High Voltage Resistor Selection Checklist

Explore our comprehensive High Voltage Resistor Selection Checklist, meticulously designed for engineers and professionals in the electrical and electronics industries.


Introduction to High Voltage Resistor Selection Checklist

The resistor is the most common and well-known passive electrical component. A resistor is a device connected into an electrical circuit to introduce a specified resistance. The resistance is measured in Ohms. As stated by Ohms Law (E=IR), the current through the resistor will be directly proportional to the voltage across it and inversely proportional to the resistance. Resistors have numerous characteristics that determine their accuracy during use. The performance indices affect the accuracy to a greater or lesser extent depending on the application. Some of these indices are: Tolerance at DC, Temperature Coefficient of Resistance (TCR), Voltage Coefficient of Resistance (VCR), Noise, Stability with respect to Time and Load, Power Rating, Physical Size, and Mounting Characteristics. Resistor networks typically require temperature and voltage tracking performance. Please refer to the application note: Glossary of Resistor Terminology for an expanded explanation of resistor terminology.

Selection Requirements

1. Determine the resistance in ohms and watts.
2. Determine the proper physical case size as controlled by voltage, watts, mounting conditions, and circuit design requirements.
3. Select the resistor that meets your needs for type, termination and mounting.

Step 1 : Determine the resistance in ohms and watts.

Ohm’s Law:

E=IR or I=E/R or R=E/I

Ohm’s Law, as shown in the above formula, enables one to define the voltage (E), current (I), or resistance (R) when two of the three terms are known. When current and voltage are unknown they must be measured in the model circuit.

 

Power Law:

W=I2R or W=EI or W=E2 /R

Watts (power) can be determined from the above formulas that are derived from Ohm’s Law. R is measured in Ohms, E in volts, I in amperes, and W in watts.

Watts must be accurately determined before resistor selection. Simply stated any change in voltage or current produces a much larger change in wattage (heat dissipated by the resistor). The effects of relatively small increases in voltage or current must be determined because the increase in wattage may be significant enough to influence resistor selection. As stated in the above formulas the wattage varies as the square of the current or voltage. Allowances should be made for maximum possible voltage.

Step 2 : Determine the proper physical case size as controlled by voltage, watts, mounting conditions, and circuit design requirements.

Power Rating and Physical Size:

A resistor operated at a constant wattage will reach a steady temperature that is determined largely upon the ratio between the substrate size (surface area) and the wattage dissipated. Temperature stabilizes when the sum of the heat loss rates (by radiation, convection, and conduction) equals heat input rate (wattage). The larger the resistor surface area per watt to be dissipated, the greater the heat loss rate and therefore the lower the temperature rise.

Free Air Wattage Rating (Maximum Power Rating) is defined as the wattage rating of resistors as established under specified standard conditions. The absolute temperature rise for a specific resistor is roughly related to the area of its radiating surface. It is also dependent upon a number of other factors such as thermal conductivity, ratio of length to width, heat-sink effects of mounting, and other minor factors.

The precise temperature limits corresponding to 100% rated wattage are somewhat arbitrary and serve primarily as design targets. Once a wattage rating has been assigned on the basis of an empirical hot spot limit, the verification of its correctness must be established through long term load life test (see Application Note: Life Test Data – High Voltage Chip Resistors) based on performance and stability standards rather than the measurement of hot spot temperature.

Step 3 : Select the resistor that meets your needs for type, termination and mounting.

✔ Resistor Selection:

Select the most suitable resistor that meets the requirements of the application. OhmCraft resistors are made to your specification. Refer to the appropriate data sheet to determine part number or call OhmCraft for assistance.

✔ Wattage Rating:

To allow for the differences between actual operating conditions and the Free Air Wattage Rating it is a general engineering practice to operate resistors at less than the nominal rating.

✔ Voltage Rating:

Determine maximum applied (working) voltage that the resistor will be exposed to and select the appropriate package size.

✔ Pulse Operation:

When a resistor is operated in a pulse application, the total power dissipated by the resistor is a function of the pulse’s duty cycle. Typically, one will define the number of joules of energy the resistor must dissipate and choose a resistor accordingly. For additional information refer to our Pulse Resistor white paper or contact OhmCraft.

