Digest Number 160
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> I am using a 7805 regulator with a small heatsink. I had it running for
> some 30 minutes, and the truth is that I couldn't touch the reg or
> heatsink, as it was so hot. I understand that heat dissipation is a
> function of input voltage, but is there a way to reduce this dissipation,
> also, what heat levels can be expected? I am using around 200mA of the 1A
> the regulator delivers, so I don't see an over-current problem. One third
> question, are the capacitors at the input and output of the regulator
> strictly necessary?
>
The power dissipated is a function of the voltage AND current. A 78xx
series regulator needs about 2.5V overhead. If you are running 12V in you
have to drop 4.5V(12-5-2.5). 4.5x.2=.9W . At lower currents you can have a
higher input voltage and vice-versa. Note that the 1A rating depends on
keeping the case at 25 degrees C.
The output cap is almost always needed. The input cap can sometimes be
left off if the source supply is near and clean DC(i.e. a battery or previous
regulator).
> some 30 minutes, and the truth is that I couldn't touch the reg or
> heatsink, as it was so hot. I understand that heat dissipation is a
> function of input voltage, but is there a way to reduce this dissipation,
> also, what heat levels can be expected? I am using around 200mA of the 1A
> the regulator delivers, so I don't see an over-current problem. One third
> question, are the capacitors at the input and output of the regulator
> strictly necessary?
>
The power dissipated is a function of the voltage AND current. A 78xx
series regulator needs about 2.5V overhead. If you are running 12V in you
have to drop 4.5V(12-5-2.5). 4.5x.2=.9W . At lower currents you can have a
higher input voltage and vice-versa. Note that the 1A rating depends on
keeping the case at 25 degrees C.
The output cap is almost always needed. The input cap can sometimes be
left off if the source supply is near and clean DC(i.e. a battery or previous
regulator).
Comments
> The power dissipated is a function of the voltage AND current. A 78xx
>series regulator needs about 2.5V overhead. If you are running 12V in you
>have to drop 4.5V(12-5-2.5). 4.5x.2=.9W .
Sorry, Mike, but the above is not correct. The dropout voltage only
reflects how much higher the input voltage must be than the output for
correct operation. The dissipation is calculated by the total voltage
across the regulator X the current. In other words, your figures above
should be (12 - 5) X .2 = 1.4 Watts.
dwayne
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
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