[ARC5] Resistor Excursions

[Richard Knoppow]

    I am not sure about how resistors are made today, I worked for a 
company who made precision metal and carbon film resistors about fifty 
years ago. For very low values, less than the raw resistance of the 
coated blanks, the coatings could be cut away in strips forming what 
amounted to parallel resistors on the blank. These would have no 
inductance beyond lead inductance, and little capacitance.
    I don't remember the values that were adjusted this way and its 
probably different now.
    Remember that when a resistor is spiralled the coiled part is very 
low Q. Generally, film resistors are good to very high frequencies, 
again, the inductance being mostly lead inductance. For modern surface 
mount parts its pretty close to zero.
     Your network analyzer probably went up to considerably higher 
frequencies than my RX-Meter (about 250 Mhz).

On 4/19/2024 10:34 PM, Tom Lee wrote:
> My measurements agree with what Jacques and Richard have said. I once 
> characterized a junkbox full of CC resistors on a network analyzer in 
> order to derive the simplest model that would be reasonably valid out to 
> ~1 GHz and easy for my students to remember when choosing components. 
> Out of that effort I came up with a crude rule of thumb: A CC resistor 
> acts very much like a resistor shunted by a capacitance whose value in 
> pF is numerically equal to the power rating in watts. So, a half-watt 
> resistor has about a half-pF capacitance, and so on. It's not a perfect 
> fit, but in terms of "bang for the buck" it serves well. It satisfied 
> the main goal of the exercise, to come up with something easy to remember.
> 
> As I recall, film resistors were quite similar; the inductive nature 
> that many expect because of the spiral construction didn't really show 
> up unless the resistances were fairly low (~100 ohms on down). The 
> capacitance tended to dominate the reactance.
> 
> The same weekend of experiments yielded another crude rule of thumb: The 
> shunt capacitance of a surface-mount resistor is (again very 
> approximately) in pF the size designation of the part, with a leading 
> decimal point. So, an 0805 resistor has about 0.08pF capacitance 
> shunting it, etc.
> 
> These approximations aren't intended for use in circumstances where high 
> accuracy is needed. They're just for answering questions like, "Would 
> this resistor likely work in this part of this 1 GHz amplifier?"
> 
> -- Cheers,
> Tom
> 

-- 
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998