PRISM Forum - Message Replies


Topic: PRISM Questions and Answers
Topic Posted by: SRC (src_forum@alionscience.com )
Organization: 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.

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Original Message:

Posted by: Sue Hedrick (shedrick@advanced-control.com )
Organization:ACC
Date posted: Wed May 3 16:43:04 US/Eastern 2006
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 (SRC)
Date Posted: Thu May 4 8:34:14 US/Eastern 2006
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: Thu May 4 10:28:39 US/Eastern 2006
Message:
Hi David, 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 resitive 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 (ddylis@alionscience.com )
Organization: System Reliabilitye Center (SRC)
Date Posted: Thu May 4 10:47:56 US/Eastern 2006
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.


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