AVR-Chat

At 08:39 AM 9/23/04 -0700, you wrote:
>i have my project on its final stage, the problem is that the 7805 voltage 
>regulator heats up the heat sink too much. is this ok? i have inputs 
>between 12V - 18V. it is connected to a 6V SPDT reed relay switch, which 
>in turn, facilitates the switching of power source (main and batt).
>
>also, the relay coil is powered by only 5V (from the regulator) instrad of 
>6V, and sometimes it doesn't toggle the internal switch. is this because 
>of the coil supply of 5V instead of 6V? but sometimes it does work 
>properly upon switching. and sometimes, i need to tap the relay for the 
>internal switch to toggle.

I agree with the suggestions of looking for a 12V relay.  Try to find an 
automotive relay they have to withstand/work in a large voltage range so 
they may be able to operate over the input range.  In any case check the 
pull-in drop-out voltage specs first.

  On your dropout problem I have my doubts that the issue is driving the 
relay at 5V, the pull-in voltage on the relays I've
worked with usually have a larger range.  Another possibility is that your 
power supply impedance is too high and the voltage to the coil is dropping 
below your supply voltage.

You need to know how much current your circuit is drawing (and how stable 
the output supply is) before you can determine what your heat source is 
(and how reasonable it is).  A simple shunt measurement will help here.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III