Àâòîðû: Frederic Cordier,Johan Montagnat
Èñòî÷íèê:http://staff.ui.ac.id/
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..
High Voltage Resistors
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.
New HV Surge Resistors
It's often difficult to find high voltage resistors rated for surge applications. Arcs can occur inadvertently in many applications, thus, requiring surge limiting resistors. Other applications have designated periodic rapid discharges. Stackpole Electronics has announced a significant expansion to their ASR and SPR series of axial leaded anti-surge resistors.
The ASR series adds a ? watt body size to the current sizes of 1 watt, mini ? watt, and mini ? watt sizes. The SPR series has significantly improved the maximum working voltage up to 2000 volts, the max. overload voltage up to 3000 volts, and the surge withstanding voltage up to 10KV. The ASR and SPR series are a power metal film technology and are really the same part. The SPR series is only for the 1 watt and mini ? watt sizes and strictly the values from 1 Meg ohms to 12 Meg ohms that are UL 1676 recognized.
The ASR series is available in resistance values down to 100 ohms and up to 33 Meg ohms and can withstand surges from 1 kV up to 10 kV depending on size and resistance value. Together they offer an excellent alternative for carbon composition and ceramic composition resistors for pulse power withstanding with low inductance. In fact, side by side testing shows the ASR/SPR series is superior to carbon comps for high voltage surges.
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