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High Voltage Resistors
Even such a mundane component as a resistor has special requirements when it
comes to high voltage applications. The usual little 1/4 watt carbon film resistor
used in most other electronics is only rated to 250 or 500 volts, a far cry from
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!
And, of course, the physical length of the device of around a centimeter means
that around 5-10 kV, arcing around the resistor body is a significant problem.
There are a number of manufacturers of resistors intended specifically for high
voltage applications, and of course, you can construct a resistor suitable for
- High resistance, low power - These are typically used for measuring high
voltages as part of a voltage divider (or as a meter series multiplier).
Generally constructed by thin film techniques on a ceramic substrate of
appropriate size for the voltage rating. Suppliers include Caddock,
- High precision - Used in precision voltage dividers for measurement
- High power - Typified by the products of the Cesewid corporation
(formerly Carborundum), these units are designed to take large peak or
average powers and are often constructed so they are non-inductive. A
typical application would be as a current limiting device in a capacitor
discharge circuit, components in pulse forming/shaping networks, or as a
energy dump load. Suppliers include Cesewid and Maxwell Labs..
- Using conventional products well beyond their catalog ratings - The
power dissipation and breakdown voltage issues may be addressed by
potting or immersing the resistor in some suitable substance: silicone, oil,
high pressure gas, etc. You'll need to do some engineering and/or testing
on the system to make sure it will work as planned, e.g. potting in
silicone may increase the breakdown voltage, but will reduce the max
power dissipation. There are some good reasons not to use wirewound
power resistors, even though their nominal breakdown voltage is fairly
- Strings of small low voltage resistors work very nicely, although their
construction is labor intensive. 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. Important design issues which need calculation include
power dissipation, breakdown voltage, and overall precision.
- Water resistors - Actually water and some ionic salt, these are usually
self constructed and are generally of low resistance. Accurate resistance
control and stability is hard to achieve, but there are many applications
which don't require accuracy, including charging resistors for capacitor
banks and energy dump loads. I've gathered some design criteria,
equations and a discussion on construction details into a summary.
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