resistors wanted

That used to be a staple product. I suggest that when searching for them, use the term "composition" rather than "carbon" to describe them.

Bill

-- Fermez le Bush--about two years to go.

" snipped-for-privacy@polka.co.za"

them,

Carbon film on ceramic substrate would be the way to go. A number of asian manfs make them but the OP may have difficulty in locating a distributor locally who carries 2W types.

Ross Herbert wrote in news: snipped-for-privacy@4ax.com:

Might not be... Metal or carbon films break down catastrophically with high voltage, but solid carbon resistors don't, until much more is forced on them. I found that out when making a limit resistor for a HeNe laser tube, I ended up with 2W solid carbon resistors, 4 in series, wrapped in adhesive lined heatshrink. It lasted for years, but previous experiments with metal film AND carbon film types ended up like little fireworks in seconds, even when selected for an apparent voltage rating that could stand the supply voltage. My guess is the supply went high to ignite the tube, and having started a burn, continued it even though tube ignition brought the voltage within safe limits. Solid carbon doesn't care about that, it just works.

The OP didn't say why he wanted them, but my guess is similar high voltage systems where the current path must be safely distrubuted through the whole volume of the resistor.

Lostgallifreyan wrote in news:Xns98FC55011E48Ezoodlewurdle@140.99.99.130:

I ought to add that it never failed. :) I dropped the tube, but the resistor assembly and PSU survived all.

Plus the construction is different. The composition resistor material fills a hollow tube; the composition varies according to resistance. The film resistors are spiral cut to get high values and can be inductive. You can also get arcing across the cuts if the voltage is high enough (despite the coating).

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I agree that the generally available metal film resistors don't tend to stand up well to high pulse voltages but I have found carbon film to be much better in this regard. Carbon composition may be preferable in this regard depending upon the application.

The problem is that fewer (none?) manufacturesrs these days make carbon composition resistors, and even then they will usually be max

For example, check out the product lines of two well known manufacturers and see if you can locate a 2W, 2.2Mohm carbon composition - or carbon film even.

Ohmite do make a ceramic composition resistor as a replacement for the carbon composition and the OY series is 2W. I would go for this option myself.

Ross Herbert wrote in news: snipped-for-privacy@4ax.com:

I pass. >:) The emphasis on high voltage stuff being commonplace is vanishing steadily along with CRT's, as LCD's and such take over.

I think there will always be specialised high-volt parts manufacturing though. You won't find (m)any oil filled multi-KV rated caps or solid carbon resistors at Digikey or RS, but people do still make them. I guess the problem is that most aren't made to general preferred values anymore, but to specific bulk customer order only.

Nice find on the ceramic composites. I like the look of those. I bet they cost a bit, though..

Since the resistor market has been cornered by a couple of companies, conventional carbon and metal film product is vanishing fast, as the demand from high-volume producers is almost over.

I can remember some years ago picking up a bulk load of resistors, probably a couple of million for less than $10 the lot. It took 2 van loads to clear and was all good quality product.

That was then. Now I find that demand is rising as manufacturers wake up to the fact that unless they start buying this stuff from the main manufacturers, their product lines are going to stop through lack of parts.

The ranges that RS Components (for example) carry these days in conventional leaded resistors is very small compared with 10 years ago.

Even specialist parts like flame-proof resistors are getting expensive and hard to find from regular distribution, and the makers don't want to know for relatively small quantities.

Peter

-- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk

On Fri, 23 Mar 2007 08:21:23 GMT, Lostgallifreyan put finger to keyboard and composed:

This has always bothered me. In TV sets there are special high voltage "focus" resistors. My catalogue lists a 1W, 10KVDC, 7KVAC RMS type with values in the range 100K to 10M. I can understand a pulse rating of 10KVDC, but I can't understand the RMS rating. Even at 10M, the maximum sustainable voltage for a 1W resistor is only 3kV.

Furthermore, my experience with switchmode PSUs in all manner of mains powered equipment suggests that there is an unusually high failure rate in the 100K-500K "startup" resistors that are used to kickstart the chopper transistor, even though these resistors are operating well within their specified wattages.

- Franc Zabkar

Franc Zabkar wrote in news: snipped-for-privacy@4ax.com:

My guess (not a very good one) is that although their RMS rating would force the resistor to dissipate more than 2W, it would at least not arc over until that voltage was reached. It might be designed as a short-term max rating.

I don't know enough about switchmode PSU's to comment other than suggesting that pulses that reach those resistors them might have a high enough peak current to erode their ability to withstand what should be a safe average current. The term 'kick start' suggests to me that some heavy peak currents might hit them.

On Sun, 25 Mar 2007 09:21:17 GMT, Lostgallifreyan put finger to keyboard and composed:

These resistors only ever see the peak rectified AC mains voltage which in 240VAC locales is 340VDC. A 330K resistor would therefore see a max current of around 1mA, and a continuous wattage of around 350mW.

- Franc Zabkar

Franc Zabkar wrote in news: snipped-for-privacy@4ax.com:

I had a look at Repairfaq.org and a couple of diagrams, and I can't see why either. The only thing I can think of is that they run hot, and might not be as good at running hot permanently as they're supposed to be. Power cycling would aggravate that. Also, the fams in many PSU's might draw gunk onto them that reacts with them. Heat would accelerate that, and if the resistors rely on a film, it won't take much to damage it if there's a heat-assisted reaction. I doubt the makers test for that much, but it would be interesting to know if those nicely glazed and shiny looking ceramic types have a lower failure rate in the same situations, compared to the more porous coatings.