Application Note

VISHAY MILWAUKEE
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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: vishaymilwaukeeresistor@vishay.com
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
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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.
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NGR datasheet
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www.vishay.com/doc?49893
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Revision: 08-Jul-15
Document Number: 21044
2
For technical questions, contact: vishaymilwaukeeresistor@vishay.com
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