What is a ceramic composition resistor?
Ceramic Composition resistors are composed of a mixture of a finely ground insulator and conductor which is compressed into a cylindrical shape. Terminals are attached and an insulation coating is applied to the outer body of the resistor. The resistance is controlled based upon the ratio of the insulator to conductor mix.
Ceramic is an excellent electrical insulator as well as a superb heat conductor. This property of Ceramic allows the cores of these resistors to endure low to moderate power electrical current flowing through them without overheating and becoming damaged. By their nature, ceramic resistors are also very chemically inert.
Ceramic’s usage is not limited only to the internal components of resistors. Ceramic insulation and thermal properties will externally insulate and maintain greater thermal endurance as compared to many other kinds of resistors.
The construction of a true ceramic resistor, known as a carbon composite resistor, is different from most other kinds of resistors, even though they may also use ceramics. Ceramic resistors are composed of a combination of finely powdered carbon and ceramic material. These two powders are combined in specific ratios to determine the final value of the resistor.
Ceramic Resistor Construction
Sintering is the process of densification resulting in the formation of a solid mass of material via heat or pressure while remaining below the liquefaction melting point. This is a key step in the manufacturing process used with ceramics.
Ceramic resistors are manufactured using a sintered body of ceramic material, with conductive particles, uniformly distributed throughout the body substance mix, to produce a resistor matrix that is 100% active and non-inductive. Ceramic resistors are also chemically inert.
When carbon is present in a higher ratio to the ceramic, the resistive value will be lower. However, a higher ratio of ceramic material to carbon leads to a higher resistive value for the resistor. When the desired ratios are established, the mixture is compressed to create the resistor body shape and then kiln fired to sinter the ceramic. These types of resistors will commonly have an external shell of ceramic material that will serve as an insulator.
A Basic Ceramic Composition Resistor Overview
Ceramic resistors are commonly used in various kinds of electronic circuits and devices. One very positive nature of Ceramic resistors is high operation temperatures. A drawback of these resistors is that they can create significant amounts of electrical noise. For this reason, a ceramic resistor will seldom be chosen for sensitive radio receivers or other devices which are susceptible to interference.
Some types of ceramic resistors are listed below:
Axial ceramic resistors can perform where high peak power or high-energy pulses need to be handled in a small size. These non-inductive resistors can also be used as alternatives to hard-to-find carbon composition resistors.
The Ohmite Tubular Bulk Ceramic Resistors provide excellent performance for high peak power or high-energy pulses. Bulk construction advantageously produces an inherently non-inductive resistor, and it allows energy and power to be uniformly distributed through the entire ceramic resistor body – there is no film or wire to fail.
Some varieties of ceramic resistors can withstand high energy and high voltages and up to 230o C operating temperature.
Another type of ceramic resistor is a material composition that is formulated to withstand high operating temperatures resulting in high power dissipation. The maximum continuous operating temperature can achieve 350°C. This type is suitable for use in oil without an oil-resistant coating.
Some ceramic resistors can withstand high operating temperatures with high power dissipation. For use in oil without an oil-resistant coating. Higher resistance values and 230°C operating temperatures are also available.
Slab Ceramic Resistors
High energy and non-inductive, this class of ceramic resistors have a unique geometry, providing slim profile to reduce resistor package size. This enables tighter grouping and provides more efficient cooling in forced air applications.
Other types of ceramic material resistors can operate in high energy and high voltage applications. Maximum continuous operating temperature can reach 230°C. The standard dielectric coating is recommended for use in air, and the oil-resistant coating is recommended for use in oil.
Another type of ceramic resistor can withstand high energy and high voltage applications where the necessary resistance value is higher than most other ceramic resistors. Maximum continuous operating temperature is specified at 230°C here as well.
Ceramic Resistor Applications
Motor drive discharge
Medium voltage motor drive systems can be discharged via ceramic resistors as a requirement for safe operation. This kind of assembly may be sized based upon the DC bus capacitance coupled with the shutdown time of the specified motor drive. The current capability of the shutdown relay, coupled with a safe discharge through the ceramic resistor, are two critical functions in this assembly design.
High load testing systems
Load bank ceramic resistors can be used to safely simulate load applications in power designs, such as testing required of EV batteries and charging station output.
Engine braking and crowbar
High power ceramic resistors are typically employed in train locomotives and trams. These resistors will safely convert high kinetic energy, of such vehicles, to heat. Ceramic resistors, in this application, will not exhibit a voltage spike across load terminations.
Neutral grounding resistors
Ceramic resistors are used in this design for the power grounding of Y-connected generators.
Linear accelerator slab
When scientists are examining particle physics in a controlled, contained environment, a huge amount of energy must be effectively and safely contained. Many times, a slab assembly can be submerged in oil. This method will enable a safer means of dissipating the heat generated when powering a linear accelerator.
