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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

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  • \$\begingroup\$ 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. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Mar 15, 2016 at 20:48
  • \$\begingroup\$ 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. \$\endgroup\$
    – Chris Stratton
    Commented Mar 15, 2016 at 20:54
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    \$\begingroup\$ @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. \$\endgroup\$
    – unix
    Commented Mar 15, 2016 at 20:55
  • \$\begingroup\$ Did the regulator on the breadboard version get hot, or did you only conclude it would from the model? \$\endgroup\$
    – Chris Stratton
    Commented Mar 15, 2016 at 20:56
  • \$\begingroup\$ @Chris It did not get hot. I was reading over the datasheets, was curious to do the math, and surprised by the resulting calculations. \$\endgroup\$
    – unix
    Commented Mar 15, 2016 at 20:59

1 Answer 1

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Yes you have calculated the temp correctly, a simple units\dimensional analysis will tell you that.
Your options are: 1) Switch to a DC to DC 7805 drop in compatible regulator. (pricey) 2) Switch to a different package (more surface area)
3) Put a heatsink on it, this could be as simple as a few square cm's of aluminum to a full blown heat sink with fins on it.

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    \$\begingroup\$ Dimensional analysis will tell you if you're wrong, but it can't tell you that you're right. I have a nice wrench that measures energy, apparently.. (Newton-meters). \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Mar 15, 2016 at 21:45
  • \$\begingroup\$ That's why I wrote unit\dimensional... \$\endgroup\$
    – Voltage Spike ♦
    Commented Mar 15, 2016 at 22:19

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