Understanding resistor temperature coefficient of resistance in applications where high-precision and temperature stability are required.

Temperature coefficient of resistance (TCR) is the calculation of a relative change in resistance per degree of temperature change. It is measured in ppm/°C (1ppm = 0.0001%) and is defined as: TCR = (R2- R1)/ R1(T2- T1). For high-precision resistors, TCR is typically expressed in parts per million (ppm) per degrees Celsius, with reference to normal room temperature, typically +25°C.

Despite the importance of this specification, individual resistor manufacturers use different methods for calculating TCR, which may not provide enough information to enable an end user to accurately predict the influence of temperature changes on resistance value. Such published TCR variances therefore have the potential to create measurement uncertainty, in applications where high-precision resistor performance and temperature stability are absolute requirements.

In high-precision devices, such as the Bulk Metal Foil resistors manufactured by the Vishay Foil Resistors brand of Vishay Precision Group (VPG), published TCR specifications include nominal typical curves, normally from -55°C to +125°C, which define nominal “cold” (-55°C to +25°C) and “hot” (+25°C to +125°C) chord slopes. Product datasheets typically specify the maximum spread for each slope (e.g., ±0.2ppm/°C and ±1.8ppm/°°C).

Temperature affects resistor component operation and resistor behavior within the installation environment. As electrical current flows through a resistor, it generates heat: the Joule effect. The resulting thermal response induces relative mechanical changes and stresses within the resistor. Stresses caused by differential thermal expansions can vary by resistor materials of construction and other factors. Ambient temperatures within the installation environment can generate heat, affecting resistor performance.

An optimal resistor design minimizes susceptibility to external and internal stresses during different usages and power loads, without sacrificing performance and reliability. VPG Bulk Metal Foil technology creates a precise thermo-mechanical balance between generated heat, materials of construction, and associated manufacturing processes. Careful design can compensate for the effects of heat and stress during operation, increasing performance stability.

For example, VPG designs ultra-high precision resistors incorporating a Bulk Metal Foil element, by first bonding a proprietary cold-rolled foil material onto a ceramic substance. That is photoetched into a resistive pattern, without introducing mechanical stresses onto the material. Then the resistors are laser-adjusted to a specified resistance value and tolerance. Because the resistive material is neither drawn, wound, nor mechanically stressed during manufacturing, the Bulk Metal Foil maintains intended design characteristics and full performance reliability, including TCR.

Other resistor manufacturing methods, such as wire winding, thin-film sputtering, or thick-film glazing, can introduce mechanical stresses and have greater potential for thermo-mechanical imbalance. When a resistor is operating higher than rated temperatures, it can fail to function or incur damage that directly compromises accuracy. Resistor over-temperature conditions throughout an extended timeframe may permanently change individual resistance values, leading to a complete circuit malfunction. Users should pay close attention to rated temperature specifications to ensure that a resistor is operating according to published specifications. By adhering closely to these values, the user can be assured of continued resistor reliability, regardless of manufacturing process.

Despite differences in designs and manufacturing processes, TCR remains one of the most commonly accepted resistor performance stability indicators. TCR is imperative for predicting resistor sensitivity to ambient temperature variations, as well as anticipated component behavior at low- and high-operating temperatures.

Bulk Metal Foil resistor TCR considers extreme theoretical conditions within individual specification limits. In contrast with technologies such as thin film, manufacturers can typically present TCR in a relatively narrow temperature range, with less emphasis on extreme temperature effects.

TCR may be further defined as a resistance change between two temperatures, divided by the temperature difference (chord slope), or TCR = (ΔR/R)/ΔT. It is common practice to define cold chord slope from -55°C to +25°C, and hot chord slope from +25°C to +125°C (in this case ΔThot = +125°C – +25°C = +100°C).

The change in resistance vs. temperature in NiCr resistors is nonlinear, normally following a parabola. Mathematically, this function can be described by:

In this case, for any temperature T, Y will express the value of the change in resistance ΔR/R, normally in ppm, from the nominal value at +25°C. In other words, for the function Y, this will be expressed by the derivative function Y'. This function defines the slope (TCR) of a line tangent to the parabola and indicates how TCR is changing. For the above parabola function: Y'= 2aX + b (Y' is expressed in ppm/°C).

