Parasitic capacitance of SMD resistors and their generated noise

google resistor voltage coefficient

Carbons can be ballpark 100 PPM/volt.

John Larkin wrote: ================

** Yawnnnnnnn ....

How pathetic.

** Published them years ago.

The excess noise as voltage was applied ( up to 20V) was marked, except for MF types

** And all your other senses too.

** Not audio.

Wot a tedious f*****ad.

..... Phil

** Google " arrogant f****it " - see yourself described. ** But are in fact not.

..... Phil

Comps can be 200.

In a tube amp with, say, a 50 volt p-p swing, that could be 10000 PPM gain change, 1% distortion. That's almost enough to hear.

I'm having fun. Are you?

Semi-metals are semiconductors, but at or near their intrinsic temperatures. A bunch of metal shavings in a tube, if shaken, has enough Shottky rectification at the contact points to be an RF detector (coherer was the antique radio term). Ceramics or metal oxides, with metal, are going to have minority carriers. At each metal connection, you get carrier injection.

Carbon resistors, especially in presence of ozone, drift to higher values because the carbon slowly turns to CO or CO2. Metals also grow oxide skins (some more quickly than others). Substrate, metal, and glaze (or at least lacquer) plus two solderable connections... a complete surface mount resistor packs a lot of engineering into that little package.

John Larkin bullshitted as usual wrote: ================================

** Bullshit ** Made up, absolute crap !!!!

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Just to refute JL's rampant insanity - I carried a test: With a sine wave at 1KHz feeding a small toroidal supply tranny in reverse. Max available = 250Vrms at 0.05% THD.

A pair of MF resistors ( 100k and 1k ) in series showed only residual THD across the 1k. Change the 100k to a ( very old ) 0.5W CC and the THD reading was then 0.33% Drop the input level to 50V rms and the reading was 0.07%.

250V rms = 705 V p-p. 50V rms = 141V p-p.

FYI

One cannot test the * lineartiy * using DC voltages. Cos the damn things heat and drop value by several %.

..... Phil

Yes! I learned that the hard way. Decided I was gonna make my own HV divider from a string of 22meg CC resistors. I was astonished by the change in resistance with voltage. So I made a new scale for the meter.

I said "comps", meaning carbon composition. We were talking about carbon resistors.

In a divider made from a string of identical resistors, the nonlinearity cancels. That's why the best dividers are all on the same substrate.

The long-term resistance shift doesn't behave that way at high voltages, because the ones at the top of the string suffer more corona discharge which can therefore erode them faster. I found this out the hard way.

John

I guess it should be coated or potted or something.

Ww have a potential customer who wants a 1500 volt power supply programmable and stable to 1 PPM. That could get interesting.

Some sort of shield could prevent corona. But 1500v isn't bad.

No, I don't think so. The resistors all have a negative voltage coefficient and they add in a string.

Given 10 identical resistors in series, each drops 1/10 of the total voltage.

Yes. But the resistance at rated voltage is much lower than it is at a couple of volts.

But in a divider with 25% matched nonlinearity, you'd have

1*0.75 / N*0.75 = 1 / N regardless.

Of course since the nonlinearity is caused by hot filaments beginning to form inside the resistive element, one wouldn't expect them to be particularly repeatable or time-invariant.

Cheers

Phil Hobbs

It's about the same as focus voltages in old CRT displays; I've replaced a lot of focus resistors.

To prevent corona around a carbon resistor, you might consider getting the resistor sealed in vacuum (glass tube package). At 45 kV auto ignition wiring just had to go to 8mm diameter silicone insulation: the corona in silicone is harder to start than corona in air, so... you build up the insulator to fill the high-field volume entirely.

Victoreen and Ohmite make vacuum packages :

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** Game ove.,

** You awake ?

At any applied voltage, all the series Rs have the *same* voltage drop. The 10:1 ratio is unchanging it they all have the same deviations.

...... Phil

Actually, once a hot channel has formed it can be pretty stable.

When there is just enough channeling to cause temperate gradients across the current path, these can move the current path around and can give you an unstable resistance. We saw this with NTC thermistors at one place where I worked - when the automated test gear tried to dissipate about a milliwatt in the thermistor the resistance - displayed to six significant figures - never showed the same last two digits for any length of time.

Measuring the resistance on a higher resistance range (with less power dissipated in the thermistor) gave a stable result, but at lower resolution.