7805 votlage regulator overheating

Thread Starter

lordofentropy

Joined Apr 10, 2010
58
I have a circuit which needs to operate on 5 VDC and measured current is 208.5 mA. The power supply generates +12V (max 2 amps). I wired the regulator as you normally would (per datasheet), .33uF cap on input voltage line to ground and a .1uF cap on output voltage to ground. However, with a heat mount on the regulator, and a high watt resistor (to bring the input voltage down some) the regulator still is over heating. I have never ran across this issue before using a 7805 (1amp max out). Any ideas, suggestions?

Thanks
 

paulktreg

Joined Jun 2, 2008
834
So how many volts have you got on the input to the 7805?

It only needs one or two watts and 7805 will feel hot but there's hot and damaging hot if you know what I mean.
 

Papabravo

Joined Feb 24, 2006
21,217
Here is the calculation:
Rich (BB code):
PD = (12V - 5V)* 0.208A ≈ 1.46 Watts
If the thermal resistance is 45°C/W and the ambient temperature 22°C then the part will be at
Rich (BB code):
22°C + 1.46Watts*45°C/W = 87.7°C
Don't know 'bout you I'm not going to put my lips on that!
 

Thread Starter

lordofentropy

Joined Apr 10, 2010
58
I've tried from 11V-7. something volts and results are all the same, at the lower voltages as an input going into the 7805 it just takes longer...and i am talking Temps above 100 degrees F. I've had 8-9 volts going into it and w/ a thermal couple saw temps reach well over 125 degrees, trying to push it and see if it ever would reach a plateau I stopped at 178 degrees and it was still rising.
 

bretm

Joined Feb 6, 2012
152
Papabravo did the math for you. 88 degrees C would be normal for the voltage and current you listed, and that's 190 Fahrenheit. Why do you consider that overheating? What is the part rated for? (125 deg C would be a common number)
 

bountyhunter

Joined Sep 7, 2009
2,512
I've tried from 11V-7. something volts and results are all the same, at the lower voltages as an input going into the 7805 it just takes longer...and i am talking Temps above 100 degrees F. I've had 8-9 volts going into it and w/ a thermal couple saw temps reach well over 125 degrees, trying to push it and see if it ever would reach a plateau I stopped at 178 degrees and it was still rising.
Which package device is the 7805?

You need to use a TO-220.
 

Papabravo

Joined Feb 24, 2006
21,217
I just looked at the 7805 datasheet. R-theta-J-A for the Fairchild Part in a TO-220 package is actually worse than I quoted above. It is 65°C/W. So plugging that into the calculation above makes it worse. This one most definitely needs a substantial heatsink.
 
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