Power wirewound resistors have steady-state power and voltage ratings which indicate the maximum temperatures that the units should attain. For short durations of 5 seconds or less, these ratings are satisfactory; however, the resistors are capable of handling much higher levels of power and voltage for short periods of time (less than the cross-over point). For instance, at room temperature, the RS005 has a continuous rating of 5 W, but for a duration of 1 ms the unit can handle 24,500 W, and for 1 μs the unit can handle 24,500,000 W. The reason for this seemingly high power capability is the fact that energy, which is the product of power and time, is what creates heat; not just power alone. Vishay Dale can provide solutions for an application if provided with the information detailed in Figure 2.
Figure 1: Vishay Dale offers a wide variety of wirewound resistors. (Image source: Vishay Dale)
Short pulses (less than the crossover point time duration)
For short pulses, it is necessary to determine the energy applied to the resistor. For pulses less than the cross-over point, Vishay Dale engineering assumes all of the pulse energy is dissipated in the resistance element (wire). In order for the resistor to maintain its performance characteristics over the life of the product, Vishay Dale bases analysis and recommendations on the amount of energy required to raise the resistance element to +350°C with no heat loss to the core, coating, or leads. The cross-over point is the time where significant energy starts to be dissipated not only in the wire itself but is now being dissipated into the core, leads, and encapsulation material. This is the point where the pulse is no longer considered a short pulse but is now considered a long pulse.
The pulse handling capability is different for each resistor model and value, as it is based on the mass and specific heat of the resistance element. Once the power and energy have been defined, Vishay Dale can determine the best resistor choice for the application.
An example of an RS005 500 Ω resistor at room temperature:
ER = Energy rating of a given model, resistance value, and ambient temperature. Provided by Vishay Dale, ER = 6.33 J.
PO = The overload power capability of the part at 1 s. The overload power capability of an RS005 for 1 s, 10 x 5 W x 5 s = 250 Ws/1 s = 250 W
Cross-over point (s) = ER (J)/PO (W)
6.33 J/ 250 W = 0.0253 s
The cross-over point for the RS005 500 Ω resistor at room temperature is approximately 25.3 ms.
Long pulses (cross-over point to 5 seconds)
For long pulses, much of the heat is dissipated in the core, leads, and encapsulation material. As a result, the calculations used for short pulses are far too conservative. For long pulse applications, the short time overload ratings from the datasheets are used. Note that repeated pulses consisting of the short time overload magnitude are extremely stressful and can cause some resistor styles to fail.
- To find the overload power for a 5 s pulse, multiply the power rating by either 5 or 10 as stated on the datasheet
- To find the overload power capability for 1 s to 5 s, convert the overload power to energy by multiplying by 5 s, then convert back to power by dividing by the pulse width in seconds
- For pulse durations between the cross-over point and 1 s, use the overload power computed for 1 s
- What is the overload power for an RS005resistor?
From the datasheet, the RS005 is rated at 5 W and will take 10 times rated power for 5 s: 10 x 5 W = 50 W
- What is the energy capability of the RS005 for 5 s?
For 5 s, the energy capability is: 50 W x 5 s = 250 W·s or J
- What is the overload power capability of the RS005 for 1 s?
For 1 s, the overload power capability is 250 W·s / 1 s = 250 W
- What is the energy capability of the RS005 for 0.5 s?
For 0.5 s, the energy capability is 250 W x 0.5 s = 125 W·s or J
Information required to determine pulse capability
Figure 2: Determining the answers to these questions pertaining to pulse capability will help determine the application solution. (Image source: Vishay Dale)
Pulse applications often fall into one of three categories: square wave, capacitive charge/discharge, or exponential decay. An example of the pulse energy calculation for each of these will be shown in the following sections.
A constant voltage or current is applied across a resistor for a given pulse duration.
Figure 3: Example of a pulse energy calculation for a square wave with an amplitude of 100 VDC for 1 ms through a 10 Ω resistor. (Image source: Vishay Dale)
A capacitor is charged to a given voltage and then discharged through a wirewound resistor.
Figure 4: Example of a pulse energy calculation for a capacitive charge/discharge application. (Image source: Vishay Dale)
Exponential decay/lightning surge
The application reaches a peak voltage and decreases at a rate proportional to its value. This is typically modeled by DO-160E WF4 or IEC 6100-4-5 and represents a lightning surge.
