Resistors

[k7yoo]

I have been replenishing my resistor assortments and have been refilling the 2 watt bin with metal film resistors from Digi-Key. Just for grins I Googled for 2 watt carbon ohmage and landed on Surplus Sales of NE website. I note that he is asking around $5 a pop for carbon 2 watters. What am I missing here?? Is this about esthetics for resto-freaks or is there some technical reason metal film is not as good? I can see where the MF resistors may have some adverse L characteristics, but the specs say otherwise. Enlighten me Oh great ones
a-ohm....aaaa--ooohhhmmm   (Tibetan Technician Chant)

[steve_qix]

Wow - $5.00 !

I use metal film resistors up to 30mHz with no problems whatsoever.  The metal film resistors are also very stable over a wide temperature range.  Sounds like another case of AudioPhoolery (or something like that) when a 2 watt carbon resistor costs $5.00 

Regards,

Steve

[KB2WIG]

  " Surplus Sales of NE website. "

I've thought fer a long time that ss of ne overprices their stuff............ 

 Fair Radio goes overboard sometimes... A while back, (1999?) I asked for a price on a connecting cable for an ART 13.  They wanted $125. At the time, the 2 end connectors were going for $50 total and the cable itself was not unobtainable. They didn't make the sale. - Nowdays, new connectors are available from Robert Downs,  wa5cab?? for around $30 a piece.   klc

[WA1GFZ]

Carbon resistors have a very high peak power rating. Films are very good at RF but their pulse power rating is a lot lower. For us hammies either works fine. Carbon resistors drift with age. I think a film will be more stable over time if you size it properly.

[KL7OF]

Carbon resistors are needed for beverage antenna termination as they will handle the current and voltage spikes that build up in beverages.  Carbon film types will work but they need to be replaced when they burn out from overload.(which is quite often, especially in the spring and summer lightning season) 

[k4kyv]

I have loads of 1/2, 1 and 2-watt carbon resistors, plus a few 5 and 10 watters, mostly used but some new, and prefer to use them in restoration  projects.  I have found that they drift considerably with age, so I always check them before putting them into a circuit.  The drift is usually to a value of resistance up to 75% higher than the nominal marked value.  In many applications, such as grid leaks and plate resistors, a 25% shift is inconsequential to performance of the circuit, while in others, you need close to the exact value.

I have found the drifted values to be stable, so if I need a 75k resistor, but can only find a 47k that has drifted up to that resistance, I may still use it and that has never caused me any problem.

Most film resistors are 1 or 2%, while typical carbon resistors are 10% or 20%.  Ones rated at 5%, with the gold band, are definitely in the minority.  If I am putting a circuit together, I use whatever resistor I can find with the proper resistance, and pay little attention to the type.  But when restoring older equipment with some "vintage" value, I prefer to avoid orange-drops and film resistors, if I can find good working period components.  If I am restoring a real antique piece, I go all out and hollow out the wax capacitors and refill the case with a modern one, and use resistors with the old-style case-tip-dot markings.

For the best peak power handling capability and stability, I prefer wirewound resistors if the built-in inductance isn't a problem.  Although the tolerance is usually not marked on the resistor, I find many of them, even ones decades old, to be precisely on the marked value according to my digital ohmmeter.

[WA1GFZ]

wire wound resistors are the most stable

[N3DRB The Derb]

when I get off my "quiet time" here I'm going to start building the lytic string for Jenny. JN sent some carbon 2 watters @ 220K
which will parallel quite nicely down to the 100K level but with twice the wattage per cap, which was a change they made in later versions. I measure them before they go in, if they are more than +- 50% percent of value I'll check the circuit and see if the value is critical for some reason. If not, then I use em, if so I dont. In a lytic string I'm going to match them up to each other and I want them to have not drifted much. I worry about them changing value unequally in the future making a cap go BANG!

It depends on what circuit how important it is. In most applications, it's not.

[WA1GFZ]

I had the lab test a number of resistor types to failure to see what happens to resistance as it fails. I had to measure the resistance of an explosive squib while in a bomb rack. We didn't want the resistance to go down and set it off under any condition. Wire wound resistors just open all others go low in resistance as the smoke leaks out.

