Re: VK2AUJ's µBITX project

Jerry Gaffke

Here's the LM7805 datasheet:

On page 4, says the absolute maximum allowable junction temp when operating is 150 C.
At the very bottom of page 4, says the junction-to-case-(bottom)-thermal-resistance
for the TO220 package is 1.7 degrees C per Watt.
Halfway down the page under note 3 it states "For the TO-220 package (NDE), θJA is 54°C/W and θJC is 4°C/W"
The  θJC  and the junction-to-case-(bottom)-thermal-resistance to be different terms for the same thing.
Oh well, let's assume it's 4 degrees C per Watt, the larger of the two.

I measure the current from my 12v supply into the LM7805 on the Raduino at 60 ma.
So the LM7805 is dissipating (12v - 5v) * 0.060amps = 0.420 Watts
And we can calculate the junction temperature to be 4 * 0.420 = 1.68  degrees C higher than the TO220 tab.
The junction temperature is just a couple degrees C above that of the metal tab you see on the outside of the package.

Conclusion:  That LM7805 should be quite comfortable well past 100 degrees C,
the temperature at which a wet finger will sizzle when you touch the tab. 

Also under note 3 on page four is this:  "If the die temperature rises above 150°C,
the device goes into thermal shutdown"
So if the LM7805 ever does get dangerously hot, the Raduino will lose power till things cool off.

The 110 degrees F that Skip reported translates to (110-32)*5/9 = 43.33 degrees C.
He's got over one hundred degrees C to go before there's trouble.
On the other hand, electronics failures become far more frequent as temperatures go up.
Nothing wrong with keeping things cool.

Many semiconductor devices spec an absolute max junction temp of 70 or 85 degrees C.
This regulator is designed to dissipate power, the 150 degree max junction temp is rather unusual.

Jerry, KE7ER

On Tue, May 1, 2018 at 02:55 pm, Skip Davis wrote:
Oh yes I remember now I added one here because I wasn’t happy how hot that regulator was getting. I think I measured the temp at over 110 degrees F. Nice set up you have.

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