Vishay has announced two new series of Bulk Metal Foil ultra-high-precision Accutrim trimming potentiometers designed to meet and exceed the requirements of MIL-PRF-39035 Char. H. The approved 1/4-in square RJ26-style 1240 series, and 3/8-in square RJ24-style 1260 series devices offer low end-to-end TCR of ±10 ppm/°C from − 55 °C to + 150 °C, + 25 °C ref., and ± 25 ppm/°C through the wiper, load life stability of 0.1 % typical under full rated power at + 85 °C for 10,000 hours, and a < 0.05 % typical setability.
Vishay’s new 1240 and 1260 series of ultra-high-precision trimmers feature a Bulk Metal Foil resistive element that possesses a unique inherent temperature and load life stability. Their advanced, virtually backlash-free (< 0.05 %) adjustment mechanism makes them easy to set quickly and accurately, and maintains that setting through temperature cycling and vibration, as well as thermal and mechanical shock. A unique feature of the Vishay Foil trimmer is its stability in the vibration environment, which eliminates the need for unwanted adjustments.
The devices offer a multi-fingered, noble metal wiper engaging a planar surface of etched Foil for low TCR end-to-end (five times tighter than required by MIL-PRF-39035 Char. H) and through the wiper. Other technologies, such as wirewound and cermet trimmers, have a fair TCR end-to-end, but cannot be relied upon to demonstrate an acceptable TCR through the wiper, which is an important element in the precise adjustment of circuits subject to temperature fluctuations.
In addition to their low TCR and excellent load life stability, the 1240 and 1260 trimmers feature a 0.1 % typical setting stability, a wide resistance range from 5 Ω to 10 KΩ, and standard resistance tolerances of ± 5 % and ± 10 %. The devices offer a 0.25-W power rating at + 85 °C and for increased reliability provide the utmost in ESD immunity, withstanding electrostatic discharges up to 25 kV.
The devices provide unidirectional, infinite resolution, which when synthesized by a combination of an absolutely uniform resistance material, multiple wiper contact points, and multiple resistance circuit paths, yields a uniquely accurate setability of less than < 0.05 %. There are no turns of wire to be traversed, causing a step function, and no contact resistance variation to cause an irregular change in resistance with mechanical adjustment.
The 1260 devices offer tin/lead terminations, while the 1240 devices feature gold-plated terminations with tin/lead terminations on request. The 1240 and 1260 trimmers provide an “O”-ring to prevent ingress of fluids during any board cleaning operation, and are available with special options including special markings, burn-in and screening operations.
Because of the stability and linearity of the foil trimmer it is not necessary to use a buffering resistor in series with the trimmer or a linearizing resistor in parallel with the trimmer. The low values available in foil trimmers are uniquely linear making them suitable for end to wiper circuit use which is not normally recommended.