# Design Heat Sink!!!!!!!!!!!!!!!!!

Discussion in 'The Projects Forum' started by sneh_km, Apr 7, 2008.

1. ### sneh_km Thread Starter New Member

Apr 7, 2008
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We are facing a problem related to heat sink design for ICs LM317 and LM7805 for power supply.....we are confused wether the design depends on the area of the heatsink or something else....please tell us how to design it.....we have to submit our project urgently....!!!!!!!!!!

2. ### DC_Kid Distinguished Member

Feb 25, 2008
638
9
i'm rather certain the complete surface area of the heatsink is a design metric when engineering the power management of a "power" device. however, common sinks have thermal ratings which need to meet or exceed the operational specs of your design, so choosing a sink becomes a fairly simple task.

but to answer your question, a short list of metrics when designing the sink:
1. area
2. geometric shape
3. sink material, thickness, etc
4. forced or free air flow
5. sink paste

LM7805 (5v 1amp to220) - a 2" piece of 1x 1/8" thick aluminum flat stock bolted to 7805 should suffice.

May 16, 2005
5,072
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4. ### Audioguru New Member

Dec 20, 2007
9,411
895
The amount of heat that the regulators dissipate is their input voltage minus their output voltage times their load current. With a huge aluminum finned heatsink their max allowsd dissipation is about 18W each.
How much dissipation is each?

5. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
One thing unmentioned, the heatsink 'coating' is important. Yes, a bare aluminum heatsink dissipates less than a black annodized heatsink.. Various coatings have different radiation rates..

Steve

6. ### John Luciani Active Member

Apr 3, 2007
477
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To determine the temperature rise you need to calculate the junction
to ambient thermal resistance (Tja) ---

Tja = Tjc + Tcs + Tsa all the dimensions are DegC/W

Tjc thermal resistance between the IC junction and IC case. This is
from the IC datasheet.

Tcs thermal resistance between the IC case and your heatsink. This
should also be in the IC datasheet. Usually you place a thermal
compound between the IC case and the heat sink to decrease the
thermal resistance. The decrease occurs because the thermal
compound fills the air gaps between the two surfaces and conducts
heat better than the air.

Tsa thermal resistance between the heatsink and ambient air. This is
in the heatsink datasheet. Most heatsink datasheets specify Tsa
with and without airflow. You can determine if you need a fan by
reviewing this number.

For a single heatsource the junction temperature would be

Tj = Tja * P + Tamb (DegC)

where P is the power dissipated in the IC and Ta is the ambient
temperature.

(* jcl *)