Traditionally Wire Wound Resistors meant coated resistors of either silicon or vitreous enameled on a Ceramic body which was wound with resistance wire which was welded or soldered to the terminals and these types of resistors were termed as professional or industrial grade Wire Wound Resistors.
However in the mid 1970’s there was a demand for commercial grade wire wound resistors to meet the growing demand for a cheaper alternative to be used in the commercial or entertainment sectors.
This demand was largely met by the introduction of ceramic encased resistors wherein the resistor core was an extruded fiber glass body which had resistance wire wound on it in a continuous process .These continuous fiber glass wound rods were cut to size and the terminals were crimped on to the fiber glass resistor core as opposed to the professional/industrial grade where the resistance wire was welded to the terminals .These fiber glass sub-assemblies were then inserted into a ceramic case and potted with a silicon resin infused potting mixture which contained inert fillers.
However with the constant escalation of the costs of ceramic products over the years the cost advantage of these ceramic encased resistors has eroded and this has narrowed to a point where many customers would now like to re-evaluate the decision as to whether to use a silicon/cement coated resistor or a ceramic encased resistor.
In order that the component user can make a comprehensive techno/economic decision, we at HTR, now give below a table showing the salient differences and advantages between the two technologies.
|Techno/Economic Parameter||Silicon/Cement Coated Resistor||Ceramic Encased Resistor|
|Moisture Absorption||Extremely limited||Should be moisture proof, provided the potting mixture is correctly utilized and formulated.|
|Tight Tolerances||Upto to even 0.05%||Limited to 5% at best.|
|TCR||Very low TCR’s as low as 10 ppm/°C are possible.||Due to the method of construction it is observed in practice that optimum TCR’s achievable would be at best 60 ppm/°C .|
|Load Life||Extremely Robust||Extremely Robust|
|Flammability||Fire Retardant Coatings are available which are now UL certified .(However under sustained overload conditions there is a possibility that the resistor may burst into flames momentarily which will immediately self extinguish)||It is Flame proof due to the nature of its construction.|
|Manufacturing Cost||Average due to low curing temperature of Silicon/Cement coating.||Higher as there is an additional process of potting the resistor in the ceramic case|
|Emission of Fumes||Very limited fumes||Very Limited fumes|
|Production Process (From an Environmental angle)||Good||Good|
|Non Inductive Winding||Possible||Not Possible due to nature of process|
|Power to size Ratio||Excellent||Fair|
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