High Voltage Resistor
A resistor has special requirements when it comes to high voltage applications. The typical little 1/4 watt carbon film resistor used in most other electronics is only rated to 250 or 500 volts, well below the kilovolt levels needed. The voltage limitation is usually set by power dissipation issues: a 10K resistor with 1 kV across it dissipates 100 watts, well in excess of the typical 1/4 watt.
Even in low current applications, the physical length of the device of around a centimeter means that around 5-10 kV, arcing around the resistor body becomes a significant problem. Also, some resistive materials can significantly change resistance in the presence of high electrical stress. Some high voltage resistors use a spiral path of darker metal oxide on ceramic. This type of high voltage resistor is often called a "tiger stripe" resistor. The peak voltage capability of these is often a function of body length, and they are available in voltage ranges of up to 50 kV. Another important type of resistor is the non-inductive tubular or bulk ceramic resistor, such as those made by Kanthal or HVR Advanced Power. Because of the larger volume of resistive material, these resistors are capable of handling significantly higher pulsed power than their wirewound or metal film counterparts, making them suitable for rapid energy dumping and high energy pulse work. There are a number of manufacturers of resistors intended specifically for high voltage applications, and of course, you can construct a resistor suitable for your application.
- Voltage dividers to measure HV with standard voltmeter.
- Charging resistors to limit the current.
- Load resistors for an impulse generator.
- Capacitor discharge and energy dumping
- Controlling the circuit Q.
In some cases, the HV experimenter can use conventional products beyond their catalog ratings by potting or immersing the resistor in some suitable substance such as silicone, oil, or high pressure gas. Using this method will require additional testing to ensure the assembled device operates as intended.
A simple but labor intensive method to construct HV resistors is to string together small low voltage resistors. This is a popular way to make high precision voltage dividers, because you can inexpensively obtain high precision resistors, and the whole assembly can be potted to reduce corona problems.
Potting in silicone may increase the breakdown voltage to avoid arcing around the body of the resistor, but will reduce the max power dissipation, because the heat cannot convect away and is trapped within the device. Therefore, select a high resistance value which will minimize current and therefore power.
Solder together a long string of resistors, insert into a length of PVC pipe, and fill with silicone. Important design issues which need calculation include power dissipation, breakdown voltage, and overall precision.
If possible, avoid wirewound power resistors for applications involving a large amounts of stored energy. Their nominal breakdown voltage is fairly high, so they may be suitable for high voltage, low current applications. However wirewound power resistors should be avoided for high power beyond their rated capacity, because if a small open develops (the typical failure mode, due to excessive power disspation or minor physical damage) an arc will form at the gap, melting the wire back, making the arc bigger. This can result in catastrophic failure modes, such as a short-circuit.
One solution for inexpensive resistors that can handle high voltages and energies is the water resistor. A water resistor is essentially a tube filled with a salt solution and an electrode at each end. You can control the concentration of the salt to control the resistance, for a given size tube. The water provides very high heat capacity and high dielectric strength, and many high energy water resistors used in pulsed power work use copper sulfate solutions and copper electrodes.