Resistors act as high-rel heating elements in space application

Resistors act as high-rel heating elements in space application

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By eeNews Europe

The THEMIS heliophysics constellation of satellites was commissioned by NASA to investigate what causes auroras in the Earth’s atmosphere to dramatically change from slowly shimmering waves of light to wildly shifting streaks of colour. Discovering what causes auroras to change is intended to provide scientists with important information on how the planet’s magnetosphere works and the important Sun-Earth connection. Now in its sixth year in space THEMIS is helping to show how even small variations in the magnetosphere can sometimes cause extreme space weather.

The THEMIS satellites have proven to be so reliable that NASA has been able to repurpose two already in-orbit spacecraft to extend their useful science mission in a new project known as ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun). This additional use has removed the need to build and launch new spacecraft, saving NASA tens of millions of dollars.

Arcol was commissioned to provide a highly reliable heat source to protect the sensitive data-gathering and processing electronics and proposed their HS range of high power resistors. The resistor design process started in 2007 when a number of the products were tested for out-gassing when exposed to (near-vacuum) 10-6 to 10-9 torr conditions. SSL Berkeley undertook the design confirmation process to ensure that the resistors provided the appropriate thermal range under worst case conditions. From those tested, the Arcol parts were found to have the lowest out-gassing characteristic. This was due to their moulding process producing a highly reliable insulation that ensures the case is completely filled, minimising voiding and imperfections.

There was also a requirement to withstand rigorous preparation processes applied from mechanical and electrical characteristics modelling to ultrasonic cleaning with isopropyl alcohol for installation in the module. It was through this process that the HS type device was tested and selected by the Space Sciences Laboratory at Berkeley for inclusion in the THEMIS module.

The HS Series is a range of high quality, high stability high power wire-wound resistors which are packaged in a compact and robust aluminium casing designed for direct heat-sink mounting or panel attachment using a thermal compound to achieve maximum dissipation of heat into the surrounding environment. The resistive element is wound onto a high-thermal-conductivity ceramic former ground to a close tolerance finish ensuring maximum contact for rapid heat transfer. The element is encapsulated into the aluminium housing by a transfer moulding process which ensures a good humidity seal and a permanent compression fit. The encapsulant is a high temperature moulding compound and the dedicated mould tool design ensures accurate concentricity of the element inside the housing ensuring a high level of voltage protection.

Key specifications include; RoHS compliance and manufacture in line with the requirements of mil -18546 and IEC 115. Standard temperature coefficients are typically < 1K, 100ppm; > 1K 25ppm. They are available in 10 standard power ratings from 10 to 600W. Liquid-cooled versions are also available extending the range from 600 to 900W. Ohmic resistance values are available over the range R005 to 100K, depending on wattage. They can have seven termination options, to cover renewable energy, power supply, traction and industrial applications including; inverter/converter, snubber, braking resistor, inrush limiter, balancing resistor, voltage divider, capacitor discharge, cross over, shunt and power supplies.


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