Can any one tell me what is the origin of voltage ratings on ordinary
resistors (i.e. film and composition types.) In every case where I had to
design a high voltage divider, the dominant factor was always the power
rating of the resistors. This is intuitively a limiting factor, since thermal
dissipation requires a certain amount of surface area and/or volume to
conduct and convect away the heat.
But I don't really see how any serious voltage stress is present within a
conductive medium, whose conductivity has been specially selected to produce
as much loss as possible within a small but finite length.
Some high-voltage resistors are simply long film resistors wound into a
spiral on a cylindrical substrate. For this topology, the inter-turn spacing
would become a factor to consider. However, I am not inquiring about
resistors of special HV design.
For instance, when you look at a 1W ceramic body DC resistor, the end caps
alone look like they could easily withstand many kilovolts. So, I am really
baffled as to the origin of this 'voltage rating'. Manufacturers have some
interesting and unusual ways of establishing specs on their products, and I
have never personally had a need to be involved in this one. But listening to
the comments here has piqued my interest to the extent that I would like to
know if there is any factual basis for it.
By the way, you can fabricate very nice HV meter multipliers using 1206 or
2012 SMT resitors. I have made up little "sticks" of them that end up looking
like ordinary fuses. I keep the voltage across each resistor down to 2 or 3
hundred volts. I think they would handle 500 easily.
Eric von Valtier K8LV
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