Heat Sink gets heat up on LM7808

GopherT

Joined Nov 23, 2012
8,012
20 posts and still no schematic. Looks like the OP doesn't want a real answer..

ak
Cut him a bit of slack, he is either an early riser ( posted at 5am) or he lives on the other side of the big blue marble.

EDIT: but he did say a schematic is coming, but nothing yet.
 

DickCappels

Joined Aug 21, 2008
6,099
Your regulator is probably fine.

From the Fairchild datasheet:
" Each type employs internal current limiting, thermal shut-down, and safe operating area protection."

This means that if your 7805 is still regulating, it is not too hot.

The maximum operating temperature is 125°C, water boils at 100°C, and it is unlikely you can hold your hand in boiling water for long at all :)
 

hp1729

Joined Nov 23, 2015
2,304
Your regulator is probably fine.

From the Fairchild datasheet:
" Each type employs internal current limiting, thermal shut-down, and safe operating area protection."

This means that if your 7805 is still regulating, it is not too hot.

The maximum operating temperature is 125°C, water boils at 100°C, and it is unlikely you can hold your hand in boiling water for long at all :)
125 deg where? On the heat sink or on the die itself? What counts?
 

dl324

Joined Mar 30, 2015
9,630
125 deg where? On the heat sink or on the die itself? What counts?
It's the junction temperature that counts. At a dissipation of 5W, the temperature differential between case and junction is 25C.

The regulator has thermal protection, so it should be able to protect itself from over temperature.
 

hp1729

Joined Nov 23, 2015
2,304
It's the junction temperature that counts. At a dissipation of 5W, the temperature differential between case and junction is 25C.

The regulator has thermal protection, so it should be able to protect itself from over temperature.
The original poster mentioned that the output had dropped a bit to 4.90 V. Initially I took that as excessive current. Could it be over temp kicking in?
 

dl324

Joined Mar 30, 2015
9,630
The original poster mentioned that the output had dropped a bit to 4.90 V. Initially I took that as excessive current. Could it be over temp kicking in?
Could normal operation, 4.8-5.2V according to the datasheet, or any combination of over temp, dropout/ripple, oscillation, ... After a couple dozen posts, we still don't have a schematic...
 

panic mode

Joined Oct 10, 2011
1,812
you can drop supply voltage to reduce voltage drop across regulator. this will distribute load and your heat sink will be cooler. for example put power resistor or couple of diodes before regulator. also consider using SMPS, there is tons of cheap DC/DC regulators that will operate in wide range and will work with considerably larger output current without any heat.
 
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