Unfortunately, resistors can fail under impulse loads. When pulsed power is dissipated to the resistive elements of the device, it generates heat and increases the temperature of the resistor. Overheating can damage resistive elements, causing resistance changes or even open circuits in the device. To avoid this in your design, here are 11 things you should know about resistors and pulsed loads when selecting components:
The resistor can withstand pulsed loads above its rated power dissipation p70 (the resistor's datasheet specifies a nominal power dissipation oc of P70 at an ambient temperature of 70).
It takes time for heat to be generated and transferred in a resistor, so the pulse load capacity of a resistor depends on the pulse duration.
For short pulses (adiabatic conditions), heat is retained in the resistive element, but the short path of the pulse limits its effect on the temperature of the resistive element, even for high pulse loads. As a result, the resistor will be subjected to a peak pulse load that is higher than its rated power consumption.
For long pulses, prolonged heating will cause a more pronounced increase in the temperature of the resistive element, although this will be limited by the heat removal that has already been set during the pulse. Therefore, for longer pulse durations, the permissible peak pulse load is close to the rated dissipation.
The difference between a single pulse load and a continuous pulse load is a function of the number of pulses and the time interval between them.
Single-pulse loads: The time interval t between pulses is long enough to allow the resistor to be cooled between pulses. The applicable pulse parameters are p p specified, u specified , and the average pulse power p 0.
Continuous Pulse Load: The time interval t between pulses is very short to prevent the resistor from cooling between pulses. The applicable pulse parameters are p p specified, u specified, and the average pulse power p is p70 p 0
The pulse shape varies from rectangular or triangular to the typical exponential attenuation or sharp surge pulse of capacitor discharge.
For low-power, long-duration energy pulses, the pulse energy is the limiting parameter, and by calculating the pulse energy and determining the duration of a rectangular pulse of the same energy and peak power, the pulse shape can be converted to a rectangle for comparison with the pulse load diagram of the resistor.
For sharp surge pulses, the impulse voltage is the limiting parameter and conversion to a rectangular shape is not applicable. Instead, common surge pulse shapes are described by standardized transients and referenced accordingly in the resistor's datasheet. According to IEC 60115-1, 427, the surge pulse is composed of 12 50 and 10 700 pulses described. According to IEC38 and IEC 61340-3-1, mannequins describe electrostatic discharge.
Finding the right resistor for a pulsed load application requires determining the actual pulse conditions. Parameters such as peak power p, pulse duration t, or period t need to be identified and compared to the specified pulse load capacity of the resistor.
The pulse load diagram is usually defined at room temperature. If your component needs to operate at a higher ambient temperature, or if your application requires an additional continuous power load to increase the temperature of the resistive element, then a resistor with a higher pulse load capacity may be required.
Information about the resistor's ability to withstand pulses is displayed in the pulse load graph. What these plots have in common is that they specify the maximum allowable peak pulse power for each pulse duration of the rectangular pulse. In addition to this, their informational value can be very different:
Check if the graph covers only the peak power per pulse duration for a single resistor value, or if it covers the full range of available resistors for the resistor family. Only in the latter case is the specified peak pulse power reliable, as it is defined by the weakest performance resistance value in the range.
The pulses put pressure on the resistor and affect its resistance value. Therefore, the pulse load specification must also account for the maximum permissible resistance variation for the pulse conditions given in the pulse load diagram, e.g., 025 % r。
Thin-film resistors use different technologies and have different pulse load capacities. The main factors influencing the pulse load capacity of a thin film resistor are the resistive film material, trimming mode, and usable resistor area.
Standard thick film resistors have limited pulse load capacity, which is related to the non-uniformity of the resistive film material and a simple trim mode that limits the usable resistor area. However, there are ways to push the envelope even further:
Printing a resistive film on the top and bottom of the resistor ceramic body allows the pulse-induced heat to be distributed over twice the resistive area and significantly reduces the pulse-related temperature rise in the resistive film. EAK uses double-sided, thick-film resistors.
The omission of fine-tuning cuts allows for full use of the resistive film area for current flow. This improves the distribution of pulse-induced heat in the resistive film and avoids hot spots. The CRCW-IF and RCS series feature non-fine-tuned thick film resistors.
The SMD champion for pulse load capacity is the carbon film MELF resistor CMB 0207. Its performance is more than an order of magnitude higher than that of resistors of the same case size, as it combines the most important feature of high pulse load capacity:
The tried-and-tested, pulse-resistant cylindrical design provides the maximum effective resistive film area.
Spiral trimming mode to avoid locally enhanced current density.
Carbon film material with unmatched thermal stability.
In wirewound resistors, the resistive element consists of metal wires wound around a cylindrical ceramic core. Due to the relatively large mass of the wire, higher pulse energies of up to 60 kJ can be dissipated in the wire in a very short pulse duration. Since the resistance value of a wirewound resistor is adjusted according to the wire diameter and length, resulting in different wire qualities, its pulse load capacity is also closely related to the resistance.
For longer pulse durations, a significant portion of the heat generated during the pulse escapes from the wire, although the energy is still dissipated in the wire. As a result, the pulse energy handling capacity of the entire resistor is much higher than the wire itself during these durations.
Protecting the wires with a glass enamel coating instead of a cement coating further increases the pulse load capacity of the wirewound resistor as it can withstand higher temperatures.