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7805: when to use a heatsink???

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csdave

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Hi all,
same circuit, new question ;)
I am using a 7805 to power a circuit using around 200mA but let's say 300mA to be on the safe side.
The input to the 7805 is a 9V transformer (after rectification obviously). Since 9V is effective voltage, the power dissipated by the 7805 should be 4*300 = 1.2W.

Does that require a heatsink? I would tend to say so, as I read somewhere, but can't remember where that the thermal resistance of the 7805 is 65K/W.
But how big should it be?

The project is to be contained in a plastic derivation box.

thanks

Davide
 

yes. it requires a heatsink.
 

There is a graph in the datasheet

LM78xx_power_dissipation.gif

As you can see you can dissipate about 1.25W without heatsink even at 70 degree ambient temperature.

Alex
 
There is a graph in the datasheet

View attachment 55989

As you can see you can dissipate about 1.25W without heatsink even at 70 degree ambient temperature.

Alex
Thanks Alex,
then I wouldn't need one... right?

---------- Post added at 18:11 ---------- Previous post was at 18:10 ----------

yes. it requires a heatsink.


thanks, but can you be a little more specific? According to the graph above I wouldn't.
 

Assuming that your ambient temperature in the box will be below that temperature, no you don't need a heatsink.

Alex
 

well if the 7805 doesn't heat it above, then it won't ... what does it mean exactly that it can dissipate power?
 

datasheets give the correct figures.
provided you operate in the region you see.

on the otherhand ,
in practice , you put a heatsink when Pd exceeds approximately 800 to 900mW(close to 1W).

you can have a safe and longlife device.
 

It can handle that amount of power consumption

Answers.com - What is Power dissipation

Alex

Yep, I knew this, but what I find hard is to predict the resulting temperature of the device, let alone of the box ;)

---------- Post added at 18:26 ---------- Previous post was at 18:25 ----------

datasheets give the correct figures.
provided you operate in the region you see.

on the otherhand ,
in practice , you put a heatsink when Pd exceeds approximately 800 to 900mW(close to 1W).

you can have a safe and longlife device.

so how should I choose the heatsink? Should I just bolt it to the device or do I need thermal paste?
 

Can't help you on the temperature calculation.
A thermal paste will always give lower thermal resistance between the semiconductor case and the heat sink, this will improve the heat transfer and will keep the device at a lower temperature.
It is up to you to use it or not since this is a very low consumption application.

Alex
 

Can't help you on the temperature calculation.
A thermal paste will always give lower thermal resistance between the semiconductor case and the heat sink, this will improve the heat transfer and will keep the device at a lower temperature.
It is up to you to use it or not since this is a very low consumption application.

Alex

yep guess so ;) I am going to shop for a heatsink tomorrow and see what they have as thermal paste.

thanks a lot!
 

Well the ROJA = 65°C/W

If your ambient temperature = 25°C
Tj = 25°C+65°C/W*(.3A * (9V-5V)) = 103°C that is below the 150°C max for Tj

And datasheet gives a 2W operation without heatsink.

However in practice there is a protection circuit inside the 7805 that avoid the operation for more than 0.5W that is for certain circuits not for all.

Regards
 

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