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High power switching can cause surge or pulse conditions. Pulse resistors suppress the pulse and prevent damage to sensitive components. Applications for pulse resistors include power supplies and motors (inrush current), Defibrillators, welding equipment, medical equipment and medical cabling.

Overload, inrush current, pulse and surge events are largely unpredictable. It is important to select the correct pulse resistor to survive any potential threats. A key design choice is between pulse voltage resistors and those capable of withstanding a high current. The design of one is the exact opposite of the other. In this post, we only consider high current pulse resistor design.

Understanding The Threat

High-energy pulses can occur in a wide variety of applications. They may be predictable or random events. It is important to understand the nature of the event, the application and environmental conditions.

Energy is power multiplied by time. In an electrical circuit subjected to an overload, surge or pulse event, the voltage and/or current will increase. Hence, given a fixed resistance, the power will also increase. The longer the duration of the event, the higher the energy.

Resistors dissipate this energy in the form of heat. Failure to select the correct thick film pulse resistor can mean the high temperature either destroys the resistor material or degrades long-term performance.

The amount of heat dissipated depends on the structure of the resistor component. It is also related to resistor cooling (if any) and the ambient temperature. With a short high amplitude pulse, the temperature of the resistor material could reach hundreds of degrees Celsius.

However, the short pulse width means there is insufficient time for the energy (in the form of heat) to transfer through the mass of the substrate material. Heat generation and transfer in the resistor take time. Therefore, a resistor’s pulse load capability depends on the pulse duration.

A high pulse load with a short duration may not have a significant heating effect. Whereas, a lower pulse load with a long
duration can cause more damage.

When a single pulse occurs with a lower peak amplitude but a longer duration, the average power is more of a concern. The substrate material will dissipate a proportion of the heat generated by the pulse event. But, it is important to ensure the maximum power rating of the resistor device is not exceeded.

The shape of the pulse and the duration between pulses also require careful consideration. Pulse shapes can vary, from rectangular or triangular to the typical exponential decay curve.

Selecting A Pulse Resistor

To select the appropriate pulse resistor for a given application, first evaluate the environment. Remember, pulse load diagrams apply at room temperature. If the resistor must operate at a higher ambient temperature, opt for a resistor with a higher pulse load capability.

Most pulse resistors are designed to handle repetitive known pulse conditions without failing. Manufacturers often provide nomo charts to help system designers understand the impact of single and repetitive pulse events.

Take into account the nature of your application and the pulse condition. Deciding if the pulse condition is repetitive or instantaneous is crucial to understanding if the pulse duration remains within the continuous-power rating of the resistor.

When dealing with repetitive pulses, begin by calculating the peak pulse amplitude. Use this information to ensure the energy generated will not damage the resistor track.

Next, establish the average power dissipation over the pulse period, ensuring it doesn’t exceed the resistor’s continuous power rating. In the case of short pulses, heat dissipation is minimal, causing the heat to remain in the resistive element. Consequently, the resistor can withstand peak pulse loads higher than its rated dissipation.

For longer single pulses, there’s more time for heat dissipation. This, in turn, depends on factors like the resistor’s mass, heatsinking, and cooling. Depending on the pulse condition the resistive element can experience a significant temperature rise. Therefore, for extended pulse durations, the permissible peak pulse load approaches the rated dissipation.

Understanding the difference between a single pulse load and a continuous pulse load involves considering the number of pulses and the time intervals between them. For short, sharp pulses, standardised pulse shapes are often provided on resistor datasheets. For high-energy pulses with extended durations, convert pulse shapes into rectangular shapes for comparison with standardised pulse load diagrams available on most datasheets.

Finally, consider heating effects on adjacent components. Also, review the impact on solder joints of repeated temperature cycling.

Pulse Resistor Applications – Solutions

Some design engineers take a cautious approach and over design circuits and components to withstand a pulse event. The alternative is to take the time to consider pulse conditions before selecting an appropriate resistor. This approach can
reduce the size and weight of the resistor component and save on system board area and cost.

The first step in choosing an appropriate thick film pulse resistor is to determine the maximum voltage of the pulse. This should be within the rated maximum voltage of the resistor. Depending on the operating temperature, the rated pulse power of the resistor should be derated accordingly.

Typical application issues to consider include the board area available for resistor components. One solution can be to use a vertical-in-line component. This approach delivers a large surface area without impacting the system board area. It also delivers effective resistor cooling due to natural convection flow.

Where the application demands a fail-safe condition, resistors can be designed to fracture (fail) if subjected to severe pulse conditions. Scribe lines placed in the substrate behind potential hotspot areas act as a fracture point.

By understanding the application, the pulse condition and the environment, it is possible to use manufacturer’s data to select a standard thick film pulse resistor device. If no standard device is available, then the TSEC engineering team is available to
discuss the design and manufacture of an application-specific pulse resistor device.

Specialist manufacturers will consider the width of the resistor track. This, coupled with minimising hot spots at resistor trimming, can prevent overheating and burn-out. They will also review if double-sided screen printing will improve surge or pulse performance. An appropriate choice of thick film ink can also help.