# Voltage Regulator Heat Concerns

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

TO-92 datasheet

TO-220 datasheet

• I don't see how the relay current or duty cycle figures into this- it's not going through the regulator, right? So your ATtiny85 circuit is drawing 54mA plus the 53mA the relay draws? Total 108mA? That's a lot. Commented Mar 15, 2016 at 20:48
• The heat generated by any linear regulator should be the same - they're just controlled power resistors after all. If you want a check on your numbers, breadboard it and see. Chances are an SOT223 regulator with moderate PCB heat sinking can handle your needs while taking up a lot less space than a TO220. Chances are also you should be able to get your current requirement to a tiny fraction of what you have now, especially in the time average - below 1 mA may well be possible by sleeping the ATtiny and not leaving voltage across resistors - mostly you'd see the regulator's quiescent current. Commented Mar 15, 2016 at 20:54
• @Spehro Pefhany Correct. Can't believe I missed that. It is a 12V relay powered directly from the 12.8V supply, not the VR. When I was operating this on a breadboard, I completely forgot that the relay shouldn't be in the measurement. That leaves only the uC and BJTs. I'll have to redo the math now.
– unix
Commented Mar 15, 2016 at 20:55
• Did the regulator on the breadboard version get hot, or did you only conclude it would from the model? Commented Mar 15, 2016 at 20:56
• @Chris It did not get hot. I was reading over the datasheets, was curious to do the math, and surprised by the resulting calculations.
– unix
Commented Mar 15, 2016 at 20:59