3/10/2024 0 Comments Lm7805 junction temperatureIf its all going to be on a board or in a box, then you have gained very little, at all. But, this strategy does not reduce the total power dissipation. Adding a series resistor (or diodes) as some may suggest, will reduce the voltage at the regulator input, reducing ITs power dissipation. You have several choices: (1) add a heat sink, (2) use a lower supply voltage, or (3) reduce the current, or (4) switch to a buck converter. This long story tells us that, for THIS component, 1W (or even 0.7W) really is quite a bit of power dissipation. But, you say, thats only inside? True, BUT, without a heat sink to remove heat, the tab and body will be ALMOST the same temperature as the chip, say 80C or so. At normal room temperature (25C), that means that the chip inside will be 25C + 65C = 90C. So, what does this mean? It means that 1W of power dissipation will make the junction about 65C hotter than local air temperature. Thats not so much you say? Well a TO220 package (like a 7805) has a thermal resistance of 65C/W from the internal chip to ambient (free air). Power dissipation is the product of the current and the voltage drop (Gee, same rule as a resistor! How strange!) But, this WAS recognized in the original post - just not carried to its logical conclusion. You are basically ignoring the critical fact about linear regulators.
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