LM7805 overheating

Discussion in 'Embedded Systems and Microcontrollers' started by Litch, Feb 26, 2013.

  1. Litch

    Thread Starter Member

    Jan 25, 2013
    86
    7
    ( In reference to thread: http://forum.allaboutcircuits.com/showthread.php?t=80117&page=2 )

    First proto (Apart from the whole power circuitry being wrong) the LM7805 1.5A VREG gets rather hot, so much so that I've raised it to stand vertical and put a heatsink on it.

    Still gets too hot.

    That's with a 9V 2A DC SM PSU feeding it, with either the RPI connected (~400mA draw), or every relay engaged (~450mA). Have not been keen to try both.

    Is there something I'm doing wrong? Should the LM7805 get that hot at < 1A load?

    What is the "Temperature coefficient of output voltage" listed in the LM7805 datasheet?

    Should I be looking at voltage "conversion" rather than "regulation" ie. DC to DC power supply rather than a wide range DC input in to a 5V vreg?

    PCB space is minimal, I need somewhere between 1.5 to 3A @ 5V - Ideas?
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Litch likes this.
  3. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    A switching regulator is my bid, however, with minimal board space you may not have room for a massive diode & inductor...

    Any regulator will heat up as it regulates, and with linear regulators(like the 7805), dropping 4V from 9V to get the 5V out @3A => 4V * 3A = 12W! Yes, I'd reckon the thing will heat up:eek:
     
    Litch likes this.
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Yes, it should get hot. Very hot.
    You are dropping 4V between input and output, and at 0.5A that makes 2W, and that is a LOT of heat without a heatsink.
    The fairchild datasheet says that the thermal resistance junction-case is 5K/W and junction-ambient is 65K/W. This means that at 2W without any heatsink, the die of the device will be at a whopping 155°C if your ambient temperature is 25°C, and the case will be 10°C less, but 145°C is still a nice way to burn your fingers.
    Also, the device has maximum junction temperature of 125°C, so you really should either get some reasonably large heatsink, or use a switching converter. But switching converters are a lot more complex and harder to do right on the first time, so if you can tolerate the few watts of heat being produced in your circuit and have means of getting it out of the enclosure like fans, ventilation holes, metal case, etc., consider just using a proper heatsink.

    Also, the figure 2W doesn´t seem as large, but imagine that it is equivalent to the power produced by a 0.2kg weight falling from 1m every second. Imagine something like a 3.5" hard drive falling from the table on your leg every second. Or lifting it up. (Ok, a hdd wieghs ~400g, but you get the idea)
     
    Last edited: Feb 26, 2013
    Litch likes this.
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Also, how about buying a wallwart with 5V 3A regulated output? Or an old ATX PC supply? It could save you a lot of hassle.
     
  6. Litch

    Thread Starter Member

    Jan 25, 2013
    86
    7
    Hmm, thanks people - seems I severely underestimated the power component of this project.

    Seems even if I was doing 6V supply, which makes it a 1V drop with a 2A drain, that's still 2W heat dissipation.

    I don't see much choice left but to attempt to squeeze in a DC-DC converter.

    Thanks to all for the info!
     
  7. Litch

    Thread Starter Member

    Jan 25, 2013
    86
    7
    Thanks to praondevou's suggestion I went hunting for a DC-DC *drop in* replacement for the VREG: http://www.farnell.com/datasheets/1359235.pdf

    It's about $9 which makes a good replacement compared to other DC-DC units, which are substantially larger (50mmx25mm) and cost around the $100 mark.

    Thanks again.
     
  8. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    8, 10, 12 watts cause surprising amounts of heat. Look at the size of a 10 watt resistor to get an idea of how much surface it takes to radiate that much heat. Confine it and the temperature skyrockets.
     
  9. skusku

    Active Member

    Aug 9, 2009
    63
    1
    I did a project where I used a PIC and a HD44780 which was powered from a 2A 12V PSU and then it ran through a 7805. I tested it for a day and the regulator maybe changed 1 degree from room temperature after 8 hours. Now I am worried that it might get too hot (I didnt even put a heatsink on). Its installed and gonna run 24/7....
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    2A 12V PSU means nothing, you need to say what is the current the circuit draws.
     
  11. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    Wait, a LCD and a PIC draws next to nothing, current-wise. You are comparing apples to high-current apples... 200mA is a lot different from 2A....
     
  12. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Use a 5V electronic transformer.
     
  13. skusku

    Active Member

    Aug 9, 2009
    63
    1
    Thanks tshuck! Thats why I included what I was using, so you experts can help in the right direction :)
     
Loading...