Heat Sink

In use, a regulator IC will get quite hot, so a heatsink will need to be attached to it to dissipate the heat.  The type of heatsink you choose depends on the regulator's case style, the amount of heat it must dissipate, and the way in which you wish to mount it.

The examples shown below all suit the 78xx, 78Mxx, and 78Sxx regulator series TO-220 case style. It has a mounting hole for the regulator, and also has a mounting hole for fixing to a board.  

Choosing a heat sink.

Heatsinks are rated by their 'thermal resistance' (R_th) given in °C/W.  For example, a rating of 2°C/W means the heatsink (and therefore the regulator attached to it) will be 2°C hotter than the surrounding air for every 1W of heat it is dissipating.  Note that a lower thermal resistance means a better heatsink.

To determine the heatsink rating required, follow these steps:

1. Work out the thermal power to be dissipated (P)
   To do this, use the following formula...
   
   
   
   where P is the power to be dissipated, V_s is the input voltage to the regulator, V_r is the regulated voltage, and I is the current being supplied to the connected circuit.  For example, a 5V regulator supplying 0.5A and powered by 12V supply, would dissipate,
   
   
   
   Note the use of (12 - 5) = 7V which is the voltage difference across the regulator.  Clearly, lower supply voltages are better so long as they are a few volts above the regulated output.
    
2. Find out the maximum operating temperature of the regulator (T_max)
   This can be found from a catalogue or datasheet.  For a typical 78xx series regulator, T_max = 125°C.
    
3. Estimate the maximum ambient (surrounding air) temperature (T_air)
   If heatsink is not in a case, T_air = 30°C is reasonable, but inside a case it will be higher (perhaps 40 or 50°C) allowing for everything to warm up in operation.
    
4. Work out the maximum thermal resistance for heatsink (R_th)
   Use the following formula...
   
   
   
   where R_th is the thermal resistance, T_max is the operating temperature from (2) above, T_air is the ambient temperature from (3) above, and P is the power to be dissipated from (1) above.
   
   So with the example figures given above...
   
   
    
5. Choose a heatsink with a thermal resistance which is LESS than the value calculated above (remember a lower value means better heatsinking!)  In this case, 10°C/W would be a sensible choice to allow a safety margin.  A 10°C/W heatsink dissipating 3.5W will have a temperature difference of 10 x 3.5 = 35°C so the temperature of the regulator will rise to 30°C + 35°C = 65°C which is safely less than the 125°C maximum.

The heatsink shown on the far right above can dissipate 9.5°C/W and so would be ideal for use in this example.