# Voltage regulator getting extremely hot

I've built a LED PWM controller according to the below schematic and the voltage regulator is getting extremely hot (I can't touch the heatsink).

I've measured input and output voltages and currents and these are:
Vin = ~24 V
Vout = ~4.85 V
Iin = 80-95 mA
Iout = 80-95 mA

If my regulator is indeed L78S05 (because the package says UL75S05, but can't find any datasheet for this) then it should handle up to 2A easily and up to 35 V on input. Also if I understand correctly, it reduces the voltage by dissipating excess power as heat, so how hot would it get if it would be supplied by 35 V and 2A?

Unless there is something wrong with my construction and it shouldn't get so hot right now. If so, could you help me find what is wrong?

• Dropping 24V down to 5V with linear regulator is extremely inefficient. You can feel this inefficiency. It can handle what it says it can handle only if properly cooled. Your heatsink is probably not sufficient. Jul 26 '19 at 14:13
• 24v - 5v = 19v being dropped by the 7805. 19v * 0.1A = 1.9W... way too much power dissipation for this device. Consider a switcher instead, which is much more efficient and will not get as hot. Jul 26 '19 at 14:20
• So I assume if I would first drop voltage to 12 V with one regulator and then with second regulator to 5V they would not get so hot, but overall temperature inside the case would be the same, right? Jul 26 '19 at 14:42
• @rdtsc I see that these convertes has no way to attach heatsink, can I be sure that they won't get hot and melt? Jul 26 '19 at 14:45
• @Marek I would not speculate about temperature in the case, as it has more affecting factors. But yes, this might help to manage the temperature on each regulator. Jul 26 '19 at 14:59

• In your first paragraph you say the thermal resistance is 50 C/W and then talk about adding a 24 C/W heatsink. You are mixing up the $\Theta_{JA}$ and $\Theta_{JC}$ without being clear. Then in the second paragraph you state a new value without clarification. I know what you mean but the OP probably does not. Jul 26 '19 at 16:25