For simplicity, one can also use the fact that a chord slope equals the tangent midpoint value of the relevant temperature range. For example, the value of the hot slope (+25°C to +125°C) equals the tangent value (Y') at the midpoint, T = +75°C.

Thin-film resistor manufacturers often target best hot slope, while keeping cold slope within the specified limit. A study conducted to compare and analyze VPG Bulk Metal Foil and thin-film precision resistor TCR, using the change rate calculation method, has shown that a change in resistance due to temperature can be significantly larger than specified TCR limits. This comparison is based on tests conducted on two groups of different precision thin-film NiCr resistors from different manufacturers, each specifying a TCR of 5ppm/°C.

Results of this study demonstrated that the maximum change in resistance (TCR) due to temperature changes across the temperature axis from -55°C to +125°C will vary in Bulk Metal Foil resistors from -2.17ppm/°C to +2.2ppm/°C, for a total of less than 4.37ppm/°C. For the same temperature range, the TCR of the thin-film resistor sample from manufacturer A varied from -3.6ppm/°C to +7.2ppm/°C, for a total of nearly 11ppm/°C; and from manufacturer B, from -9.1ppm/°C to +4.99ppm/°C, for a total of 14ppm/°C.

So, precision thin-film resistors may exhibit a TCR that is much higher than specified limits on the manufacturer’s datasheet.

Bulk Metal Foil resistor TCR is achieved by matching two opposing effects of the inherent increase in resistance due to temperature increase vs. the compression-related decrease in resistance caused by the temperature increase. The two effects occur simultaneously, resulting in an unusually low, predictable, and repeatable TCR specification. As a result, Bulk Metal Foil resistors achieve intrinsic maximum stability and near-zero TCR, a specification not reliant on screening or other means for achieving uniform high-precision resistor performance and temperature stability. VPG employs this stringent TCR calculation method to ensure high-precision resistor reliability across full resistance values and operating temperature ranges.

For engineers selecting high-precision resistors, TCR specifications can help them to better predict reversible shifts in component resistance from an ohmic value within the application, under intended operating temperatures and within the installation environment. As TCR calculation methods can vary by manufacturer, manufacturing process, materials of construction, and other aspects, it is important for users to understand any nuances in the chosen method, to better understand TCR data as a component reliability metric. VPG methods for calculating TCR follow strict protocols to help engineers to be confident in the long-term reliability of components within demanding applications.

About the author: Molly Chamberlin, president of Embassy Global, is a 20-year expert in sensors, electronic components, and motion control, and can be reached at molly@embassyglobalpr.com. Reuven Goldstein, director of engineering and R&D for the Vishay Foil Resistors brand of VPG since 1997, also provided technical contributions to this article.

Junkosha, a provider of peelable heat shrink tubing (PHST) technology, offers ultra-small, high-shrink ratio PHST, delivering a solution for accessing to the most exacting parts of the vasculature with minimal impact on the patient. Suitable as tubing for laminating jacket coating in tiny guide wires, 0.011" and 0.014", PHST has a recovered ID as low as 0.009". Miniature guide wires navigate vessels to reach a lesion or vessel segment within the brain or heart. The high-shrink ratio PHST (2:1) supports processes where tapered microcatheter shafts are used or where tolerance take-up is an issue.

“Our aim is to answer customers’ unmet needs through technology innovation. As a part of this, we are being asked for solutions that take small to the next level,” says Joe Rowan, president and CEO of USA and Europe, Junkosha.

Makrolon Rx3440, a medical polycarbonate from Covestro LLC, offers durability and chemical resistance to prevent cracking for safer delivery of oncology drugs, and demonstrates retention of stress to provide more reliable IV connections.

Products covered range from low-cost OEM-type flexure actuators to 6-axis integrated nanopositioning systems, ultra-fast laser steering systems, and photonics alignment systems. Along with the piezo mechanic systems, digital controllers and sub-nanometer precise position sensors are part of the solutions available.

