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.
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.
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.
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.