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.

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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. 

Published on 09 Jan 2024 by Ali BARI

What you should know about Ceramic Capacitors ?

▲ What you should know about Ceramic Capacitors ?   1. Materials expert For 50 years and as a market leader, EXXELIA’s comprehensive knowledge of the materials properties and performances have enabled us to design capacitors in Porcelain, NPO, BX, 2C1, BP, X7R and –2200ppm/°C ceramics. > See our capacitors in catalog   2. Custom Designs Our catalog products don’t meet your application?  Based on the valuable experience accumulated over the design of 2,000+ specific ceramic capacitors, you can trust EXXELIA to define a qualitative custom solution in a time effective manner.   3. No Obsolescence Choosing a standard or custom EXXELIA product means you won’t have to worry about obsolescence.   4. Typical Applications Aerospace & Defense: cockpit panels, flight control, radio systems, missile  guidance systems… Space: military and commercial satellites, launcher… Medical: MRI, external defibrillators, implantable devices… Telecommunications: base stations… Oil and gas: drilling tools, MWD, LWD, wellheads…   5. ISO 9001 And AS9100C Quality is at the core of Exxelia’s corporate culture. Each sites has its own certifications.    6. Certifications Capacitors manufactured by EXXELIA comply with American and European standards and meet the requirements of many international standards. For Space qualified parts (ESA QPL), please refer to our catalog «Ceramic capacitors for Space applications».   7. Quality & Reliability EXXELIA is committed to design and manufacture high quality and reliability products. The test cycles reproducing the most adverse operating conditions over extended periods (up to 10 000 hours) have logged to date well over 5.109 hours/°Component. Failure rate data can be provided upon request. 8. Conflict minerals EXXELIA is committed to an approach based on «Conflict Minerals Compliance». This US SEC rule demands complete traceability and a control mechanism for the mineral procurement chain, encouraging importers to buy only «certified» ore. We have discontinued relations with suppliers that procure from the Democratic Republic of the Congo or an adjoining country.   9. Environment EXXELIA is committed to applying a robust environmental policy, from product design through to shipment. To control its environmental footprint and reconcile this with the company’ functional imperatives, our environmental policy provides for the reduction or elimination of hazardous substances. We also focus on compliance with European Union directives and regulations, notably REACH and RoHS. 10. RoHS Compliancy SMD CAPACITORS The capacitor terminations are generally protected by a nickel barrier formed by electrolytic deposit. This barrier gives chip capacitors leaching performance far exceeding the requirements of all applicable standards. The nickel barrier guarantees a minimum resistance to soldering heat for a period of 1 minute at  260°C in a tin-lead (60/40) or tin-lead-silver (62/36/2) bath without noticeable alteration to the solderability. It also allows repeated soldering-unsoldering and the longer soldering times required by reflow techniques. However nickel barrier amplifies thermal shock and is not recommended for chip sizes equal or greater than CNC Y (30 30) - (C 282 to C 288 - CNC 80 to CNC 94). LEADED COMPONENTS As well as for SMD products, leaded capacitors ranges can also be RoHS. These products, which are characterized by the suffix «W» added to the commercial type, are naturally compatible with the soldering alloys used in RoHS mounting technology. The connections coating is generally an alloy SnAg (with a maximum of 4% Ag). However, on a few products that EXXELIA will precise on request, the coating is pure silver.   11. MLCC Structure   12. Equivalent circuit Capacitor is a complex component combining resistive, inductive and capacitive phenomena. A simplified schematic for the equivalent circuit is:   13. Dielectric characteristics  Insulation Resistance (IR) is the resistance measured under DC voltage across the terminals of the capacitor and consists principally of the parallel resistance shown in the equivalent circuit. As capacitance values and hence the area of dielectric increases, the IR decreases and hence the product (C x IR) is often specified in Ω.F or MΩ.µF. The Equivalent Series Resistance (ESR) is the sum of the resistive terms which generate heating when capacitor is used under AC voltage at a given frequency (f). Dissipation factor (DF) is the ration of the apparent power input will turn to heat in the capacitor: DF = 2π f C ESR When a capacitor works under AC voltage, heat power loss (P), expressed in Watt, is equal to: P = 2π f C Vrms2 DF   The series inductance (Ls) is due to the currents running through the electrodes. It can distort the operation of the capacitor at high frequency where the impedance (Z) is given as: Z = Rs + j (Ls.q - 1⁄(C.q)) with q = 2πf When frequency rises, the capacitive component of capacitors is gradually canceled up to the resonance frequency, where : Z = Rs and LsC.q2 = 1 Above this frequency the capacitor behaves like an inductor.   