The traditional method of encapsulating resistors through the vitreous enamelled route which was in vogue till about 30 years ago has now been largely replaced with Fire Retardant Silicon/Cement coating.
It has now been observed in the market that apart from some very limited applications, where the environment is extremely harsh such as the areas of where “salt spray “or where the resistors will be immersed in oil, the new Fire Retardant Silicon/Cement coated resistors are as suitable or better than the vitreous enamelled variety at a fraction of the cost.
Further in order to meet the gap in cost between the Silicon/Cement coated Wire Wound resistors and Vitreous Enamelled resistors, some manufacturers of Vitreous enamelled resistors use a low melting temperature enamel which is contra in quality to High Quality Vitreous coating.In addition some vitreous coatings are not RoHS compliant and this must be examined in detail by the component user when their requirements are for RoHS compliance.
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 between the two technologies.
|Techno/Economic Parameter||Silicon/Cement Coating||Vitreous Enamelled|
|Moisture Absorption||Extremely limited||Though in theory, these enamelled coatings are supposed to be moisture proof but in practice due to the nature of production process very often minute hairline cracks are observed, reducing its ability to be moisture proof.|
|Power to Size Ratio||Excellent due to availability of thermally efficient raw materials.||Comparable but Higher temperature may be observed.|
|Tight Tolerances||Upto to even 0.05%.||Limited to 0.5% 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 50 ppm/°C.|
|Load Life||Extremely Robust due to reduced curing temperature.||Some failures are possible due to the fact that the enamel has a melting point in excess of 600°C and during this process there is a possibility of weakening the weld joints due to differences in the thermal coefficients of expansion of the different metals involved.|
|Flammability||Fire Retardant Coatings are available which now UL certified are.||It is Flame proof.|
|Cost||Average due to low curing temperature of Silicon/Cement coating.||Very High due to large energy cost owing to high melting temperature of Vitreous Enamel.|
|Extreme Overload Condition||Can withstand internal heat generation upto 350°C without significant deterioration of coating||Can withstand internal heat generation upto 500°C without significant deterioration of coating, though hairline cracks have been observed.|
|Emission of Fumes||Very limited fumes which are almost Nonexistent in the case of Fire Retardant coating.||No Fumes|
|Production Process (From an Environmental angle)||Good||Poor due to nature of process.|
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