TekScopes@groups.io | Glass resistors - characteristics?

Understanding Glass Resistors – Key Characteristics

The term "glass resistors" is commonly associated with high voltage applications, offering exceptional performance in terms of breakdown resistance. Glass resistors are distinguished components in the realm of electronics, noted for their ability to withstand high voltage while maintaining stability against moisture and other environmental factors.

Currently, I am involved in refurbishing a vintage B&K tube tester from the late 1950s. Reviewing its schematic and components, it’s evident that the majority of resistors are categorized as 'glass', with tolerance levels of 5% to 10%. Throughout my experience in electronics, I've never encountered a specific classification like this before. I’m seeking clarity on what precisely characterizes a glass resistor and its primary features. Are they wire-wound, carbon film, or perhaps another type?

Physical Characteristics and Construction

My first thought was that these resistors are encased in a glass tube to mitigate moisture’s impact. However, given their 5% tolerance, it's likely that moisture-induced resistance changes would be minimal in comparison to inherent tolerance variations. Could it be that long-term stability is the primary concern?

Replacement Considerations

While it’s yet unclear if there’s a need to replace any of these resistors, understanding the correct replacement material is crucial. Would I be better off utilizing metal film or something else entirely? The choice of replacement could significantly affect the performance and reliability of the unit.

Insights from Experts

Contributing to this discussion, one user mentioned that Corning manufactured a glass-encased metal film resistor, typically observed in aircraft UHF transmitters, highlighting their longevity and credibility. Other participants noted that Victoreen produced high-value glass-encapsulated carbon film resistors as well.

High Voltage Applications

As pointed out, modern hermetically sealed glass resistors can handle extremely high values with tight tolerances, particularly in environments where contamination risks are significant. Therefore, for high voltage applications, thin or thick film resistors may be appropriate alternatives. Many experts agree that ensuring adequate voltage handling is essential for reliable operation.

Understanding Glass in Resistor Design

One community member explained that the terminology surrounding glass resistors may be outdated. However, their ability to withstand higher voltages compared to lacquer-coated counterparts remains significant, with some 'non-special' glass-coated resistors rated for up to 3kV.

Concluding Thoughts

In summary, while the concept of glass resistors may not be prevalent today, their importance in historical electronics, like tube testers, remains noteworthy. Understanding their properties and characteristics is essential for anyone looking to refurbish or maintain vintage electronic equipment.

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