Experiment 7 - Heat Generation of the Voltage Regulator

A voltage regulator becomes very warm when under a heavy load. To avoid problems with your circuit, you should be aware of some details...

In the Experiment 2 you found that with a current of I_out = 0.5 A not only the load restistor, but also the voltage regulator heat up significantly.

This is not a threat to the voltage regulator 78xx since it has an overheating protection and shuts down. However, this can cause all sorts of problems with your circuit when a voltage regulator to shut down unexpectedly. You should understand how the heat is generated and how to take care of it.

Circuit diagram heat generation voltage regulator 7805.

Circuit heat generation voltage regulator 7805. (Enlarge)

The power dissipation P_D of the 78xx, and thus the generation of heat, is given by the formula...

P_D = (U_in - U_out) × I_out + U_in × I_GND

You can neglect the part (Uin × IGND)  as IGND is only a few μA.  Therefore, the calculated power dissipation... PD = (Uin - Uout) × Iout.      = (9.0 V - 5.0 V) × 0.50A = 2.0 Watt. The resulting heat must be dissipated and you might need a heat sink to dissipate enough heat, for the voltage regulator to remain functional. However, you can also try to limit the power dissipation...

Heat sink with voltage regulator 78xx (TO220-casing). (Enlarge)