Heat Sink gets heat up on LM7808

Thread Starter

Vindhyachal Takniki

Joined Nov 3, 2014
483
I have 12V input on LM7805, & output load current is aroung 600-700mA.
I have connected a large aluminium heat sink with fins on it. Problem is that in 5 minutes of operation heat sink gets too hot, so that i cannot even hold it with hands for more than few seconds.
Can my circuit works for continuous 1 day for that? or I should go for some dc-dc.
 

nerdegutta

Joined Dec 15, 2009
2,676
Hi.

Please upload you schematic diagram, and a picture of your circuit. Did you solder it yourself, or is it on a bread board?

Is it a 7808 or a 7805?
 

hp1729

Joined Nov 23, 2015
2,304
I have 12V input on LM7805, & output load current is aroung 600-700mA.
I have connected a large aluminium heat sink with fins on it. Problem is that in 5 minutes of operation heat sink gets too hot, so that i cannot even hold it with hands for more than few seconds.
Can my circuit works for continuous 1 day for that? or I should go for some dc-dc.
LM7805?
Is dropping 7 Volts at 800 mA. Yep, might get warm. It could also be oscillating, but I suspect you would know that.
 

GopherT

Joined Nov 23, 2012
8,012
I have 12V input on LM7805, & output load current is aroung 600-700mA.
I have connected a large aluminium heat sink with fins on it. Problem is that in 5 minutes of operation heat sink gets too hot, so that i cannot even hold it with hands for more than few seconds.
Can my circuit works for continuous 1 day for that? or I should go for some dc-dc.
The 7V drop x 700 mA is 5 watts. That is a big heat sink. It should also get forced air cooling from a fan.
You could use a DC/DC converter and that will reduce the heat.

You could also get a different SMPS supply to power your project (e.g. Use a cell phone charger for the 5v source) or any other DC source that is not 12 V

Finally, you can make sure that your circuit needs 5 V. Some CMOS chips can handle higher, LEDs can be powered with higher if you use appropriately sized current limiting resistors. They way, all the heat is dissipated over more components instead of one poor little 7805.
 

dl324

Joined Mar 30, 2015
9,637
Problem is that in 5 minutes of operation heat sink gets too hot, so that i cannot even hold it with hands for more than few seconds.
Being too hot for you to touch isn't a practical indication of a component being stressed.

LM7805 will operate at a junction temperature of 125C. Thermal resistance from junction to case is 5C/W, so it won't start temperature throttling until the case is 100C. Unless you're a professional chef, you'll find that temperature too hot to tolerate even briefly; but the part will be within it's specs.
 
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