TODO - copy from Example - Heatsink Sizing Thermal Calculations
My friend Emery. told me that really for a 5V linear regulator you shouldn’t exceed really more than 7V, since it just throws away all the excess as heat.
Dave Jones also showed a design flaw in a $400 rigol power supply that where the regulator was dissipating 5W which caused ripple on the output feeding the display circuitry and forcing the whole PSU to reset.
Since the power dissipation is just , and the maximum for e.g. a LM7805 is around 125C, and the rise is like 50C/W. We can see that we can only dissipate less than 3W without a heat sink.
So if you want to push the 5V linear regulator to drive a raspberry pi, which can consume up to 1.5A, you can only get away with a voltage difference of 2V, so only a 7V input.
So without going through the math, Emery was right.
Here are the calcs, per the guide at https://learn.sparkfun.com/tutorials/understanding-thermal-resistance/all:
