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This question already has an answer here:

I'm using 9v regulator (l7809cv) with 12 1,5v AA batteries in series so that my application gets both 18v and 9v for my Arduino Uno r3 board.

I've been running it for about 15min without any problems, but when I touch the l7809 it feels really hot after 10min or so.

I've measured a current between the batteries and the l7809 to be steadily around 100mah. I haven't exceeded the maximum voltage input ratings, and as far as I can see, neither the current ratings (it says in the datasheet that it's "internally limited")

Maximum temp. appears to be 0 - 150 celcius, which seems to be more than enough.

So my question is, is it OK that the component gets so hot that it hurts touching it although it probably isn't 150 c, or should I make some kind of heat sink for it?

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marked as duplicate by Phil Frost, Nick Alexeev, PeterJ, Matt Young, Dave Tweed Jul 24 '13 at 23:10

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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As a ballpark figure a TO220 component can dissipate 1 Watt without a heatsink (but proivided that the air can flow around it). 9V * 100 mA = 0.9W, so you are near the edge but still within the limit.

When dissipating 1W the component will definitely get too hot to touch. If that is a problem (for other components, or for humans) you will have to provide some heatsink. Even a very small heatsink, let's say rated at 20C/W, will reduce the temperature considerably.

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