Parallel Ceramic tubular resistor assemblies are an effective solution as a replacement for wire-wound resistors, which have not safely functioned properly in electrostatic precipitator applications. These types of systems may exhibit peak voltages higher than 300kV, while dissipating more than 2 kW of power.
Encapsulated ceramic resistors
When standard slab resistors are not robust enough to handle the application, designers will seek a more robust thermal and mechanical solution. The use of an encapsulated assembly will help to eliminate the risk of shock or vibration-induced disconnections and contaminants.
The use of a machined aluminum shell design will enable containment of multiple axial or slab resistors in series or in parallel. After being encapsulated, the assembly will be mounted to a heatsink which is liquid-cooled to allow for continuous power, (2-3x normal rating) safely powered through the completed assembly.
Resistor hardware and heat sinking
Every mounting style will have their unique and optimum advantages depending upon the application.
Ceramic resistors range from ½ watt to 1,000 watts in a single component. As power levels increase, the type of mounting requirements will change. Low level wattage resistors, under 3W, typically will be in a surface mount format.
As the resistor power level rating increases from 3W to 10W, axial thru-hole will be the mounting style of choice.
Tubular-shaped power resistor substrates have a large surface area on which the ceramic resistor material can be applied. Mounting hardware is necessary which will help raise the tubular resistor off the PC board.
Selecting Ceramic Composition Resistors
Designers need to pay attention to power rating. Resistors are self-heating components, so they must be able to handle the power dissipation while meeting all key specifications. Heat sinks may be required as well.
When mounting resistors, high dissipation may lead to a larger physical resistor size. Designers need to decide whether a resistor needs additional mounting support
When selecting a resistor, do not over-specify which will add cost and take up more board space. Pay close attention to derating curves in the data sheet. When looking at a resistor temperature curve, does it specify ambient air or heat sink temperature?
Resistors may need to be cooled via normal passive convection or even using forced air. Beware of large components that may block airflow from the fan to the resistor.
Beware of pulsed load applications. Power bursts can greatly exceed ratings. Thick film and wire wound resistors are best suited here.
Self-inductance: A resistor inductance will not be a factor in a DC design. However, at high frequency, the self-inductance may be a problem. Designers may want to look for a resistor with low self-inductance in this case.
In high voltage applications, designers will need to select the right voltage resistor rating and adequate lead spacing to prevent arcing in high voltage designs.
The temperature coefficient of resistance (TCR) of a resistor may change the accuracy of the reading, especially when the resistor is measuring current flow by the voltage drop. Self-heating may cause the resistor ohmic value to change.
Resistor soldering must be performed within the specified temperature of the resistor, the maximum rated time of exposure to soldering, as well as the number of times that a resistor is re-soldered, or the device may be damaged.
When resistors are stored, they need to have a safe temperature, and humidity, with no exposure to sunlight, or corrosive gases/chemicals.
Power resistors may be extremely hot to the touch.
Resistor insertion and mounting must be carefully performed, especially with automated assembly mounting.
Before installing resistors on a PCB, ensure that the board is clean and free of ionic substances. These substances can accumulate during the manufacturing process. For example, the copper etching process can leave chemicals behind. Ionic substances become conductive when exposed to moisture, leading to changes in circuit performance.
Ceramic Composition Resistor Applications
High Current and Braking
Ceramic resistors have a property that will dissipate heat. This resistor capability can be used to slow down a mechanical system. The process is known as dynamic braking and the resistor used in this design is called a dynamic braking resistor. When decelerating an electric motor, kinetic energy is transformed back into electrical energy. Brake resistors in trains and variable frequency motor drives are good examples.
Braking resistors are usually rated at high power with low ohmic values. Cooling can be via cooling fins, fans, or in much higher power cases, water cooling.
Transportation and Automotive-Grade Power Resistors
Some ceramic power resistors, in 500 watts and up, are supplied in silicon ceramic with a tubular ceramic core for these applications. These resistors have wattage ratings from 12 watts to 1kW. Lug terminals are provided for soldering or sturdy bolt connections. These resistors can tolerate an overload of 10x the rated wattage for 5 seconds.
High power resistors may be used in Electric Vehicle (EV) charging systems, power control, power distribution, motor control, and battery modules.
The energy which is dissipated during dynamic braking into a power resistor may be used to heat the vehicle cabin, reducing the requirement for electrical heating, which would place an extra load on the battery. See “High current and braking” and “Transportation and automotive-grade power resistors” sections above.
Counterfeit vs. Originals
The obvious way to detect a counterfeit resistor is to perform a visual and dimensional inspection of a large sample at incoming inspection.
Performing some basic electrical tests at minimum, ambient, and maximum rated temperature per the data sheet on a substantial sample size is an excellent means of detecting counterfeit components
Purchasing from a trusted, reputable supplier such as Ohmite is a sure way to get valid resistors
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