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7351Re: [nslu2-general] NSLU2 using 7805s and a 12V battery

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  • Philip Pemberton
    May 4, 2008
      efil vasers wrote:
      > So far, I've tried a few different regulators (rated 1 or 1.5 amps), and
      > when I turn the NSLU2 on, after about 30 seconds, the regulators are too hot
      > to touch and soon shut themselves down.

      OK, this is going to be a long post, mainly because I don't like saying that
      something is going wrong, I also like saying exactly *why* it's going wrong
      and how I came to the conclusion. If you've seen Big Bang Theory, think
      'Sheldon with slightly better social skills' :)

      Two things that could cause this:

      You're trying to drop too much power over the 7805s.
      7805s are linear regulators (and horribly inefficient ones at that, but
      they're cheap and pretty much the 'jellybean standard' so everyone uses them).
      That means that any voltage drop has to be dissipated in the form of heat...
      So let's do the math:

      Your input voltage is 13.8V (assuming a fully charged SLA battery). You're
      using 7805s, so your output is 5V. That means a voltage drop of 8.8V by virtue of:
      13.8V - 5.0V = 8.8V

      You're also dumping a peak of 2A over the the regulators (though ~1A typical;
      the USB ports are rated for 500mA each for a total of 1A). That's 17.6 Watts:
      8.8V * 2A = 17.6W

      Or with the USBs 'open' (nothing connected), we assume 1A because:
      2A - (500mA * 2) = 1A

      8.8V * 1A = 8.8W

      So your regulators have to dissipate between 8.8 and 17.6 Watts of energy as
      heat. And probably more than that because of internal inefficiencies. That's a
      lot of heat!

      Your heatsink isn't big enough
      You need to get rid of 17.6W of heat, and keep the temperature of the chip die
      itself below the thermal cutoff point (70 Celsius if I remember correctly). I
      honestly can't remember the calculations for heatsinks off the top of my head,
      but you can probably find them in 'The Art of Electronics' by Horowitz and
      Hill. Or on the internet.

      OK, so your possible solutions would be:
      - A switching regulator. These are pretty much turn-key these days, but you
      have to be careful with them. Good PCB layout is essential, heatsinking less
      so. Also, because you're not wasting as much energy as heat, your battery will
      last longer.
      - A bigger heatsink. Probably something about the size of a PC processor
      heatsink, with a fan, and drilled to take your regulator. You'll need some
      thermal grease and an insulator pad too.
      - A pre-regulator. Add another regulator in front of the 7805 to
      pre-regulate the 12V down to 8V, then feed the 8V to the 7805. Still horribly
      inefficient, but you're distributing the load over two separate chips, which
      reduces the heat output per chip.

      > I connected the regulators in parallel with each other to provide up to 3
      > amps (more than the NSLU2 power supply), so I figured that I wouldn't need
      > heatsinks for them. Am I wrong, or is my NSLU2 just behaving strangely?
      > How many people out there have tried this?

      You shouldn't parallel 7805s. One chip will end up with a lower voltage than
      the others (due to internal tolerances), and that'll take most of the load.
      Result being that it ends up getting roasted, then when it eventually blows
      you get a cascading failure of all the other 7805s. Lots of fun, involving
      burned plastic, melted silicon and smoke that really isn't good for your
      health or the chips'.

      You can add an external pass transistor though -- though it's been ages since
      I've done it. I do recall it involves a 10-ohm resistor and a beefy PNP
      transistor though. Cost is likely to be more than a switcher, though.

      Phil. | (\_/) This is Bunny. Copy and paste Bunny
      ygroups@... | (='.'=) into your signature to help him gain
      http://www.philpem.me.uk/ | (")_(") world domination.
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