Heat Sink getting VERY hot

Discussion in 'General Electronics Chat' started by RodneyB, Sep 11, 2013.

  1. RodneyB

    Thread Starter Well-Known Member

    Apr 28, 2012
    I would like to find out if the below is normal.

    I have a circuit that drops voltage from 26.4 Volts to 13.9 volts, which then drives a day night switch which switches a load of 0.733 Amps at 10.2 watts.

    When I connect the voltage controller the heat sink does not get hot. As soon as I connect the day night switch without the load the heat sink starts to get hot. When the day night switch comes on the heat sink get very hot.

    I have put a fan inside the housing it is mounted 100mm away from the heat sink blowing air onto the heat sink and then out through holes behind the hat sink.

    I would like to find out the most efficient way of cooling the heat sink, where to place the fan to get the most efficient cooling and if the generated heat is correct.

    I would like to understand a more efficient way of dissipating the heat.
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    1) get a better heatsink

    2) increase airflow

    3) reduce power dissipation
    RodneyB likes this.
  3. Metalmann

    Active Member

    Dec 8, 2012
    Can't you move the fan closer than 4"?
    RodneyB likes this.
  4. Shagas

    Active Member

    May 13, 2013
    Maybe you can find a more efficient way to drop
    the 26.4 volts to 13,9 ... Maybe using PWM .

    Otherwise : Bigger heatsink+fan , or use a peltier if you can facilitate it
    RodneyB likes this.
  5. richard.cs

    Active Member

    Mar 3, 2012
    It sounds like 1) your day/night switch uses a significant current on its own, maybe 100-200 mA for noticable heating and 2) your circuit is some kind of linear regulator which is a pretty wasteful method of dropping the voltage.

    Consider replacing the circuit with a small switching module like this one: http://www.ebay.com/itm/1PCS-NEW-LM...-step-down-module-GOOD-QUALITY-/300901295435? (better quality ones are available from Farnell/digikey/etc for more money). Much less heat will need to be removed and you shouldn't need the fan.
    RodneyB likes this.
  6. RodneyB

    Thread Starter Well-Known Member

    Apr 28, 2012
    I am moving the Fan closer and I have improved the air flow by making the draw for the fan bigger. I am also blocking any other holes so that all air flow around the housing can only escape through the holes behind the heat sink. I have not measured the current on the day night switch but am using a 741 as the IC switching on a relay.

    In conclusion I believe that it is essential for me now to learn the theory of switched mode modules and build my own.

    Thank you to everyone for the assistance

  7. MrChips


    Oct 2, 2009
    A switched mode power supply provides power only when it is required. Hence it runs cool and is efficient.

    In contrast, a linear voltage regulator has to waste and dissipate the unwanted power. That is why it gets hot.
  8. RodneyB

    Thread Starter Well-Known Member

    Apr 28, 2012
    I would like to try and make one. I googled "Switched Mode power supply" found lots of excellent theory but no circuit diagrams.

    Can you point me in the right direction?

    Many thanks

  9. Papabravo


    Feb 24, 2006
  10. sheldons

    Well-Known Member

    Oct 26, 2011
    how about a schematic of what you are using.....complete schematic of each item then maybe the items i have uploaded here will have a good chance of seeing what is going on....
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Go to any of the major IC manufacturers sites.. I like TI..
    They have a webbench design app on their site and you simply input your variables like Vin/Vout Aout,etc.. and it will automatically build it for you..

    Also the application notes in most modern datasheets are typically enough to be able to easily reproduce whatever you want. They walk you right through the functions/calculations,etc... Almost any idiot can build stuff now with just enough knowledge to be dangerous just by reading the datasheest/application examples section..

    I had zero experience doing switchmode stuff and used the webbench and easily got what I needed and built it and even did extensive safety/EMI/EMC testing on it through a testing lab and it passed with flying colors. Mine was 60V max in, 12V out @500ma.. It was walk in the park easy.