[W3DBB]

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[WA1GFZ]

I've used both film types with good results. They both will have low pulse ratings. I have a transient generator with 2 1 ohm 2 watt  carbon comp.resistors in series as part of a shaping network. I'm throwing 200 amps through them for a couple microseconds at a rate of 1 HZ.
(DO-160E  600v Transient generator)
I run it for hours. When we built it I found these cool film resistors rated for many watts and a heat sink interface. Looked like they were made for switcher current sense applications. The guy who built it for me went through pains to mount them correctly. We blew them out in about 30 seconds. Carbon resistors take a beating in pulse applications

[WA1QHQ]

Just like Frank said, I did a lot of lightning supressor design work at a previous job and carbon comp resistors were the only thing that would hold up to the tremendous peak currents caused by lightning hits. My problem was finding carbon comp resistors. This was about 15 years ago and even back then most resistor manufacturers had long since stopped making carbon comp resistors thus the high price.

[WA1GFZ]

Mark,
I was doing some lightning testing a few months ago at around 1KA and need to shape the pulse a bit. I found some nice 7 watt film resistors (the size of a 10 watt WW) for an LR network. I had 7 of them in parallel and after a few hours and a couple dozen strikes I took all of them out. A single 2 watt carbon would have sailed through it.

[WBear2GCR]

There is a very good reason that they are charging so much.

Actually two... that place is generally overpriced, and more importantly afaik no-one is making Carbon Composition resistors anymore!!

Why?

No demand. Everything is surface mount. Film, not carbon comp!

The two types of metal film are the standard MF, spiral cut, the other is "bulk film" as made by Vishay and Caddock and others... non inductive and very good, very expensive too!

The other type that might be included is metal oxide...

20 secs into the future there will be articles about series paralleling many surface mount resistors to equal a single 1/2 watt carbon resistor for replacement purposes. 

Times are changing friends.
Think about how easy it was to find one of those TV doorknob caps?
Try to find one today.
How about some sweep tubes??

Simply stated, standard size components of all sorts are going away.

                  _-_-WBear2GCR

[W1VD]

The main reason for the high price is that domestic manufacturers stopped making them...although there is some indication that at least one manufacturer may start again. There are a couple foreign manufacturers currently supplying them.

While designing and building resistively-loaded, wide-band UAV receive antennas a few years back I had to settle for NOS AB 1/8- and 1/4-watt carbon comps. Virtually all resistors were out of tolerance - right out of the sleeve. The distributor tipped me off to a mil procedure for baking carbon comp resistors before use - apparently that was SOP for mil manufacturers back in the day. Took over the wife's oven for a couple days and almost all resistors came back into close tolerance. Can dig up the mil paper if anyone is interested... 

[k4kyv]

The main reason for the high price is that domestic manufacturers stopped making them...although there is some indication that at least one manufacturer may start again.

Now if they would just improve the formula and make them stable enough that the resistance wouldn't change over long periods...

Their peak power handling capability and non-inductivity are the advantages they have going for them.  There will always be certain uses for those characteristics.  But if there is no longer any massive demand, the price will be high.

Ever look at the prices of newly manufactured Western Electric 300B's? "The domestic retail price for the 300B is $400.00 per tube and $900.00 for matched pairs." Of course, a little audiophoolery is involved in the calculation.

I save every nut, bolt, capacitor, transformer, choke, tube and resistor that I can salvage out of any piece of point-to-point wired electronic equipment that I discard, before throwing the rest away.  I even save bits of hookup wire if I think they might ever come in handy.  Ditto for any other component if it is easily removable and potentially worthwhile.  Sometimes even the stripped down chassis base is saved, especially if there aren't too many holes.

[WA1GFZ]

Jay,
One of our techs told me that the lab can buy them for us. It is common to bake boards to get moisture out before coating is applied. I know a lot of my stash has increased in value. There are a number of different non-inductive power resistors
out there today and many can be mounted on a heat sink.