Wednesday 4 September 2013

Resistors provide heat in space probe

Yesterday we talked about Keeping Cool in heated enclosures so we couldn't avoid mentioning this very different application where the opposite is required!

ARCOL Resistors has a great deal of experience of providing products for custom requirements but faced a unique challenge when asked to provide resistors for a space programme.

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.

These 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 were 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 10-6 - 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 ARCOL’s unique moulding process producing a highly reliable insulation that ensures the case is completely filled, minimising voiding and imperfections.

Further, there was the 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.

ARCOL's HS Series are 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 special ARCOL 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 Std. > 1K 25ppm.

Tony Daly, Sales Manager of ARCOL Resistors, comments; “Arcol has over 60 years of experience in manufacturing resistors and regularly provide custom designs to meet extreme requirements. Our HS series of power resistors are accepted as the industry standard design and footprint manufactured in the UK, very competitively priced and available on low minimum order quantities.”

The HS series of resistors are available in 10 standard power ratings from 10 to 600 watts. Liquid-cooled versions are also available extending the range from 600 to 900 watts. Ohmic resistance values are available over the range R005 to 100K, depending on wattage.

Designed for ease of assembly the HS series offers 7 different termination options, more than any comparable product. A wide range of thermal transfer accessories are available as are customised resistor values and terminations.

ARCOL’s HS series power resistors are optimised for stability, linearity and reliability, making them suitable for many 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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