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Old 28th June 2012, 08:09 AM   #1
mkusan is offline mkusan  Croatia
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Default LM317 safe operating temperature

I replaced 7805 regulator in my CD player with a small pcb with LM317 (TO-220) on it. I am measuring 80C/176F with an ambient temperature of 26C/78F. Obviously those temperatures were measured with the top off, so I can imagine when the lid is closed they are even higher.

I did not use the original heat sink on which 7805 was mounted to, that was not possible without further modification which is what I am trying to avoid obviously.

Data sheet mentions 125C/257F as max. operating temp, but since max. and optimal is not necessarily the same thing I would like to hear your advices.
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Old 28th June 2012, 08:36 AM   #2
MarianB is offline MarianB  Romania
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The heat sink you used is smaller than the original one? i'm asking this because LM317 supports about 0,5A more than 7805 so in theory it should stay cooler...What is the voltage that powers the 317's stabiliser?
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Old 28th June 2012, 08:44 AM   #3
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Quote:
Originally Posted by MarianB View Post
The heat sink you used is smaller than the original one? i'm asking this because LM317 supports about 0,5A more than 7805 so in theory it should stay cooler...What is the voltage that powers the 317's stabiliser?
That is NOT how things work!

The temp rise, other things being equal, is a function of actual dissipation (V across x I through).

Nothing to do with what it WILL support.

If you use a smaller heatsink it will get hotter. Personally, I do not like anything getting too hot to touch comfortably. 80degC is!
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Old 28th June 2012, 08:57 AM   #4
MarianB is offline MarianB  Romania
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Maybe i didn't make myself clear, as LM317 ( T220 device ) supports more current ( 1,5A as to 1A for 7805 ) it also supports more Pd, that is why i sayd it should stay cooler ( maybe a poor choice of words ), but i think it is clear what i ment.

On the other hand, i did NOT recomanded a smaller heat sink, i've just asked if he used such thing... if so than it is not ok, but we need to know the voltage that powers the stabiliser cus if it is possible to lower it down ( No more than Vout+3 or 4V ) than the Pd would be smaller so the device would stay cooler.

Last edited by MarianB; 28th June 2012 at 08:59 AM.
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Old 28th June 2012, 09:19 AM   #5
mkusan is offline mkusan  Croatia
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Quote:
Originally Posted by MarianB View Post
The heat sink you used is smaller than the original one? i'm asking this because LM317 supports about 0,5A more than 7805 so in theory it should stay cooler...What is the voltage that powers the 317's stabiliser?

Hi MarianB,
you are right, I used modest heat sink and the voltage input is 9.47V, so vin-vout difference is 4.47V.

Thanks
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Old 28th June 2012, 09:23 AM   #6
wwenze is offline wwenze  Singapore
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The 125C limit is for junction temp, which will at all times be higher than the case temp that you're measuring.

I'd keep case temperature to less than 100C.
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Old 28th June 2012, 09:24 AM   #7
MarianB is offline MarianB  Romania
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That is just why i asked about the heatsink cus if it were the same than it would make no sense for 317 to heat up more than 7805, also if the Vin-Vout difference were bigger than it could do with lowering it down so Pd would have been maller but as you say it is than it cannot be smaller than that, the only thing to do is get a bigger heatsink.
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Old 28th June 2012, 09:55 AM   #8
mkusan is offline mkusan  Croatia
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OK bigger heat sink it is then. What should I be aiming for, less then 60 degrees Celsius, or even lower is preferred?
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Old 28th June 2012, 10:02 AM   #9
MarianB is offline MarianB  Romania
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Yes, less than 60 degrees it would be best.
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Old 28th June 2012, 10:13 AM   #10
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I guess the other question that should be asked is safe for the device or safe for it's surroundings cooler means less stress on nearby components and also less likelyhood of developing dry joints.

Long legs, and a decent distance from other components should allow a higher operating temp, than mounted close to the board and close to other components.

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