AVR-Chat

At 10:39 AM 9/23/2004, jay marante 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.
> 
>thanks for the help.

Power the relay from the 12V side, and add a series resistor if needed. 
You obviously need to shed some load from the 5V supply.

If you can lower the input voltage by any amount, that will help. 
If nothing else, several 1n4001 diodes in series will help spread out the heating.