LM2940 is only rated to max 26V. The 7805 and 7812 etc... might have an absolute rating of 35..40V, but it doesn't have enough margin to operate with a 36V input.
You cannot put a resistor on the front of it to fix it. You could try a zener regulator combo to reduce the voltage prior to the regulator (transistor + zener) which would help burn some of the power off... but it's just the wrong approach.
Using a massive heatsink has limited benefits. Your creating a lot of heat and stressing the silicon with high temperatures.
The answer is... it's not a good solution to derive 5 and 12V from a 36V power supply with a linear regulator. Perhaps with a switch mode regulator, but the complexity of choosing the right part and associated components , wiring configuration and making sure it works across the desired current range is not something people want to mess with for hobby CNC. Plugpacks are cheap, reliable, easy to get and inexpensive.
It would be realistic to have a 5V regulator running from the 12V plugpack... otherwise, just get another plugpack for the 5V.  |