calculating heatsink size

Discussion in 'General Electronics Chat' started by Fenris, Nov 9, 2007.

  1. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    hi all

    having succesfully got my sound to light circuit
    working, thanks to the forum, i know have to address
    heatsinking. im hoping i have understood the maths of
    heatsink calculation so here goes.
    circuit supply 12VDC

    A TIP41 with a max of 6A continuous, Switchs 2*12V/20W
    halogen G4 bulbs.

    the bulbs draw 3.33A leaving 2.67A across the TIP41

    2.67A*12V=32.04W

    i need to keep the temp of the TIP below 100oC inc
    ambient tempreture. so......................

    100oC/32.04W=3.12oC/W
    meaning the spec of my heatsink must be =<3.12oC/W

    this seems ok i dare say i have it wrong and explianing it to me wont help
    my english is better than my maths, my concern is i cant find a heatsink
    of the value or less as calculated.

    Regards

    Fenris
     
  2. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Are you using the TIP41 as a switch, or as some kind of regulator. I don't understand the 6A - 3.33A = 2.67A ??? Also, why would the power be 12V * 2.67A ?? Isn't the 12V across the bulbs, not the xstr. Are you using 24V and dropping half across the xstr?

    If you are using the TIP41 as a switch, and the total load is 3.33A, then the current through the TIP41 is 3.33A. Power dissipated in the xstr would be the CE "on" voltage of the transistor times the current through it (provided it's in saturation, and is not being used as a linear regulator). However, it would be easier to look at the TIP41's dissipation curves for 7A or so (this is about twice the current and provides headroom).
     
  3. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    hi heres a daigram of the circuit
    for the TIP41C, from your
    comment its obvious i have
    failed to understand the
    maths and relevant data
    i need :confused:

    regards

    Fenris
     
  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    So, it appears you are varying the bulb brightness based upon the input?

    If so, then worst-case would be at 6V across the xstr or:

    3.3A = 2 * 20W / 12

    3.64 ohms = 12V / 3.3A

    1.65A = 6V / 3.64 ohms

    10W = 1.65A * 6V

    The TIP41 will dissipate 10W if it is constantly half-on, the 13mA of LED current is not significant.
     
  5. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    hi

    thanks for that so if i have followed the calculation side
    correctly im going to need a heatsink =<10oC/W?????
    lots to choose from in that case, cheers


    regards

    Fenris
     
  6. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    What is the max ambient temp?
     
  7. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    hi nano

    ambiant temp for the uk is less than 25oC most of the time
    the unit will be in a variaty of enviroments as in indoors, out-
    doors. something to take into acount also is that the unit will
    probably only operated in short bursts, so i think its way over
    spec'd for the heatsink but well covered.

    Regards

    Fenris
     
  8. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    You should consider an ambient temperature "inside the case" of 40ºC. It is a good safety factor.
     
  9. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    hi there

    the case will be well vented with slots top and sides near the heat sink
    to allow good airflow. that said i should probably go for a heat sink
    rated @ =<5-6oC/W? im figuring off the top of my head that that will
    do the trick (cant do the maths im inumerate with anything over and above
    normal maths)

    regards

    Fenris
     
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