Manufacturing steps > See our capacitors in catalog SMD environmental tests Ceramic chip capacitors for SMD are designed to meet test requirements of CECC 32100 and NF C 93133 standards as specified below in compliance with NF C 20700 and IEC 68 standards: Solderability: NF C 20758, 260°C, bath 62/36/2. Adherence: 5N force. Vibration fatigue test: NF C 20706, 20 g, 10 Hz to 2,000 Hz, 12 cycles of 20 minutes each. Rapid temperature change: NF C 20714, –55°C to + 125°C, 5 cycles. Combined climatic test: IEC 68-2-38. Damp heat: NF C 20703, 93 %, H.R., 40°C. Endurance test: 1,000 hours, 1.5 URC, 125°C. > See our capacitors in catalog   STORAGE OF CHIP CAPACITORS TINNED OR NON TINNED CHIP CAPACITORS Storage must be in a dry environment at a temperature of 20°C with a relative humidity below 50 %, or preferably in a packaging enclosing a desiccant.  STORAGE IN INDUSTRIAL ENVIRONMENT: 2 years for tin dipped chip capacitors, 18 months for tin electroplated chip capacitors, 2 years for non tinned chip capacitors, 3 years for gold plated chip capacitors. STORAGE IN CONTROLLED NEUTRAL NITROGEN ENVIRONMENT: 4 years for tin dipped or electroplated chip capacitors, 4 years for non tinned chip capacitors, 5 years for gold plated chip capacitors. Storage duration should be considered from delivery date and not from batch manufacture date. The tests carried out at final acceptance stage (solderability, susceptibility to solder heat) enable to assess the compatibility to surface mounting of the chips.   LEAD STYLES   SOLDERING ADVICES FOR REFLOW SOLDERING   Large chips above size 2225 are not recommended to be mounted on epoxy board due to thermal expansion coefficient mismatch between ceramic capacitor and epoxy. Where larger sizes are required, it is recommended to use components with ribbon or other adapted leads so as to absorb thermo-mechanical strains. RECOMMENDED FOOTPRINT FOR SMD CAPACITORS  Ceramic is by nature a material which is sensitive both thermally and mechanically. Stresses caused by the physical and thermal properties of the capacitors, substrates and solders are attenuated by the leads. Wave soldering is unsuitable for sizes larger than 2220 and for the higher ends of capacitance ranges due to possible thermal shock (capacitance values given upon request). Infrared and vapor phase reflow, are preferred for high reliability applications as inherent thermo-mechanical strains are lower than those inherent to wave soldering.    SOLDERING ADVICES FOR IRON SOLDERING Attachment with a soldering iron is discouraged due to ceramic brittleness and the process control limitations. In the event that a soldering iron must be used, the following precautions should be observed: Use a substrate with chip footprints big enough to allow putting side by side one end of the capacitor and the iron tip without any contact between this tip and the component, place the capacitor on this footprint, heat the substrate until the capacitor’s temperature reaches 150°C minimum (preheating step, maximum 1°C per second), place the hot iron tip (a flat tip is preferred) on the footprint without, touching the capacitor. Use a regulated iron with a 30 watts maximum, power. The recommended temperature of the iron is 270 ±10°C. The temperature gap between the capacitor and the iron tip must not exceed 120°C, leave the tip on the footprint for a few seconds in order to increase locally the footprint’s temperature, use a cored wire solder and put it down on the iron tip. In a preferred way use Sn/Pb/Ag 62/36/2 alloy, wait until the solder fillet is formed on the capacitor’s termination, take away iron and wire solder, wait a few minutes so that the substrate and capacitor come back down to the preheating temperature, solder the second termination using the same procedure as the first, let the soldered component cool down slowly to avoid any thermal shock.   14. Packaging TAPE AND REEL The films used on the reels correspond to standard IEC 60286-3. Films are delivered on reels in compliance with document IEC 286-3 dated 1991. Minimum quantity is 250 chips. Maximum quantities per reel are as follows: Super 8 reel - Ø 180: 2,500 chips. Super 8 reel - Ø 330: 10,000 chips. Super 12 reel - Ø 180: 1,000 chips. Reel marking complies with CECC 32100 standard: Model. Rated capacitance. Capacitance tolerance. Rated voltage. Batch number.   15. Dimensional characteristics of chips tray packages   16. High Q Capacitors Tape and Reel Packaging Specifications   17. EIA standard capacitance values Following EIA standard, the values and multiples that are indicated in the chart below can be ordered. E48, E96 series and intermediary values are available upon request.   18. EIA capacitance code The capacitance is expressed in three digit codes and in units of pico Farads (pF). The first and second digits are significant figures of the capacitance value and the third digit identifies the multiplier. For capacitance value < 10pF, R designates a decimal point.  See examples below:   19. Part marking voltage codes Use the following voltage code chart for part markings: 20. Part marking Tolerance codes Use the following tolerance code chart for part markings:   21. Reliability levels Exxelia proposes different reliability levels for the ceramic capacitors for both NPO and X7R ceramics.   As the world’s leading manufacturer of specific passive components, we stand apart through our ability to quickly evaluate the application specific engineering challenges and provide a cost-effective and efficient solutions. For requirements that cannot be met by catalog products, we offer leading edge solutions in custom configuration: custom geometries, packaging, characteristics, all is possible thanks to our extensive experience and robust development process, while maintaining the highest level of reliability. Where necessary, special testing is done to verify requirements, such as low dielectric absorption, ultra-high insulation resistance, low dissipation factor, stability under temperature cycling or under specified environmental conditions, etc. > See our capacitors in catalog