Vertical lift is required in a range of medical equipment where accessibility and comfort can improve care and patient outcomes; wheelchair lifts, examination tables, dentist’s chairs, adjustable nurses’ stations, and optician instruments such as non-contact tonometers. Thomson lifting columns come in a range of models with features to match application needs for speed, range of movement, or capacity loading. Available solutions can help improve equipment performance, cost, and design, benefitting users and patients. Following are features and options to consider when looking for the right vertical lift.

The LC Series offers speed and performance for wheelchairs, examination tables, and ergonomic workstations. The series provides up to 2,000N loading and can operate between 15mm/s and 19mm/s. Custom-designed units can be manufactured for higher speeds up to 25mm/s. For example, the LC2000 can improve machine performance by providing high load capacity, long stroke length, and speed. The column’s telescopic lead screw with nesting, three-piece extrusion gives a small retracted length and large extension-to-retraction ratio.

The LC3000 is designed for bariatric, chiropractic, and other applications where high load capacity is important. Using a ballscrew with a three-piece extrusion, with additional extrusion overlap allows for extended-length bushings. This design provides up to 3,000N load capacity with high moment loading in a compact frame.

All components within the lifting columns are designed for low, consistent noise and smooth operation for a pleasant working environment and to keep patients relaxed. Units use high-performance linear actuators with a noise-deafeating cover, so no other noise attenuation to the equipment is needed. Each column uses a single motor and screw for smooth operation and patient comfort, while also allowing lower amp draw and longer life.

Telescopic lifting columns maximize the extension-to-retraction ratio, giving flexibility in range of movement to improve ergonomics for the operator, patient, and doctor. Powered wheelchairs built with these lifting columns feature a minimal retracted height, making it easier for patients to get into the chair. Their high extension lets users reach high objects. Examination tables or dentistry chairs can be adjusted for easy patient access, and then quickly and smoothly moved to optimum height for user comfort.

Lifting columns are stand-alone actuators that provide lifting force while also handling potential moment loads up to 400Nm. The design across the range uses engineered polymer slide bushings to provide column moment loading capability. These bushings slide along the extrusion as the column telescopes and are keyed to enable up to 40% more moment load than alternate designs. A loading brake ensures the load is held even during power failure for user safety.

In applications where there is too much side load for a single column, two or four units may be synchronized using a combination of Thomson encoder and DCG control or a suitable third-party controller. This offers stability and more load and moment-load capacity, while still facilitating installation and system integration.

The columns’ exterior is an aluminum extrusion designed to be visible, so no additional covers or shrouds are needed. This simplifies the machine design and reduces manufacturing costs. Pre-drilled mounting holes and end-of-stroke limits, require no external limit switches.

The fully enclosed columns use high-viscosity grease to prolong life with up to 10,000 cycles at full load.

Three models of Thomson lifting columns offer features to balance extension-to-retraction ratio, load capacity, speed, and cost for the application. All columns have quiet, smooth operation; install easily; are maintenance free; and self-supporting. For applications such as baby incubators, optician testing machines, and mobile carts where the extension-to-retraction ratio is not as critical, the LC1600 uses a two-piece extrusion to provide stroke lengths of up to 400mm with a loading capacity of up to 1,600N and quiet operation.

Lifting columns are designed to give the flexibility in height required to avoid back injuries as well as meet high load and extension requirements. All units come as a complete system with attractive housing, making installation and machine design easier. In addition, the columns provide rapid movement for the loads they can lift without compromising productivity. Their design allows a single, central column to handle significant moment loads or, if required, multiple columns to be synchronized into overall machine design.

Specifying the proper lifting columns ensures that machine builders can get the features they need for their application in a cost-effective, easy to install solution, delivering better overall equipment performance and user experience.

About the author: Kyle Thompson is a Thomson Systems Group global product line manager. He can be reached at 631.218.5016 or kyle.thompson@thomsonlinear.com.

With manufacturing operations in the United States and Europe, Thomson lifting columns can be customized to precise application requirements. Provision for higher retracted lengths, mounting plates with customer specific mounting arrangements, special cable lengths, and double or extended bushings to provide greater moment load can be produced with adequate lead times.

SyncThink was granted patent US 14/530,598 by the U.S. Patent and Trade Office, allowing them to use systems and methods for creating and changing dynamic content based on eyegaze analysis. The patent broadens the application of its core technology, especially in the growing arena of virtual reality.

