In high voltage and power applications, resistors are often used as circuit protection elements. As such, they must continue to perform when subjected to overload, inrush. surge or pulse events. A high energy resistor is one possible solution.
In this post, we discuss the nature of the threat and compare resistor technology performance. We then cover resistor design for high energy event survivability.
What is a high energy resistor
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 usually dissipate this energy in the form of heat.
High energy resistors must also be capable of surviving the transient high currents generated by the event.
Establishing the threat
All calculations must take into account the ambient operating temperature of the application. Resistor manufacturers usually quote power ratings of resistors at 25͒℃. It is, therefore, important to derate the power rating based depending on the actual temperature.
High energy pulses or surges may 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.
For repetitive pulses, the first step is to calculate the peak pulse amplitude. As this may generate sufficient energy to damage the resistor track. With this concern eliminated the next step is to establish the average power dissipation over the period of the pulses. This value must not exceed the continuous power rating of the resistor.
Thick film resistor pulse withstand calculations are discussed elsewhere on this blog. For simple pulse conditions, these can be sufficient. But for more complex situations it is important to consult the thick film resistor manufacturer.
Many manufacturers provide nomo charts. These allow the system designer to understand the impact of single and repetitive pulse events.
Types of high energy resistor
The type of resistor chosen for an application where overload, pulse or surge events occur will depend on normal operating conditions and the nature of the threat.
Ceramic (Carbon) Resistors – Tend to perform well when subjected to pulse or surge events and this is one of the major applications of ceramic resistors. However, they have electrical performance limitations compared to the technologies listed below.
Wirewound Resistors – can withstand high power pulses for short durations with little resistance shift in resistance. However, they tend to be relatively large devices with high inductance.
Thick Film Resistors – thick film high energy resistors deliver a relatively cheap, non-inductive, solution in a small footprint. Their pulse handling capabilities may be less than other technologies but they deliver good overall performance.
Thin Film Resistors – Due to their construction they are more susceptible to high energy pulse or surge events than the technologies listed above.
Bulk Metal Foil Resistors – These devices tend to have good surge survivability characteristics. They tend to be used in lower power (precision) applications.
In higher power, general-purpose applications where cost and size are consideration thick film is often the preferred choice.
Thick film resistor manufacturing issues
Thick film resistors fail under pulse conditions because they are unable to dissipate the heat generated in the resistor device by the electrical energy of the pulse.
With a short high amplitude pulse, the temperature of the resistor material could reach hundreds of degrees Celsius. But 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.
Failure to select the correct thick film resistor device can mean the high temperature either destroys the resistor material or causes a long-term degradation in performance.
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.
For high pulse power and pulse current handling it is important to maximise the width of the resistor track. This, coupled with minimising hot spots at resistor trimming will prevent overheating and burn-out.
Double-sided screen printing can improve surge or pulse performance in some applications. An appropriate choice of thick film ink can also help
The selection of appropriate protection resistors capable of surviving high energy events is key. Overload, inrush current, pulse and surge events are largely unpredictable. It is important to select the correct high energy resistor to survive any potential threats.