What is a High Power Resistor？

First understand, what is a high-power resistor?

High-power resistors mainly refer to resistors with higher power than conventional resistors. The size of the power is relative to the conventional, not the absolute amount of resistance above the power.

Our common special high-power resistors include: high-voltage resistors, general ceramic pipe wire-wound resistors, aluminum shell wire-wound resistors, flat non-inductive film resistors, etc. So, how to choose high-power resistors? What are the classifications?

Resistor selection criteria: According to the different materials used, resistors and potentiometers can be divided into alloy type (wound wire, alloy foil), film type (carbon film, metal film), synthetic type (synthetic solid core, synthetic film, Glass glaze) three categories, in addition, according to the structure and use can also be used for other classifications. According to the different requirements of use, the preferred types of fixed resistors are also different. When choosing a fixed resistor, the precautions are as follows:

Resistor Stability

If the function of the circuit has high requirements for resistance stability, such as precision attenuators, sampling voltage divider circuits, etc., attention should be paid to the selection of resistors according to the different loading conditions. When working with DC loads, the order of wirewound resistors, carbon film resistors, metal film resistors, metal oxide film resistors, synthetic film resistors, and synthetic solid core resistors should be preferred.

Synthetic film resistors and synthetic solid core resistors series rarely fail open-circuit and short-circuit, but the resistance value is not stable enough. For insensitive circuits with tolerance design, the use of this type of resistor prevents equipment failure caused by sudden failure of the resistor. Therefore, these two types of resistors are often used in certain circuit designs that require high reliability.

The metalized film resistor series is subject to reverse oxidation reaction when subjected to DC loads, and this reaction is aggravated in hot and humid environments. When the resistors are subjected to AC loads, if the operating frequency is low, they can be selected according to the wirewound resistor, metal oxide film resistor, metal film resistor, carbon film resistor, synthetic film resistor, and synthetic solid core resistor series.

When the operating frequency of tens of kilohertz above, to consider the high frequency characteristics. Some circuit design engineers regardless of the application conditions, wherever the requirements of high stability circuit are selected metal film resistor series. In general, the drift of the metal film resistor series of film resistors for the carbon film resistor series of three times.

Therefore, in the power derating application, carbon film resistor series seems to be lower than the metal film resistor series of drift failure work. There is a type of precision alloy resistor that combines the advantages of both metal film and wirewound resistors, with a very stable resistance value and good frequency characteristics. When the operating frequency is below 5 MHz, it is basically a pure resistor, and is therefore often used as a standard resistor.

Operating Frequency

When resistors are applied to AC loads, frequency characteristics should be considered. When the frequency increases, the resistance value will deviate significantly from the nominal value due to the distributed capacitance, the skin effect, the dielectric loss resistor and the inductive effect caused by the leads. The operating frequency of wirewound resistors is generally not higher than 50 kHz, while the operating frequency of non-inductive wirewound resistors can be as high as 1 MHz or more.

Load characteristics

For AC and DC loads, the resistance value shall not be greater than the power rating PR. When the resistor is used at an ambient temperature Ta greater than the allowable ambient temperature TR, the relationship between the used power P and the nominal rated power is: P = PR (Ta - TR ) (Tmax - TR ), that is, PR should be used with derating.

Under the pulse power, the peak power of the resistor may be hundreds of times the average power. The main limiting factors are: A. The voltage is too high to cause interlayer breakdown; B. The pulse current is too large and exceeds the allowable current density. The order of the resistor's ability to withstand pulse loads is wire wound resistors, carbon film resistors, metal film resistors, metal oxide film resistors, synthetic solid core resistors, and high voltage glass glaze resistors.

Because, in the case of the same power wattage, the film thickness of carbon film resistors is greater than that of metal film resistors, so the ability of carbon film resistors to withstand pulse current is stronger than that of metal film resistors.

Resistance Value Nonlinearity

Resistance value nonlinearity, also known as voltage coefficient, is defined as the average relative change in resistance value for every 1V change in voltage over a specified voltage range. Any resistor has more or less some nonlinearity, that is, the voltage coefficient is not zero.

It is just that some resistor values have negligible nonlinearity and some are more serious. When the voltage coefficient is not zero, the resistance value of various voltages are different, this phenomenon is particularly serious in the impact of high voltage loads. Therefore, high-voltage load resistors must be preferred to good linearity of the resistor.

The reason for nonlinearity is mainly the presence of a decentralized structure in the resistor body, resulting in gap resistance. When the voltage is greater than a specific value continues to rise, the gap resistance gradually decreases. When the voltage rises, the resistor as a whole heating and resistance value changes with the nonlinear effect is different, the former need a certain amount of time to reflect, the latter is an instantaneous effect. In the manufacturing process of the resistor, if there are internal defects, the nonlinearity increases, so this feature can be utilized for screening. The order of preference according to linear requirements is wirewound resistors, metalized film resistors, metal oxide film resistors, carbon film resistors, high-voltage vitreous enamel resistors, synthetic solid core resistors and synthetic film resistors.

noise

For some high-sensitivity equipment, the noise figure is an important indicator, especially the front-end circuit of the equipment, and the noise figure of each part has an impact on the system. The resistors in the pre-stage circuit should be selected in the order of wirewound resistors, metal oxide resistors, carbon film resistors, high-voltage glass glaze resistors, synthetic film resistors and synthetic solid core resistors according to the noise potential. In addition, frequency range and other factors should also be considered.

And how to categorize high power resistors by material?

(1) Wirewound resistors are made of resistive wire wound into a resistor with high resistance alloy wire wound on an insulating skeleton, coated with heat-resistant glaze insulation or insulating varnish. Wirewound resistors have a low temperature coefficient, high precision resistance, good stability, heat and corrosion resistance, mainly used as precision high-power resistors, the disadvantage of poor high-frequency performance, large time constant.

(2) Carbon Synthetic Resistors are made of carbon and synthetic plastic.

(3) Carbon Film Resistors are made by plating a layer of carbon on the ceramic tube and depositing crystallized carbon on the ceramic rod skeleton. Carbon film resistors have low cost, stable performance, wide resistance range, low temperature coefficient and voltage coefficient, and are currently the most widely used resistors.

(4) Metal Film Resistors are made by plating a layer of metal on a ceramic tube and evaporating the alloy material on the surface of the ceramic rod skeleton by vacuum evaporation method.

Metal film resistors have higher precision, better stability, noise, and smaller temperature coefficient than carbon film resistors. In the instrumentation and communication equipment in a large number of adopted.

(5) Metal Oxide Film Resistors are made by plating a layer of tin oxide on the ceramic tube and depositing a layer of metal oxide on the insulating rod. Because of its own that is oxide, so the high temperature stability, heat shock resistance, strong load capacity. According to the use of points, there are general-purpose, precision, high-frequency, high-voltage, high-resistance, high-power and resistance network.

In summary, resistors are the most widely used components in electronic circuits, usually forming different series according to power and resistance value, which are used by circuit designers. Resistors in the circuit is mainly used to regulate and stabilize the current and voltage, can be used as a shunt and voltage divider, can also be used as a circuit matching load.

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