SRC Forum - Message Replies

Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

Original Message:

Posted by: Sue Hedrick
Date posted: Mon Mar 5 16:01:08 US/Eastern 2007
Subject: Resistor Calculation
Message:
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?

Thanks, Sue

Reply:

Subject: Theta-EC
Reply Posted by: David Dylis (ddylis@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Mar 5 17:47:22 US/Eastern 2007
Message:
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.

Reply:

Subject: Resistor Calculation
Reply Posted by: Sue Hedrick
Date Posted: Mon Mar 5 17:48:33 US/Eastern 2007
Message:
Thanks for your reply. Perhaps you could help me to pick which method to use. We have considered them all and each has its own difficulties in this application. The default rise of 30 degrees C is too high. The power stress on these parts is under 10%. We have no way of measuring the actual temperature rise of these parts. We have contacted the manufacturer and they do not have Thermal resistance data for these parts, neither theta EA or theta EC. To calculate Delta T of the parts, I need to know the thickness of the layers of resistive material and substrate material along with the thermal characteristics of each. Perhaps there are easier ways to calculate these parameters that I am not aware of. Any help would be appreciated. Thanks, Sue

Reply:

Subject: Resistor Temperature/Temperature Rise
Reply Posted by: David Dylis
Organization: System Reliability Center
Date Posted: Mon Mar 5 17:51:37 US/Eastern 2007
Message:
In lieu of actual information, I would recommend using the PRISM default. Appendix G of the PRISM User Manual includes defaults for resistor temperature rise. These defaults vary based on the type of resistor being used. Defaults are as follows:

Resistor, Fixed, Carbon Composition: Temperature Rise = 20 Resistor, Fixed, Carbon Film: Temperature Rise = 30 Resistor, Fixed, General: Temperature Rise = 30 Resistor, Fixed, Metal Film: Temperature Rise = 30 Resistor, Fixed, Network: Temperature Rise = 30 Resistor, Fixed, Thermistor: Temperature Rise = 0 Resistor, Fixed, Wirewound: Temperature Rise = 20 Resistor, Variable: Temperature Rise = 25 Resistor, Variable, Film: Temperature Rise = 25 Resistor, Variable, Non-Wirewound: Temperature Rise = 20 Resistor, Variable, Trimmer, Composition: Temperature Rise = 20 Resistor, Variable, Wirewound: Temperature Rise = 0 Resistor, Varistor: Temperature Rise = 50

If you have additional questions, feel free to contact me.