Thank god Roman was joking about the baby, me thinking *do i have to go throgh that again?* regards Art ----- Original Message ----- From: Dan Michaels To: Sent: Wednesday, January 24, 2001 11:07 AM Subject: Re: !Re: [EE]: +12V and +5V in automoble > Barry Gershenfeld wrote: > > >>Not the most elegant or cheapest solution, but you might also > >>consider small-V 5W zeners instead of a series R. I have one > >>item where I do this, but is Vz = 4.3v @ .3A, so only about > >>1.5W. > >> > > > >No matter how you do it, if you aren't switching then you are > >dissipating it. And the combination of resistors or zeners > >or pass transistors all add up to that 7 watts of heat and > >you will have to get rid of it somehow. And heat sinks > >aren't free, either. > > > >And a mobile unit that's too hot to touch is rather embarrassing > >(don't ask how I know this:) > > > > Yes, my scheme was simply a way to distribute the heat, > so the 7805 wasn't dissipating all of it. In my system, > [15-5v]*.3A = 3W in 7805 w/o distribution. Removing 1W+ > to the zener helped cool everything down "enough" to apply > Roman's test --> take a baby's finger and hold it on the > hottest part of the ckt for 30 seconds; measure sound output > at 3 feet using Radio Shack sound meter; if >=105 dB, turn off > power. [jus kidding about the baby, folks]. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.