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7805 and 555 timer circuit heat. Answered

Hi all!

I made a test led blink circuit with a 555 timer on a bread board. Just a basic two led blink circuit you can find all over the place. I used a 7805 for a supply regulator with power from a 9v batt. The circuit functioned fine (to my knowledge), but the 7805 got REALLY hot. I've used 7805 in opamp circuits with a 7.2v batt and no detectable heat. Is what I'm experiencing normal? Or do you believe something I did was wrong. If I remember correctly (I'm really tired as I write this) i was using 100nf caps on both ends.

Thank you



8 years ago

The 7805 does tend to get hot, especially when there is a large voltage between the V_in and V_out. As long as you stay with in the 1A current limit you should be fine. (I doubt a blinking LED circuit is drawing anywhere near that.) If the case of the 7805 is getting very hot you should consider adding a heat sink to dissipate some of the heat. The regulator also has built in thermal protection, so if it was severely overheating the chip would have shut off.

The reason It gets hotter when being supplied by a 9V battery is because the energy from the voltage difference between the desired 5V needs to go somewhere.  In the case of linear regulators like the 7805 the energy is dissipated as heat.  If you want to know how much power is being dissipated as heat you can measure the current supplied by the regulator and multiply it by the 4V that the regulator is dropping before the circuit to get the power ( P = I*V ).