HV behaviour of carbon resistive element - Page 1
Forum section: electronics / projects

author: Manul
author_rank: Super Contributor
timestamp: October 16, 2021, 07:53:18 pm
content: Hello,specifically, I'm interested in carbon track trimmer pots. How carbon track element would react if maximum voltage rating is (hugly) exceeded? If power dissipation is out of equation (very short pulse), is it likely, that:1. It will experience huge non-linearity (sort of coherer action), because of micro-arcing between individual particles (with permament damage?)2. It will stay linear until arcing between end contacts and/or wiperTo give example, AVX HC 414/30-N 10K trimmer pot. Rating 300V. Lets say pulse of 500ns at 1.5kV.Any comment on physics or your personal experience is appreciated. If question is stupid and can only be answered experimentally, it's ok.Thanks

author: Gyro
author_rank: Super Contributor
timestamp: October 16, 2021, 08:13:08 pm
content: From personal experience... The carbon film track (and underlying phenolic substrate) will arc, burn and blister and the smell will be unpleasant. I guess this counts as long term damage.Linearity will occur somewhere else!P.S. Sorry, I haven't tried it at 500ns.

author: Benta
author_rank: Super Contributor
timestamp: October 16, 2021, 08:25:53 pm
content: This is the kind of thing you'll need to experiment with yourself in your private lab.Why someone would want to operate components way outside their specifications is a mystery to me. If you need HV components, then buy such parts. It may be expensive,but that's the way it is.No one here is going to give recommendations on this.

author: Manul
author_rank: Super Contributor
timestamp: October 16, 2021, 09:25:20 pm
content: My thinking process was (assuming open-air construction):If resistive element of some length is made of totally uniform resistive material, then I think it would always arc between it's ends. That would be the voltage limit (maybe I'm wrong on this).In contrast - if resistive element is, for example, wirewound, then it would arc between adjacent windings. So sum of all gaps between windings becomes limit.I was just interested where in this picture falls carbon track pots or carbon composition resistors.Quote from: Benta on October 16, 2021, 08:25:53 pmI'm far from advocate of using parts outside specification, I always pursue a quality design. Sometimes, aside from just doing things, I'm curious about various physical aspects (scientifically). I thought that this one is interesting.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 16, 2021, 09:34:39 pm
content: Does this align with a transient/lightning type standard? Even with anecdotal support, it sounds like you may end up having to do sample testing, which usually means you need to set up a transient generator that is based on a standard circuit/parts - and that may or may not be practical for you.

author: Gyro
author_rank: Super Contributor
timestamp: October 16, 2021, 09:36:49 pm
content: Quote from: Manul on October 16, 2021, 09:25:20 pmNow carbon composition resistors are a completely different animal - they have great surge capability...https://www.eevblog.com/forum/projects/resistor-extreme-pulse-overload-experiments/

author: Benta
author_rank: Super Contributor
timestamp: October 16, 2021, 09:45:52 pm
content: Quote from: Manul on October 16, 2021, 09:25:20 pmFair enough, thanks for clearing that up.The key words are "carbon composite", which means a homogene mix of carbon particles and something else, the "something else" usually being some kind of polyxxx/epoxy/whatever (lots of trade secrets here).The breakdown mechanism is not end-to-end, but internal sparking/discharge between the carbon particles, leading to either higher resistance or a total flashthrough. This seems to depend on the level of overload. Additionally, the internal sparking may also make the binding material carbonize, making the situation worse.I'm not an expert, but this the kind of failure mode I've seen when carbon resistors get overloaded.

author: Manul
author_rank: Super Contributor
timestamp: October 16, 2021, 09:47:09 pm
content: Quote from: Gyro on October 16, 2021, 09:36:49 pmI see you are the authority on the subjectThanks for the link.

