PRISM Forum - Message Replies
Topic: PRISM Questions and Answers
Topic Posted by: SRC
Date Posted: Wed Jan 12 8:33:33 US/Eastern 2000
Topic Description: Welcome to the PRISM forum! Please feel free to post your questions and comments about the PRISM assessment software here.
Posted by: Sue Hedrick
Date posted: Wed May 3 16:43:04 US/Eastern 2006
Subject: Resistor calculation
With regard to resistor calculations, how can I determine theta EC. I had thought that it was calculated using (storage temp - max operating temp) / resistor power rating. This gives me huge numbers, greater than 600 deg C / W. What should I be doing?
Reply Posted by: David Dylis
Organization: System Reliability Center (SRC)
Date Posted: Thu May 4 8:34:14 US/Eastern 2006
PRISM provides five different methods for determining the part temperature of resistors:
Ambient Temperature + Default Temperature Rise
Ambient Temperature + Actual Temperature Rise
Ambient Temperature + Thermal Resistance (Theta-EA) * P
Case Temperature + Thermal Resistance (Theta-EC) * P
Ambient Temperature + Delta-T * Stress
The thermal resistance, theta-EA (element-to-ambient) and theta-EC (element-to-case) are are typically not calculated by a user. Information for these values would need to be obtained from the device manufacturer. When information is not available, it is recommended that one of the alternative methods be used to determine resistor part temperature.