Sulphur-resistant thick-film resistors for harsh environments

The chemical industry, refineries, the oil and gas industry, environmental monitoring, mining, agriculture and the transport sector have one thing in common: sulphur compounds or sulphur gases play a role in their environment. If conventional resistors are used in the electronics of devices and machines, they can be damaged by this sulphurous environment. The result could be a massive impairment of the reliability of devices and machines, which can even lead to total failure.

This can be avoided by using sulphur-resistant resistors, such as the new WRIS-RSKS product series from Würth Elektronik. These resistors are often found in long-life applications in which stable component parameters play a central role. At the same time, the parameters must be stable over a wide temperature range and high humidity.

Nickel-chromium protective layer

Typically, when conventional resistors with silver-based internal electrodes are used in a sulphur-containing environment the reactive silver will to react with sulphur to form silver sulphide. Silver sulphide is characterised by a considerably lower conductivity, which significantly increases the contact resistance of the silver-based electrode and changes its surface structure. This is accompanied by an undesirable change in the resistance value. This can be avoided by using sulphur-resistant resistors.

With the WRIS-RSKS sulphur-resistant resistors from Würth Elektronik (Fig. 1), a protective layer of nickel-chromium alloy is applied to the silver-based electrode (here silver with palladium - Ag/Pd), which protects the silver-based electrodes in sulphur-containing environments. Testing of the components in accordance with the ASTM B-809 standard confirms their reliable function.

More than 500 component types in the range

Würth Elektronik's product range of sulphur-resistant resistors comprises of 247 components without a minimum order quantity and 260 additional components that can be supplied on request in quantities of 50 k, 100 k or more.

The resistance values range from 1 Ω to 10 MΩ with tolerances of ±1 % or ±5 %. Typical sizes are 0402, 0603, 0805, 1206 and 1210. The sulphur-resistant resistors are designed for outputs of 0.1 to 0.5 W and are suitable for a temperature range of -55 to +155°C.

Würth Elektronik will offer three design kits for the WRIS-RSKS product series of sulphur-resistant resistors, divided into the 0402, 0603 and 0805/1206 sizes.