✔ High Frequency:

OhmCraft resistors, due to their design and construction, have very low capacitance and are inherently a non-inductive design. For additional information refer to our High Frequency Attributes Application Note.

✔ Military and Other Specification:

The special physical operating and test requirements of the applicable industrial or military specification must be considered. Contact OhmCraft for additional information.

Effect of the power ratings on components

All the components of an electrical apparatus including resistors, capacitors, rectifiers, and semiconductors have their own limitations as to the maximum temperature at which they can reliably operate. The attained temperature in operation is the sum of the ambient temperature plus the temperature rise due to the heat dissipation in the equipment.

Ambient Temperature Derating, below defines the percent of full load that power resistors can dissipate as a function of ambient temperature.

Temperature Coefficient of Resistance

Temperature Coefficient of Resistance (TCR) is expressed as the change in resistance in ppm (0.0001%) with each degree of change in temperature Celsius (C). MIL STD 202 Method 304 is often referenced as a standard for measuring TCR. This change is not linear with temperature. TCR is typically referenced at +25C and changes as the temperature increases or decreases. It can be either a bell or S shaped curve. It is treated as being linear unless very accurate measurements are required, then a temperature correction chart is used. A resistor with a TCR of 100 ppm will change 0.1% over a 10-degree change and 1% over a 100-degree change. An example of a TCR curve can be found in the application note: Glossary of Resistor Terminology.

The following formula expresses the rate of change in resistance value per 1 C in a prescribed temperature range.

TCR (ppm/°C) = (R-R0)/R0 X 1/(T-T0) X 106

- R: Measured resistance (Ω) at T °C

- R0: Measured resistance (Ω) at T0 °C

- T: Measured test temperature °C

- T0: Measured test temperature °C

In the context of a resistor network, this TCR value is called absolute TCR in that it defines the TCR of a specific resistor element. The term TCR tracking refers to the difference in TCR between each specific resistor in the network.

Voltage Coefficient of Resistance

The Voltage Coefficient of Resistance is the change in resistance with applied voltage. This is entirely different and in addition to the effects of self-heating when power is applied. A resistor with a VCR of 100 ppm/V will change 0.1% over a 10 Volt change and 1% over a 100 Volt change. VCR becomes very important in high Ohmic value resistor (100M Ω and above) where typical VCRs can be greater than 1000 ppm/V to specify the voltage that will be applied. Failing to do this may result in a resistor that will not meet your specification.

The rate of change in resistance value per 1 volt in the prescribed voltage range is expressed by the following formula:

VCR (ppm/V) = (R0-R)/ R0 X 1/(V0-V) X 106

- R: Measured resistance (Ω) at base voltage

- R0: Measured resistance (Ω) at upper voltage

- V: Base voltage

- V0: Upper voltage

In the context of a resistor network, this VCR value is called the absolute VCR in that it defines the VCR of a specific resistor element. The term VCR tracking refers to the difference in VCR between each specific resistor network. Please refer to the application note: Voltage Ratio Tracking and Voltage Coefficient of Resistance.

Summary

When specifying a resistor, the following parameters MAY be of interest. Please use this chart to help you define the operating characteristics for your specific application. All of them may not important for your specific application. Also, please do not hesitate to contact Ohmcraft for application help.

At Exxelia Ohmcraft, our commitment transcends the creation of resistors. We are dedicated to empowering the visionary innovations that define the future of military technology. Our team is poised to collaborate and customize solutions that perfectly align with the evolving needs of military applications.

In a landscape where reliability is non-negotiable and precision is imperative, Exxelia Ohmcraft stands as the beacon of unwavering support, fortifying military operations with resilient, high-performance resistors.

Download our White Paper

 

Contact Exxelia

Exxelia Ohmcraft Exxelia Ohmcraft’s sister division, Exxelia Micropen Medical is at the forefront of medical device product development, providing design engineers with unique insights on conceiving and implementing new designs and features. 

