Heat Sinks

In the last post, I briefly mentioned that I planned on adding a 'heat sink' to my motor driver board in order to prevent it from over heating.  In this post I will cover why a heat sink is often needed and what it does.

Definition 
A heat sink (a.k.a heat exchanger) is a part which transfers heat into the surroundings (air in most cases).  It is typically an additional metal part, usually Aluminum but can be any highly conductive material, which is adhered to the thing that is getting hot.  Additional explanation... Wikipedia definition.
The heat (or 'thermal') energy is exchanged (or 'transferred') in to the air through a mode called convection (mostly).  There are also two other modes which do contribute to some transfer, but tend to be relatively small.  They are known as conduction and radiation.

• Convection is the thermal energy transfer through fluids, like air (Link)
• If you hold your hand above to something hot, you can feel the heat mostly because of convection
• Conduction is the thermal energy transfer through a material, like metal (Link)
• If you touch something hot, energy is transferred into your hand by conduction
• Radiation is basically waves of thermal energy (e.i. infrared light) which are emitted by the thing that is hot (Link)
• If you hold your hand next to something hot, you can feel the heat mostly because of radiation.
• The earth is warmed by radiation from the sun.  In order for radiation to be of concern, the difference in temperature between the two objects (like the sun and earth) must be very large.

Overview of a Heat Sink

So, when do you need a heat sink?  
Well, that is not always an obvious question... it depends what you are doing, what you are doing it with, and where you are doing it.

What you are doing?
Are you driving a 0.5 Amp (at stall) motor with a 10 Amp motor driver?  Then no.
Are you driving a 6.0 Amp (at stall) motor with a 2.4 Amp motor driver (like me)? Then yes!
Are you driving a 6.0 Amp (at stall) motor with a 6.0 Amp motor driver?  Then probably...

What are you doing it with?
When using a motor driver, pull up the specification sheet and determine the maximum constant operating temperature.  In my case, I am using the L293D chip (Link to data sheet) which has an operating temperature from 0C to 70C.  If you have an IR thermometer, measure the temperature during heavy use.

Where are you doing it?
If you had a motor driver chip on a RC helicopter which is exposed to the wind created by the blades (that is key), you may not need a heat sink.  In this case, the wind causes forced convection into the air. Forced convection allows more thermal energy to be transferred into the air.  That is why you will often find a heat sink with a fan strapped to the top (like the CPU on the motherboard of your computer).

In general, if it gets hot to the touch during operation, I would recommend adding a heat sink.  If it is added correctly, you will only help prolong the life of the chip.

Finding a heat sink
You will typically find a heat sink in your local electronics shop or maybe even a hardware store.  If not, try pulling apart old electronics like computers or an Xbox.  After my Xbox showed me its red 'ring of death', I took the liberty of ripping it apart to salvage anything.  There were two of the coolest (ha) heat exchanges inside.

In most cases, finding a heat sink with fins is preferable.  Some of the really cheap versions are simply a bent piece of sheet metal... essentially creating two fins.  The key to an effective heat sink is surface area.  The more surface area your heat sink has, the better it will transfer the heat to the surroundings.
The image below shows a 'good' heat sink.  However, I am somewhat bias to the cool look of these.  An inexpensive bent sheet metal heat sink may be acceptable in some applications.

Heat Sink with Fins

In order to attach it to the chip, you should find yourself some thermally conductive grease or epoxy (a.k.a heat sink compound).  Simply using common household glue or bolting the heat sink to the chip is not sufficient (The generic glue could create a thermal barrier, having low conductivity, and restrict the flow of energy.  The same is true with an air gap between the chip and heat sink).  Conductive grease or epoxy usually has a high content of conductive particles, like silver, to help the heat transfer through it.  Apply a thin layer to the chip (avoiding bubbles).  The thinner the gap the better (unless you have bubbles). (Search Google for heat sink compound for more info).

Hope that helps.