Human Touch Temperature

A lot times, we use our finger to judge how hot a device is and whether it has adequate heat sink. The late Bob Pease describes a 5-second rule in his well-known book "Troubleshooting Analog Circuits":  "A good rule of thumb is the 5-second rule: If you can hold your finger on a hot device for 5 seconds, the heat sink is about right, and the case temperature is about 85°C."

NASA Johnson Space Center actually conducted a study on human touch temperature, "A New Approach to Defining Human Touch Temperature Standards" by Ungar and Stroud.  The onset of pain threshold is about 44°C.  The touch temperature is material dependent.  For Aluminum, 1 second touch corresponds to about 70°C, and 5-second touch corresponds to about 55°C.  Bob was probably talking about plastic case, 5 seconds is about 85°C, then 1 second is about 140°C, 2 seconds is about 110°C.