Publié le 09 Jan 2024 par Ali BARI

EXXELIA ACQUIERT LA SOCIETE MICROPEN TECHNOLOGIES CORPORATION

EXXELIA ACQUIERT LA SOCIETE MICROPEN TECHNOLOGIES CORPORATION ET AJOUTE LES RESISTANCES A SON OFFRE DE COMPOSANTS PASSIFS DE HAUTE FIABILITE   Micropen, leader technologique dans la conception, le développement et la fabrication de résistances de haute précision et de capteurs spéciaux rejoint Exxelia Exxelia ajoute les résistances à son portfolio de condensateurs et produits magnétiques de haute fiabilité, et élargit son offre aux capteurs critiques pour les marchés médicaux et de sécurité Exxelia renforce son empreinte aux États-Unis avec un quatrième site de fabrication Micropen renforce son périmètre en dehors des États-Unis grâce à la présence locale d'Exxelia, en particulier en Europe et en Asie   Paris, France et Honeoye Falls, New York, USA – 6 Janvier 2020 - Exxelia, leader dans la conception et fabrication de composants passifs et sous-systèmes de haute fiabilité dédiés aux environnements sévères, annonce avoir finalisé, le 23 décembre 2019, l'acquisition de Micropen Technologies Corporation. Micropen est un concepteur et fabricant de résistances de haute technologie, commercialisées sous la marque Ohmcraft®, très implanté sur les marchés du médical, de la défense, du spatial et de l’industrie. Micropen fabrique également avec la même technologie des capteurs spéciaux uniques dédiés aux applications médicales et de sécurité.   Fondée en 1982 à Honeoye Falls, NY, Micropen Technologies a développé un processus unique de dépose de matériaux fonctionnels critiques, tels que des électrodes conductrices ou des métaux précieux, sur divers substrats, tels que la céramique, le plastique et autres. Ce système breveté est  au cœur de la conception et la fabrication de la totalité de la gamme de résistances Ohmcraft® ainsi que de certains capteurs médicaux d’observation et de stimulation les plus innovants au monde. « Nous sommes ravis d'accueillir l'équipe talentueuse et impliquée de Micropen, et très fiers d'ajouter la technologie unique de Micropen à notre portefeuille déjà vaste de composants passifs et de capteurs de haute technologie. Grâce à ce procédé inégalé, Exxelia entre dans le monde des résistances à haute fiabilité et renforce sa position de fournisseur incontournable de  produits clés auprès de clients ayant des applications critiques », a déclaré Paul Maisonnier, Président-Directeur général d'Exxelia. Ed Petrazzolo, Président-Directeur Général de Micropen Technologies, déclare « Micropen  se réjouit d’intégrer la croissance d'Exxelia et de son vaste portefeuille produit. Les points communs entre nos deux sociétés en matière d'excellence, de culture, d'ingénierie, de technologie et de fabrication sont exceptionnels et nous sommes ravis de la portée mondiale qu'Exxelia offre à nos résistances Ohmcraft et à nos capabilités d'impression Micropen ». ### À propos de Micropen Technologies: Micropen Technologies Corporation conçoit, développe, fabrique et vend des gammes de résistances à couche épaisse spécifiques, haute tension et hautes performances , et des composants électroniques de précision, utilisant son système électronique d'impression breveté. La gamme de résistances de précision de Micropen est commercialisée sous la marque Ohmcraft, leader reconnu des résistances hautes-performances spécifiques pour les marchés médical, défense, spatial, détection et test & mesure. La division Micropen fabrique et développe en sous-traitance des produits pour les marchés des dispositifs médicaux et de la détection. Pour plus d'informations, visitez www.ohmcraft.com et / ou www.micropen.com À propos d'Exxelia: EXXELIA est l'un des leaders mondial dans la conception et fabrication composants passifs et sous-systèmes de hautes performances. La large gamme de produits d'Exxelia comprend des condensateurs à film, au tantale, en céramique et électrolytiques, des inducteurs, des transformateurs, des composants radiofréquence, des capteurs de position, des collecteurs tournants et des pièces mécaniques de haute précision. Reconnue mondialement pour son excellence en conception et son expertise technique, EXXELIA développe des produits «catalogue» et «spécifiques» destinés exclusivement aux marchés de haute fiabilité tels que l'aérospatiale, la défense, le médical, les transports, les infrastructures de télécommunications et les applications industrielles avancées. Toutes les informations supplémentaires sur https://exxelia.com