Lm317

[audioguru]

With proper cooling, an LM317T can dissipate 21.25W of heat but its chip will be at its maximum allowed temperature which is very hot.
If it is turned on and off many times then it can fail from thermal fatigue.
I never operate anything at its maximum rating.

 

[netfreak]

um ok sir, and one thing more
Lets suppose im having the input output differential voltage to be = 9V and my current to be= 1A. Then my power dissipation would be equal to 9V x 1A= 9 Watts right?
And I can work on while using this chip continuously for even straight 10 hours right(while this chip is in operation with the same amount of 9watts power dissipation)?

 

[audioguru]

um ok sir, and one thing more
Lets suppose im having the input output differential voltage to be = 9V and my current to be= 1A. Then my power dissipation would be equal to 9V x 1A= 9 Watts right?

Yes.

And I can work on while using this chip continuously for even straight 10 hours right(while this chip is in operation with the same amount of 9watts power dissipation)?

Only if it is cooled enough so that its chip is below its maximum allowed temperature.

 

[netfreak]

Thankyou very very much Sir You just don't know how much have you helped me a lot
I am working on some mini projects and with the help of your guidance I am able to step more and more ahead

 

[netfreak]

Hello Sir, I am again here with another question now. What if I use only a heatsink and no cooling system/fan etc? How much would be the permissible amount of power to be dissipated in that case then? How can I calculate this Sir?

 

[audioguru]

I already showed you how easy it is to calculate a heatsink requirement. I don't know the spec's for the biggest heatsink available in your country.
Ask your teacher to explain it again or look in your text book.

 

[netfreak]

No no Sir. . you had told me how to calculate the allowed dissipation of power for having the chip without heat-sink or having the chip with heatsink+proper cooling
Am asking how do I calculate my allowed dissipation of power for having the chip with heatsink ONLY & no cooling. What would be the allowed value of dissipation of power in that case for lm317?

 

[audioguru]

Look at the datasheet for the LM317:
1) It says its maximum allowed chip temperature is 125 degrees C but you probably should not operate it so hot.
2) It says that its typical thermal resistance from its chip to its case is 4 degrees C per Watt for its "T" case.
3) You probably want an insulator that has a thermal resistance of 0.5 degrees C per Watt but it might not be available in your country.
4) Your ambient temperature might be 40 degrees C.
5) Then the allowed temperature rise is about 80 degrees C and the total thermal resistance is 4.5 degrees C per Watt.
6) Then with a "perfect" heatsink (impossible) the dissipation can be 80/4.5= 17.8W.

Simply do the arithmatic to determine the thermal resistance of a suitable heatsink.
If you can buy a heatsink with a thermal resistance of 2 degrees C per Watt then the total is 4.5 + 2= 6.5 degrees C per Watt. The maximum allowed dissipation is 80/6.5= 12.3W.

Maybe you want to use a small heatsink rated at 10 degrees C per Watt. Then the total is 4.5 + 10= 14.5 degrees C per watt and the maximum allowed dissipation is 80/14.5= 5.5W.

 

[netfreak]

Um ok sir, thankyou thankyou
and one thing more, I have got these few heatsinks but I don't know what is it rated at .. nor did the shopkeeper know. So by which way I can find its thermal resistance Sir?

 

[alec_t]

Look on the web for heatsinks of similar size and shape to yours and check their specs.

 

[audioguru]

I have got these few heatsinks but I don't know what is it rated at .. nor did the shopkeeper know. So by which way I can find its thermal resistance Sir?

I KNEW that you would say this!
Your world is completely different to my world.

 

[netfreak]

I have looked around on the internet and came to know that this is my heatsink that I have with myself at home too : https://www.futurlec.com/Heatsinks/TO220LGBA.shtml but the problem is that I cannot find its datasheet anywhere :/

 

[audioguru]

Your heatsink is tiny. I also could not find a datasheet for such a cheap little one. Yours looks like it was cut from a longer one with a saw.
Its thermal resistance is probably 15 degrees C per Watt so the TO-220 device will be at its max allowed chip temperature with only 6W of dissipation.

 

[bountyhunter]

Um ok sir, thankyou thankyou
and one thing more, I have got these few heatsinks but I don't know what is it rated at .. nor did the shopkeeper know. So by which way I can find its thermal resistance Sir?

That's actually very easy: you attach a power diode (rectifier) to the heatsink then use a power supply to put current through it. Measure the voltage drop across the diode, multiply times the current and that tells you the power bein dissipated. Use a thermal probe or themocouple to measure the heatsink temperature near the diode and then you can calculate the thermal resistance.

 

[netfreak]

@ audiogoru,
Ok thankyou Sir That was quite helpful

@bountyhunter
ok thankyou very much Sir It really increased my knowledge and am surely gona do this