# Heatsink calculation

#### Rooskey

Joined Oct 18, 2011
3
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

#### Alec_t

Joined Sep 17, 2013
10,589
Personally, I'd check the manufacturers' datasheets for advertised heatsinks and see what sizes/formats have the required thermal resistance or better.

#### t06afre

Joined May 11, 2009
5,934
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?

#### Rooskey

Joined Oct 18, 2011
3
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

#### Alec_t

Joined Sep 17, 2013
10,589
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.

#### Jony130

Joined Feb 17, 2009
5,089
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-search/en?pv357=2&pv922=14&FV=fff40012,fff80068&k=Heat+Sinks&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

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