From: Winfield Hill
Subject: Re: LM317 or 7808 is better performance for 9V output ?
Date: 17 Dec 2002 03:33:00 -0800
Organization: Rowland Institute
References: <3DEDABCA.906FB592@fanwap.com> <3DFEB1CF.C472E3DA@fanwap.com> <3DFEE13A.414240C9@scazon.com>
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Paul Burke wrote...
> Tong Narak wrote:
>> Does 317 more hotter than 78xx ?
>> I don't care about noise.
>> I would like to know about power loss and heatsink.
>> I get 9v:
>> 317 need 14v+
>> 7808 need 10v+
>> Which one low power loss and require a smaller heatsink ?
> Look, someone told you his after your first post!
> If the voltage in is the same, the voltage out is the same,
> and the current is the same, both will generate EXACTLY the
> same amount of heat. Schoolboy physics. If you want a
> solution that runs cooler, you need a switched mode supply.
Tong, what are you trying to make? Perhaps we can help.
Are you still confused about the required minimum input -
output headroom difference, or "dropout voltage" for the
two parts? Both ICs are nearly the same, about 1.75V at
300mA at 25C, and a bit less as the parts warm up. If
you see 10v given as a 7808 minimum input, that's only
because it's putting out 8 rather than 9V. If you use
some resistors, etc., to boost the 7808's output by 1V,
it'll need 10.75 or 11V minimum input, just like the 317.
Actually, taking into account the issue of tolerances,
you'd want to design with a somewhat higher nominal input
voltage, because you want to provide for its tolerances,
e.g. a low ac line voltage (120Vac drops to 95V brownout),
or a dying end-of-life discharged battery, etc. In the
case of a ac line input, you also need to provide room
for rectified ripple voltage on the storage capacitor.
On Wed, 04 Dec 2002 Tong Narak email@example.com wrote:
> Hi all,
> A. LM317 require 5V head room.
> B. 78XX require 2V head room ( Is these correct ? ).
> I would like to set the output to 9V. Load ~300mA.
On Wed, 04 Dec 2002 Paul Burke firstname.lastname@example.org answered:
A. Not in my data sheet, it only wants about 1.75V at 25deg,
and just over 2V over the range (typical values). The 780x
are also quoted as 2V typical.