Wednesday, July 25, 2007

Precision High Voltage Resistors

When selecting and subsequently evaluating precision HV resistors, it is important to consider how rugged the parts are, with respect to surges and rapid voltage transitions. Both conditions can occur in typical applications.

Precision HV resistors are often used in high voltage divider applications. Thus, you are relying on the absolute resistance to remain constant over long term conditions.

If there is significance capacitance across the divider, then it is possible to have a large surge current if there is a momentary fault within the capacitor. THis is not unusaul with high voltage film capacitors, as they are self-repairing (if the internal fault is minor). High surge current can cause localized heating, resulting in permanent resistance change (as well as a temporary change).

If there is a rapid dV/dt voltage change, such as when an external arc occurs, it is possible for the resistor to suffer a permanent change in resistance value. In addition, depending on parasitic capacitances, voltage during a transient will most likely not distribute evenly over the total divider resistance. Thus, the localized dV/dt may be very high, and the absolute voltage across some portions of the resistor(s) can be higher than its ratings.

Thus, it can be worthwhile to run surge and dV/dt tests on precision resistors that are being considered for your applications.

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