I wouldn’t recommend putting your electronics in a high-temperature oven, but some devices need to operate in or around high-temperature environments. Reliability becomes a major concern in such situations, especially when temperatures become so hot that solder could melt. Board design for these environments is its own beast, but designers still need to find components that can withstand these temperatures and operate reliably. Some areas include high-power electronics that will operate in a vacuum, industrial monitoring equipment, and scientific equipment.
Thanks to high-temperature semiconductor components built on GaN/SiC, the reliability focus now shifts away from semiconductors and onto passives. There is a broad range of high-temperature resistors available on the market, and these components help ensure overall reliability for your new devices. Here’s what you need to know when selecting high-temperature resistors and some example components for your next high-temperature system.
What Makes High-Temperature Resistors Different?
Resistors come in a number of forms, including metal and metal-oxide films, foil and carbon resistors, ceramics, and wire-wound resistors. Resistors that are designed for low power ratings and low to moderate temperatures can exhibit a large change in resistance when these ratings are exceeded. Basically, the resistor can become a short or open circuit once it burns out. What makes high-temperature resistors different from other resistors is the encapsulation material. This is the primary point of failure in a resistor at high power dissipation/high temperature.
In essence, high-temperature resistors are built with an encapsulation material that will not crack, melt, or place mechanical stress on the internal film/wire at high temperatures. Cracks in the encapsulation material can form due to differences in the thermal expansion coefficient of the various materials in the resistor. Mechanical stress can also be directed on the internal film/wire at high temperatures.
Because these resistors are built to withstand volume changes during operation or to simply expand by a smaller amount, they often have lower variation in resistance as their temperature changes. High-temperature resistors also tend to be more rugged; aside from the ability to withstand higher temperature, they are also through-hole, pluggable, or chassis-mount components. This is because they need to withstand temperatures where the solder will melt, so they are not available as SMD components.
Important Specifications of High-Temperature Resistors
When selecting high-temperature resistors, there are some important specifications that should receive attention. The important specifications for these resistors all center around reliability in terms of resistance and power rating under electrical and thermal loading. As the temperature of the component changes, so will its resistance. All resistors dissipate electrical power as heat, which then increases the temperature of the component. Because as the resistance of the component changes at high temperature, the power it dissipates as heat also changes!
These relationships between power dissipation, ambient temperature, and resistance can be summarized in two important specifications:
Temperature coefficient of resistance. Just like thermistors, all resistors have a temperature coefficient of resistance. This specification tells you how the resistance changes when the temperature of the component changes. Typical values for high precision high-temperature resistors are hundreds of ppm per °C.
Power derating vs. temperature. This specification tells you how the maximum power dissipation rating of the component changes as the component heats up. This can be specified in terms of ambient temperature, which is the easiest way to understand power derating. When the ambient temperature is higher, the maximum power dissipation by the component will be lower. This is normally summarized on a linear graph with a negative slope.
The other important specification to consider is mounting style. Chassis-mount components are by far the most rugged, but they won’t mount to a PCB unless you include mounting holes. These components are most often available as through-hole components, which can then be plugged into a mechanical socket or attached directly to a PCB. The examples below show some inexpensive high-temperature resistors.
TE Connectivity, SBC Series
The SBC series of ceramic wire-wound resistors from TE Connectivity is built to withstand high power dissipation and are rated to operate at temperatures up to hundreds of °C. These resistors are through-hole components and very desirable power derating characteristics for several low-power industrial applications. They also have a low-temperature coefficient of resistance of 200 ppm/°C (400 ppm/°C below 18R) and only +/- 3% resistance variation over 1000 hour load life at 70 °C. An example is the SBCHE15330RJ, which has a maximum power rating of 17 W and a maximum temperature rating of 350 °C.
SBC series surface temperature and derating curves, from the SBC series datasheet.
TE Connectivity, SQ Series
The SBC series of ceramic wirewound resistors from TE Connectivity are smaller resistors with maximum temperature rating of 250 °C. These resistors are ideal for precision high power applications; they exhibit no resistance change at 1000 V for 1 minute loading. These through-hole components have +/- 5% resistance variation over 1000 hour load life at 70 °C. They are also inflammable at 16x power rating for 5 minutes. Compared to the other high temperature resistor from TE Connectivity shown above, these resistors have zero power derating up to ~80 °C. An example component is the SQMW7100RJ 100 Ohm/7 W wirewound ceramic resistor.
Types of through-hole SQ series high temperature resistors, from the SQ series datasheet.
Stackpole Electronics, KAL Series
The KAL series of chassis-mount wire-wound resistors from Stackpole Electronics is another set of components that is ideal for industrial settings at high-temperature. An example component is the KAL50FB50R0 50 Ohm resistor, which has a power rating of up to 50 W and a maximum temperature rating of 275 °C. This component also has a very low-temperature coefficient of resistance of 20 ppm/°C. The chassis mounting style for this resistor makes it ideal for industrial settings, inside of vehicles, or in equipment with significant vibration or motion.
KAL series chassis-mount high-temperature resistor and derating graph, from the KAL series datasheet.