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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
 

srizbf

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yes. it requires a heatsink.
 

alexan_e

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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
 

csdave

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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.
 

alexan_e

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

Alex
 

csdave

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

srizbf

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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.
 

csdave

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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?
 

alexan_e

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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
 

csdave

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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!
 

Miguel Gaspar

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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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