author: TimFox
author_rank: Super Contributor
timestamp: October 16, 2021, 10:05:34 pm
content: I believe that traditional "carbon composition" resistors, such as the Allen-Bradleys, are a mixture of carbon and talc powders to achieve the required resistivity. Their thermal capacity allows a reasonable pulsed-power capability over the steady-state power rating. Back in the late '70s, a factory engineer from A-B told us that for high-resistance units (22 megohm, 2 W), the unit would flash over (along the outside) before it would suffer an internal high-voltage arc. The use of talc dielectric powder does give a reduction in effective resistance at high frequencies, however, as the talc dielectric tends to short out the carbon particles.

author: Manul
author_rank: Super Contributor
timestamp: October 16, 2021, 10:32:54 pm
content: Quote from: Benta on October 16, 2021, 09:45:52 pmSo a sudden drop of impedance under overload condition can happen, earlier then clearance/creepage would otherwise allow. I was suspecting that. Thanks for information.Out of curiousity, I'm still running this thought experiment / simulation in my head and weirdly strugling to make a clear view. So element has finite (and quite low, lets say 10K) resistance, but also at the same time some dielectric strengh in V/m. I kind of understand, that dielectric strengh is a separate property and no one cares, that there is conductivity at the same time. Equivalent circuit I think should be dielectric gap in parallel with ideal resistor. Still, something is slightly disturbing when I think about it. Maybe I should stop thinking about these things and go work...

author: Manul
author_rank: Super Contributor
timestamp: October 16, 2021, 10:46:37 pm
content: Quote from: TimFox on October 16, 2021, 10:05:34 pmThat's interesting information, thanks. So it seems that these high value resistors had higher internal dielectric strength then the air gap between the ends.

author: TimFox
author_rank: Super Contributor
timestamp: October 16, 2021, 11:04:36 pm
content: So long as there are no air bubbles within the mixture, the dielectric strength (in V/mm) of the talc (which makes up most of the mixture for high-megohm resistors) should be higher than that of air, and the flashover of the package is a "creepage" path (along the boundary between phenolic plastic and air) that is much weaker than air itself. Allen Bradley (now part of Rockwell Automation) no longer manufactures these resistors.

author: CaptDon
author_rank: Super Contributor
timestamp: October 17, 2021, 02:52:29 am
content: I have used metal cased carbon pots (Clarostat, Centralab, Etc.) Where the failurein High Voltage pulse mode showed up as an arc fault between the 'hot' connectionand metallic body of the pot which happened to be grounded for safety. These werevarious ohmic values from 50k to 500k. The pots got used in other projects in normalfashion and they didn't seem to suffer. (Well, one was destroyed when the flashoverbecame a constant burning arc to ground through carbonized phenolic base material.)

author: Terry Bites
author_rank: Super Contributor
timestamp: October 17, 2021, 02:37:58 pm
content: Old CRT scopes and TVs had focus pots with Kv across them. The limiting factors is power dissipation and insulation. Creapge and cleance inside the pot will set the upper bound. Weve been here before.https://www.eevblog.com/forum/beginners/potentiometers-what-determines-the-max-voltage-rating/

author: TurboTom
author_rank: Super Contributor
timestamp: October 17, 2021, 02:57:34 pm
content: To shine a light form the "other side of the wall" on the subject: Jet engine igniters (read: spark plugs) use a rather wide gap compared to IC engine plugs. That's required to provide reliable ignition of an "not-so-flamable" fuel at rather low ambient pressure / temperature conditions. To accomplish this, a high energy capacitive discharge is used (several joule pulses at a few pulses per second). Older systems use series-ignition coils to superimpose a high voltage peak to the capacitor charge voltage (which is usually in the 2kV ballpark). These systems used plain ceramic circular-gap plugs. Later on, the engineers found out that a semiconductor surface across the gap lowers the breakdown voltage considerably so the series ignition coil could be eliminated, the 2kV charge voltage would suffice to break down a 2~3mm semiconductor shunted gap reliably. So nowadays, most such engines use these shunted gap igniters.If this means that a carbon film potentiometer (of probably >10mm circumference) would be at risk of breaking down across the resistive suface if overloaded at 1.5kV for microsecond periods is a different question, but at least the breakdown voltage across the resistive film will be lower than an air gap of the same length. Otherwise I'ld repeat what others recommended before: Conduct some experiments of your own