Figure 5: Example of a pulse energy calculation for a lightning surge incident. (Image source: Vishay Dale)
Equally spaced repetitive pulses
When calculating pulse handling capability for repetitive pulses, the average power, as well as the individual pulse energy, must be considered. This is because the average power establishes some average heat rise on the part, which uses up some percentage of the part’s energy capability. That portion of the energy not used by average power is then available to handle the instantaneous pulse energy. When the two percentages (average power to rated power and pulse energy to pulse handling capability) are added together, they must not exceed 100 % of the part’s overall rating.
The following example is provided based upon an equally spaced repetitive square wave pulse.
Figure 6: This example is based on an equally spaced repetitive square wave pulse. (Image source: Vishay Dale)
- The pulse power, P = V2/R or I2R, is calculated for a single pulse
- The average power is calculated as follows: PAvg = Pt/T
- Calculate the pulse energy: E = Pt
- Calculate the percentage of average power to rated power (PR): Percentage (power) = 100 x PAVG/PR
- Vishay Dale engineering can provide the pulse handling capability (ER) given a resistor model, resistance value, and ambient temperature
- Calculate the percentage of pulse energy to pulse handling capability: Percentage (energy) = 100 x E/ER
- Add the percentages in (4) and (6). If the percentage is less than 100 %, the resistor chosen is acceptable. If the percentage is greater than 100 %, a resistor with a higher power rating or higher pulse handling capability should be selected. Contact Vishay Dale engineering to determine the best resistor choice for your application.
A series of equally spaced square wave pulses with an amplitude of 200 VDC, a pulse width of 20 ms, and a cycle time of 20 s, is applied to an RS007 100 Ω resistor at an ambient temperature of 25°C.
- The pulse power is: P = V2/R = (200 V)2/100 Ω = 400 W
- The average power is: PAVG = Pt/T = (400 W x 0.02 s)/20 s = 0.4 W
- The pulse energy is calculated: E = Pt = 400 W x 0.02 s = 8.0 W-s, or J
- The RS007 resistor has a rated power (PR) of 7 W. The percentage of average power to rated power is calculated: PAVG/PR x100 = ((0.4 W)/(7.0 W)) x 100 = 5.7%
- The pulse handling capability (ER) provided by Vishay Dale engineering at an ambient temperature of 25°C is 15.3 J
- The percentage of pulse energy to pulse handling capability is calculated:
100 x E/ER = 100 x ((8.0 J)/(15.3 J)) = 52.3%
- The percentages calculated in (4) and (6) are added: 5.7% + 52.3% = 58%
Since this percentage is less than 100 % of the overall rating, the RS007 style resistor will sufficiently handle the pulse.
Non-inductive power resistors consist of two windings, each of which is twice the finished resistance value. For this reason, the energy capability will nearly always be greater than a standard wound unit. To calculate the energy capability needed for non-inductive styles, compute the energy per ohm (J/Ω) by dividing the energy by four times the resistance value.
What is the energy per ohm pulse handling capability required to handle a 0.2 J pulse applied to a 500 Ω resistor?
The energy per ohm needed is: E/4R = (0.2 J)/(4 x 500 Ω) = 100 x10-6 J/Ω
This can be provided to Vishay Dale engineering in order to find the best product for the application.
Short pulses – No overload voltage rating has ever been established for wirewound resistors when pulsed for short durations. Sandia Corporation has performed a study on our NS and RS resistors using 20 µs pulses. This study indicates that this type of unit will take about 20 kV per inch as long as the pulse handling capability is not exceeded.
Long pulses – For pulses between the cross-over point to 5 s, the recommended maximum overload is √10 times the maximum working voltage for the 4 W size and larger, and √5 times the maximum working voltage for sizes smaller than 4 W.
If the goal of the application is for the resistor to fuse open under a specific condition, Vishay Dale offers fusible resistors. Reference page seven for common RS fuse resistor types, or click the following link for the entire RS fuse datasheet.
Fast-acting, molded styles, custom designed for specific applications
Vishay Dale has a wide variety of wirewound resistors available. They also have the ability to provide custom, molded style, fast-acting resistors for specific applications. While Digi-Key does stock some of these types of resistors, there are literally hundreds of possibilities available. See Figure 7 for some examples and the part number chart that can be used to customize an appropriate resistor for a specific application.
Figure 7: The example resistors shown on top represent a handful of hundreds of possible variants. For a custom resistor designed for a specific application, the part number chart on the bottom can be used. (Image source: Vishay Dale)