Hi. I'm finishing a project, and I'll take the atmega328 apart and use it without the arduino board
The whole thing will have a 15VAC PSU with a pcb with 7815 and 7915. I use this because a part of the project uses a 16f628 pic and some lm358 with +/- 15v rails, plus some DACs. So i'll distribute the power in two, +15 and -15 to this module with operationals, and +15 to the arduino part. Will the 7805 arduino regulator get hot? Maybe if I use a small heatsink it'll be better?
I think max current will be about 100mA on the pic module, and I don't know how much current the arduino needs. It has 3 4067, two 74hc595 and a lcd display. The transformer PSU theorically is 1A.
So, is this configuration OK?
thanks
Yes it will be a little hot, you should make a voltage divider near voltage regulator so it won't get excessive voltage. There is actually one topic in this forum about it, it should answer to your question. ATmega328 power requirements. - Project Guidance - Arduino Forum
The 7805 will get hot as the differential voltage will be in the order of 10 volts (@100mA).
Why not use an intermediate 7809 regulator between the 7815 and the 7805?
Yes there will be the extra small cost of an extra regulator but if you use the TO-220 packages and place a small heatsink on the regulators, it will be fine.
By the way, you should know that Arduino has 500mA fuse and you should not exceed this limit. 
Thanks. I suppose i'll use a 7809 before the 7805. Or a voltage divider would be the same?
I highly doubt it'll be over 300mA. Although I don't know how much current lcd displays need(will check)
thanks
capicoso:
Or a voltage divider would be the same?
Not really.
The resistors will also heat up and getting the heat away from resistors is much more difficult (and inefficient) than with a TO-220 which has been specifically designed for use with a heatsink.
B.T.W. don't forget the decoupling capacitors on the regulators.
EDIT:
Another option is to use a DC-DC convertor to bring the 15V down to 5V, although it's the most efficient method, it will be the most costly, might need extra filtering on the supply especially if you intend to use the ADC and some DC-DC convertors have a minimum load requirement.
Well I'll power it from 12v dc instead of 15v. I suppose with 12v the 7805 will be fine with a heatsink?
Why does arduino have a 500mA fuse?
capicoso:
Why does arduino have a 500mA fuse?
To protect the USB port of the computer powering it, so you don't destroy it if you use too much power.
// Per.