above
    that inside.
     
  9. Eeyore

    Eeyore Guest

    No, the figure on the 'plate' is meant to be the actual power drawn.

    Have you been able to measure 2 x 100W continuously ? It may have circuitry to
    stop you doing that, I've seen that trick before.

    And what is this unit ?

    Graham
     
  10. Do you know or recommend any app notes for a small class D amp, say
    50W, running off a non automotive 12V lead acid battery?


    martin
     
  11. Eeyore

    Eeyore Guest

    12V won't do 50W with direct coupling.

    I'd be inclined to use a SMPS to provide suitable +- supplies to do the job then use
    any of the many claas D designs thar are kicking around.

    Graham
     
  12. Rich Grise

    Rich Grise Guest

    .
    Well, is the unit failing? It sounds from your description like it was
    designed to run hot.

    If it works, don't fix it. :)

    Cheers!
    Rich
     
  13. BH

    BH Guest



    N Cook:

    Why not tell us make & model. There is always a chance that someone
    out there has experience with the unit.

    Bruce
     
  14. Thats what I thought, I'm using the Phirrips/NXPee BTL stuff at the
    mo, with the big Boost caps and they can get a bit warm, and haven't
    played with D amps. Any recommendation on TI devices etc. or what to
    avoid?
    As they all claim to be near perfect!


    martin
     
  15. Eeyore

    Eeyore Guest

    What are these big boost caps ?

    Honestly I haven't any experience of them.

    But of course !

    Graham
     
  16. James Sweet

    James Sweet Guest

    I think National Semiconductor has some, take a look on their site.
     
  17. Not on this particular design.

    But I conclusively proved that the vent holes on the LinkSys cable modem
    are WAAAAY too small. Creating all sorts of long term reliability problems.

    And truly bizarre symptoms.

    And easily compounded if you stack a router on top of it.
    Cisco denies the problem.

    The only way I've found these to remain reliable is to lean them at a 45
    degree angle with free flow above and below.



    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  18. Class H where they charge a couple of4700u caps to Vcc, and *add*
    them to the supply lines, to handle peaks, like TDA1560


    martin
     
  19. N Cook

    N Cook Guest

    Its a Yamaha Stagepas 300 , made 2005
    someone's pic, no scaling on any pics I found, but only 10x6x2.5 inch box
    http://www.keyboardplayer.com/images/home/0305_stagepas_300.gif

    Mosfets knocked out maybe by recovered punch-through in softened,
    suspiciously small size 470uF 85 degree C output filter cap.
    Interestingly the pic i've referred to above would suggest an enlarged grill
    area in the bottom left corner. The bit that looks like a bar code is
    probably and extended grill that is not on the one I am dealing with, the
    grill is only from the chrome handle to the extreme left corner.
    I will probably bend up the corner of the cover where the outlet vent is,
    propped up on a 1/4 inch stand-off, and add a bit of grill. So increase the
    airflow by double, decrease the temperature by half and reduce the strain on
    the main fan.
    If it was my amp I would probably cut a hole in the case where the internal
    fan is to ameliorate matters as well.
     
  20. Eeyore

    Eeyore Guest

    Do they indeed ? I wondered. It must have a slightly adverse effect on the audio output I'd
    have thought.

    Graham
     
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