lm7805 getting really, really hot

[rickselectricalprojects]

hi everyone
i made a 5v power supply using a 7805 regulator that i salvaged from an old tv. i have an input voltage of 12v and when i connected it to a dc motor which draws 0.8a the regulator got over 110 Celsius (or 230f for americans) and i have a heatsink on the regulator. i know that linear voltage regulators are very inefficient but should they get that hot?

there is a 10uf cap on the input and a 10uf cap and a 0.1uf ceramic cap on the output.

 

[Colin Mitchell]

7 x .8 = 5.6watts. Anything over 3 watts takes a lot of heatsink to keep the regulator cool.

 

[duke37]

Nothing to do with your problem but the 0.1μF capacitor should be close to the regulator, not spaced away from it.

 

[(*steve*)]

the temperature depends on the amount of heat generated and the rate at which it can escape (the temperature rises until the two are equal).

In your case, the heat comes from the voltage difference multiplied by the current.

Either:

a) use a larrger heatsink
b) reduce your input voltage to 8 volts or so
c) use a switchmode regulator.

 

[rickselectricalprojects]

Nothing to do with your problem but the 0.1μF capacitor should be close to the regulator, not spaced away from it.

why is that? will is help make the regulator more stable?
thanks for your help

 

[rickselectricalprojects]

the temperature depends on the amount of heat generated and the rate at which it can escape (the temperature rises until the two are equal).

In your case, the heat comes from the voltage difference multiplied by the current.

Either:

a) use a larrger heatsink
b) reduce your input voltage to 8 volts or so
c) use a switchmode regulator.

thank you for your help, i will try using a bigger heatsink.

 

[duke37]

The 0.1μF capacitor, together with its leads forms a tuned circuit. The regulator consists of a reference and a very high gain amplifier. If the tuned circuit is at a low enough frequency where the amplifier gain is high, then instability can occur.

Saying that, I have never encountered instability.