High Value Resistors Manufacturing And Application Issues
There are many resistors available with values in the mega-ohm range. Resistors in this resistance range are available in a wide range of package styles to suit a variety of applications. Making high value resistors for specialist applications in the Giga-ohm range is more complicated.
High Value Resistor Applications
Very high value resistors are generally used to minimize measurement errors. Their primary application is in low electronic signal measurement equipment. Applications include bio medical systems where measurements in the picoampere range are required and high gain amplifiers such as photodiode applications.
High value resistors are also found in high voltage applications. A high voltage across a low value resistor would (according to Ohm's Law) cause a very high current to flow. This could cause resistor damage unless the resistor had a large mass and/or significant thermal management measures were in place.
It is impractical to construct high value wirewound resistors due to the maximum resistivity and minimum wire diameter of wire alloys. The wire length required for very high value resistors makes manufacture within an acceptable form factor impractical. Materials in film form have higher resistivities than wire.
Due to their construction Bulk metal foil and thin film resistors maximum resistance tends to be in the 20MOhms range. Although some specialist devices are available up to approximately 100MOhm.
It is possible to manufacture thick film high value resistors in the GigaOhm range with specialist devices available with low TeraOhm resistance values.
Thick Film High Value Resistor Manufacturing
The resistance value of a thick film resistor depends on the composition of the resistor paste. Specifically the composition of the glass, the metal oxide conductive particle size and the number of conductive particles per unit area.
The higher the metal oxide to glass ratio the lower the resistivity. To manufacture high value resistors requires high resistivity paste with a relatively low number of metal oxide conductive particles.
High resistivity pastes require careful processing during the manufacturing process. The pastes are particularly sensitive to the firing temperature which must be closely controlled. The resistors electrical performance depends on the choice of paste and substrate material and the manufacturing process. Manufacture of high value resistors should therefore generally only be entrusted to specialist manufacturers with the required experience of the thick film technology manufacturing process.