VISHAY MILWAUKEE www.vishay.com Resistive Products Application Note Grid Resistors: Determining Nominal Resistance By Daniel Featherstone SCENARIO The customer wants to dissipate 100 V at a rate of 100 A continuously. 100 V x 100 A = 10 kW 100 V / 100 A = 1 FACT Nearly all resistor materials change resistance as temperature increases. Depending on the material, this change can range from a few percent to 50 % and higher at 100 % duty on the resistor (100 % duty raises the resistor temperature to ~ 375 °C over ambient). In the scenario above, there are two ways to handle the resistor design (assuming a material with a 33 % change in resistance at 100 % power and that the voltage applied across the resistor is constant). Design 1 Voltage across resistor (cold) = 100 V Voltage across resistor (hot) = 100 V Cold resistance (0 % power) = 1 Hot resistance (100 % power) = 1.33 Initial current (at 1 ) = 100 A Hot current (100 % power) = 75.2 A Initial power rating = 10 kW Continuous duty power rating = 7.52 kW 120 1.4 100 1.2 1.0 Resistance (Ω) 60 40 0.8 0.6 0.4 20 0 0.2 0 20 40 60 80 100 Time (s) Revision: 08-Jul-15 120 140 160 180 0 0 20 40 60 80 100 120 140 160 180 Time (s) Document Number: 21044 1 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE Current (A) 80 Application Note www.vishay.com Vishay Milwaukee Grid Resistors: Determining Nominal Resistance Design 2 Voltage across resistor (cold) = 100 V Voltage across resistor (hot) = 100 V Cold resistance (0 % power) = 0.752 Hot resistance (100 % power) = 1.0 Initial current (at 10 ) = 133 A Hot current (100 % power) = 100 A Initial power rating = 13.3 kW Continuous duty power rating = 10 kW 160 1.2 140 1.0 Resistance (Ω) 120 Current (A) 100 80 60 0.8 0.6 0.4 40 0.2 20 0 0 20 40 60 80 100 120 140 160 180 Time (s) 0 0 20 40 60 80 100 120 140 160 180 Time (s) Design 1 is good when there is a desire to limit the current flow. Design 2 is good when a nominal current flow or power dissipation is desired at the steady state / continuous conditions. The customer should decide which way they would prefer the resistor be designed based on their application. In the above scenario, Design 2 would likely be better because at continuous duty (100 % power) the resistor will be dissipating the desired power at the desired current rate. www.vishay.com/doc?31833 GRE2 Application Form www.vishay.com/doc?31875 NGR datasheet www.vishay.com/doc?31873 Load Bank Application Form www.vishay.com/doc?49893 Dynamic Braking Application Form www.vishay.com/doc?49887 NGR Application Form www.vishay.com/doc?49904 Harmonic Filter Application Form www.vishay.com/doc?49889 Revision: 08-Jul-15 Document Number: 21044 2 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE GRE1 datasheet