High Voltage Chip Resistors

It's often tough to find good resistors with high enough voltage and resistance ratings. Stackpole has expanded its HVC product line in both areas. Their surface mount resistors are now available up to 10Gohms, and with voltage ratings up to 25kV.

Stackpole claims that their manufacturing process yields resistors with lower noise and better stability.


Other specifications include:

• Temperature coefficient ratings are as low as 25 ppm per degree C.


• Tolerance as low as 0.5%


• Voltage ratings from 5kV to 25kV for chip sizes 0603 to 2512, respectively.

High Voltage Resistor Types

Once the resistor voltage rating goes above a few hundred volts, there are very few manufacturers to choose from. The main types of high voltage resistors are:

• Surge high voltage resistors, which are used in series with circuit elements that might experience arcs or intentional transients. Examples include resistors in series with an arc lamp or the resistor in an RC filter since they will need to carry current due to arcs.
  
• Precision resistors (used for high voltage dividers). Typically these are thin film resistors.
  
• General purpose high voltage resistors, generally thick film. Compared to precision high voltage resistors, these are not as stable, have worse voltage coefficients, worse temperature coefficients, and worse tolerances. However, they are lower cost and can be rated at higher power. Applications include high voltage dividers where precision is not important, load resistors and bleeder resistors.

More than with low voltage resistors, it is important to select a high quality manufacturer to obtain a part with good performance and reliability. Also, it is important in many applications to specify non-inductive types.

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.