# Heatsink calculation

Discussion in 'General Electronics Chat' started by Rooskey, Mar 31, 2014.

1. ### Rooskey Thread Starter New Member

Oct 18, 2011
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0
Hi, I am using a lm317 voltage regulator to power a pump.Vin to the regulator is 15V, Vout =11V.Current to the pump =2A.

So I take it that power dissipated = (15-11)*2 = 8W

Tmax = 60, Tamb = 20, so the max thermal resistance of the heatsink would be (60-20)/8W =5 degrees C/W.

How do I size a suitable heatsink from this?

Thanks,
M

2. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Personally, I'd check the manufacturers' datasheets for advertised heatsinks and see what sizes/formats have the required thermal resistance or better.

3. ### t06afre AAC Fanatic!

May 11, 2009
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Is not the max current for LM317 at the best 1.5A and less in some cases depending on package type. Most often electric motors have a broad voltage range. Are you sure you need a regulator at all. What kind of pump are you using any link or datasheet?

4. ### Rooskey Thread Starter New Member

Oct 18, 2011
3
0
I am controlling a diaphragm pump between 6 and 11V and when it runs at 11V it take approx 2 Amps.From the datasheet the LM317 has a max of 1.5 (Iwas looking at the typical Io =2.2A.Thanks,

http://www.fairchildsemi.com/ds/LM/LM317.pdf

5. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,973
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Be aware at startup the pump current draw is probably a lot more than 2A. The LM317 will be stressed even at 2A and should have an auxiliary transistor to pass the extra current.

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
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In LM317 datasheet we can find Rthjc = 5°C/W for TO220 case and Tjmax = 125°C
http://www.st.com/web/en/resource/technical/document/datasheet/CD00000455.pdf

The power dissipated in your circuit is equal to 8W.

So RTh = (120 - 20)/8W = 12.5°C/W.

So you need a heatsink with Rth < 12.5 - 5 = 7.5°C/W
For example this will do the job
http://assmann.us/specs/ASS_0867_HSrev03(V7477xC).pdf
And you can pick heatsink here
http://www.digikey.com/product-sear...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

http://pl.mouser.com/Thermal-Management/Heat-Sinks/_/N-5gg0?P=1yzxb21

7. ### Rooskey Thread Starter New Member

Oct 18, 2011
3
0
Thanks a million for that,really appreciate it,M