“With this new patent, the EYE-SYNC platform can incorporate tracking and monitoring of the user’s attention to modify content. We believe the potential applications for this patent are wide and significant; from customized content delivery to advertising,” says Founder of SyncThink, Jam Ghajar M.D., Ph.D.

SyncThink plans to use its eye tracking metrics to identify and analyze the attention gaze of the user and will modify the viewing content instantaneously based on the eye analytics. This may be used for training, performance optimization, and understanding how users view and use content in VR most effectively.

The Streamliner XT, Zeus Industrial Products’ thin-walled polytetrafluoroethylen (PTFE) catheter liner, delivers maximum wall thickness of 0.00075" (0.01905mm). Next in the series, the StreamLiner VT, features a maximum wall thickness of 0.001" (0.025mm) in an ID size range of 0.004" to 0.120" (0.107mm to 3.05mm).

The StreamLiner series makes a sturdier, more robust finished device while retaining torqueablity, pushability, and flexibility. The lubricious StreamLiner XT PTFE liner improves deployment force through the lumen of the catheter for improved minimally-invasive procedures. Sub-Lite-Wall StreamLiner XT and StreamLiner VT allow easier access to small vessels, including peripheral and neurovasculatures.

It’s time to consider the major advantages of producing medical devices by injection molding, and some points to take into consideration.

The injection molding process lends itself to producing high-precision, complex components for medical applications, especially when part-to-part integrity is required in high volumes.

For medical applications, the primary material recommended for molding is silicone. It is versatile and, as a liquid raw material, it offers advantages in processing for technical components in high volumes.

Silicone is the highest purity and demonstrates biocompatibility to ISO 10993 and USP Class VI standards, and is compliant with European Pharmacoepia 3.2.9. The material easily withstands common sterilization methods.

Liquid injection molding (LIM) is often used to produce silicone medical devices. Producing components by simultaneous injection of liquid silicone rubber (LSR) in combination with engineered plastics is commonly referred to as 2K, 2-shot, multi-component injection molding, or co-injection. Trelleborg calls it 2C LSR and it allows combining two, three, or more materials into one fully-bonded, component.

2C LSR enables complex components, ideal for parts with a large amount of detail, and offers designers great latitude, integrating multiple components into one, such as for medical applications where dead space needs to be avoided and hygienic designs created.

These advantages can only be realized if there is close collaboration between the device maker’s engineering team and the injection-molding engineering experts at the silicone component manufacturer.

At the root of any successful injection molded component is the tool. However, developing a tool-grade steel mold, hot- or cold-runner blocks, and related process automation equipment can be expensive, but must be considered. The high initial tooling cost should never be judged on cost alone, but calculated as an investment throughout the tool’s lifetime. Liquid injection molding can produce high-integrity parts in high volumes, making the tooling relatively inexpensive when considered piece-by-piece throughout a program’s life.

Mold efficiency depends on cavitation, parting-line geometry, gating, venting, surface finish, and supporting automation. It’s important to ensure the mold is robust enough to mitigate the inherent lot-to-lot variability of raw silicone. The mold will need to integrate seamlessly with equipment that pumps, mixes, injects, compresses, heats, and ejects components.

In processing, watch points include restrictions on part thickness to avoid shrinkage problems. Parts can also require de-flashing with added cost. These issues can be mediated through close collaboration between the device designer and silicone component manufacturer. For instance, if the tool and process equipment are expertly designed, flash can be nearly eliminated.

3d Mold Design Suppliers for Sale

Injection molding of liquid silicone can produce compounds with complex geometries and designs unachievable by other manufacturing methods. Beyond LIM, 2C LSR technology can provide opportunities for part design and additional benefits.

To take full advantage of possibilities offered by silicone injection molding, suppliers must understand the details involved with manufacturing silicone rubber parts and complex medical devices. The key to best solutions, extended life, hygienic design, meeting standards, and cost-effectiveness throughout total product life is early and close collaboration between the medical device designer and the silicone component manufacturer.

About the author: Drew Rogers is the global director of healthcare & medical at Trelleborg Sealing Solutions.

Injection Moulding, Plastic Molding, Rapid Prototyping - Hitop Industrial,https://www.mold-toolmaker.com/