7354Re: NSLU2 using 7805s and a 12V battery
- May 5, 2008First off, I'd like to thank you all again for your help.
I should have done a bit more research before jumping head first into
Next time around, I'm definitely going with a switching regulator.
My quick solution is this:
A 2N3055, bolted to an old CPU heatsink (which still had some thermal
compound on it), with the base connected to a 7805 (with a small heat
sink), collector to the battery, and emitter to the NSLU2 (and other
Also, the ground pin of the 7805 is connected to the real ground
through a 1A diode; when I measured the emitter voltage without this,
it read about 4.4 volts, went up to about 5.2 after adding it (runs
about 4.9 with the NSLU2 on).
The 7805 heatsink doesn't even warm up, while the 2N3055 heatsink
warms up after a couple of minutes, but went back down to near room
temperature when I placed an extra 80mm fan next to it.
I'm using the same solution for the 9 volt regulator (both sharing the
--- In email@example.com, Philip Pemberton <ygroups@...>
> efil vasers wrote:
> > So far, I've tried a few different regulators (rated 1 or 1.5
> > when I turn the NSLU2 on, after about 30 seconds, the regulatorsare too hot
> > to touch and soon shut themselves down.saying that
> OK, this is going to be a long post, mainly because I don't like
> something is going wrong, I also like saying exactly *why* it'sgoing wrong
> and how I came to the conclusion. If you've seen Big Bang Theory, thinkbut
> '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,
> they're cheap and pretty much the 'jellybean standard' so everyoneuses them).
> That means that any voltage drop has to be dissipated in the form ofheat...
> So let's do the math:You're
> Your input voltage is 13.8V (assuming a fully charged SLA battery).
> using 7805s, so your output is 5V. That means a voltage drop of 8.8Vby virtue of:
> 13.8V - 5.0V = 8.8Vtypical;
> You're also dumping a peak of 2A over the the regulators (though ~1A
> the USB ports are rated for 500mA each for a total of 1A). That's17.6 Watts:
> 8.8V * 2A = 17.6Wenergy as
> 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
> heat. And probably more than that because of internalinefficiencies. That's a
> lot of heat!the chip die
> Your heatsink isn't big enough
> You need to get rid of 17.6W of heat, and keep the temperature of
> itself below the thermal cutoff point (70 Celsius if I remembercorrectly). I
> honestly can't remember the calculations for heatsinks off the topof my head,
> but you can probably find them in 'The Art of Electronics' byHorowitz and
> Hill. Or on the internet.days, but you
> OK, so your possible solutions would be:
> - A switching regulator. These are pretty much turn-key these
> 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, yourbattery will
> last longer.processor
> - A bigger heatsink. Probably something about the size of a PC
> heatsink, with a fan, and drilled to take your regulator. You'llneed some
> thermal grease and an insulator pad too.horribly
> - 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
> inefficient, but you're distributing the load over two separatechips, which
> reduces the heat output per chip.up to 3
> > I connected the regulators in parallel with each other to provide
> > amps (more than the NSLU2 power supply), so I figured that Iwouldn't need
> > heatsinks for them. Am I wrong, or is my NSLU2 just behavingstrangely?
> > How many people out there have tried this?voltage than
> You shouldn't parallel 7805s. One chip will end up with a lower
> the others (due to internal tolerances), and that'll take most ofthe load.
> Result being that it ends up getting roasted, then when iteventually 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 foryour
> health or the chips'.ages since
> You can add an external pass transistor though -- though it's been
> I've done it. I do recall it involves a 10-ohm resistor and a beefy PNPpaste Bunny
> transistor though. Cost is likely to be more than a switcher, though.
> Phil. | (\_/) This is Bunny. Copy and
> ygroups@... | (='.'=) into your signature to help him gain
> http://www.philpem.me.uk/ | (")_(") world domination.
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