Resistors for high voltage applications
I thought it might be useful to share some research and practical measurements relating to resistors for high-voltage probes and similar.
You can find resistors advertised with quite impressive-looking specifications (voltage rating several kV, temperature coefficient <50 or 25ppm/K etc.), but what is rarely quoted is the voltage coefficient, i.e. the relationship between the applied DC voltage and the resistance defined at that voltage.
I was unable to find a resistor with a quoted voltage coefficient less than 2ppm/V, and this was only achieved up to a certain resistance value: as the resistance approached the specification limit (for a particular resistor size) , the coefficient rose to (typically) 10ppm/V.
At first glance 10ppm sounds like a small ratio, but if you're wanting to build a fairly precise probe for a DVM, 10ppm/V with 5kV applied amounts to a change in resistance of 5%!! So all that money you spent on a 1% part with a stable temperature dependence was wasted.......
I think the lesson here (for critical applications) is
1) to ensure you have this parameter specified in the first place, and
2) try to use a number of lower-value resistors in series to obtain the desired resistance. I've just purchased some 33M VR37's, which were 5% tolerance but measured closer to 2%.
Vishay quote <10ppm/V, but as these are nominally 3.5kV capable, running them below 1kV resulted in much less variation with voltage (c 1.5ppm/V)
John
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