I've read some very good threads here on power and heat dissipation in VRs and believe I understand (most of) it. I have a 5V circuit operating a 12V relay (nominal operating current of 53mA), an ATtiny85, a couple BJTs, and a few passive components. I measure 54mA across the circuit. I didn't give much thought to VR-generated heat but did the math nonetheless.

TO-92 The input voltage is 12.8V so the power consumed is 7.8V * 0.054A = 421mW. With a TO-92 case and Rja(ambient) of 157.4C/W, I get a 66C temp rise and when coupled with an ambient temp of 25C results in 91C if I understand correctly. Although it's not approaching the max junction temp of 125C, it's quite hot. Other LDO VRs (e.g., 7805) generated even more heat. I could reduce the heat with a series resistor but that can create other problems according to other threads I've read.

TO-220 The TO-220 has a Rja of 23.3 yielding a total temp (including ambient air) of 35C. I have it mounted horizontally using the PCB as a heat sink. More PCB space required but much lower temp.

Have I calculated the temperatures correctly? The relay is not energized 100% of the time (closer to 75% worst case) but the circuit is in a small enclosed ABS box. Is it fair to say that a TO-92 would be a poor choice for this circuit? Anything I haven't considered?

Relay datasheet:http://www.farnell.com/datasheets/1719945.pdf

TO-92 datasheet:http://www.farnell.com/datasheets/2019099.pdf

TO-220 datasheet:http://www.kynix.com/uploadfiles/pdf...M2940CT-15.pdf