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are insufficient power resistor ratings common?

chazix

chazix

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I'm starting to think so. I've seen clear-cut examples in a Belles and a classic-era Luxman, just to shame a few. And I can't say how many resistors I've seen that have overheated in power amplifier Zobel networks. (I've pretty much stopped worrying about that, unless the Zobel parts are out of spec or there's some reason to think the amp might be oscillating.)

If my name were going on the design docs, and there was s a hot-running resistor, I would specify parts with at least 2X the nominal power rating, I'd make darn sure that the part was raised off of the PCB, and if any bean-counters objected I would totally play the "fire hazard" card.

Am I going overboard?

Oh! Maybe this is a vintage thing? Now that circuit simulations are common, hopefully it is hard to mistake a power rating?
 
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they certainly would have known what the dissipation was even without sims, but there is a lot of corner-cutting that goes on in manufacturing just for cost reasons. 1/2 watt part dissipating not greater than 1/2 watt? Send it. By the time it causes a problem it'll be old enough that nobody cares.
 
It seems those "Zobel" 10R resistors are often rated for "normal" operating conditions but not necessarily for abuse or even for full bandwidth power testing. But hey they're flameproof so all is good... right?
 
I think the replies above have pretty much nailed it - thanks.

I don't know why I'm still naive about these things. My last day job was actually with a manufacturer of electronic gizmos, and I learned first-hand what can happen after a design gets passed to the "manufacturability" team.
 
chazix said:
... My last day job was actually with a manufacturer of electronic gizmos, and I learned first-hand what can happen after a design gets passed to the "manufacturability" team.
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Not good, if your implication is manufacturing had freedom to change engineering design specs without oversight.
 
whoaru99 said:
Not good, if your implication is manufacturing had freedom to change engineering design specs without oversight.
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Well, not quite that bad. Changes DID had to be reviewed by the designer. But it's easy for me to imagine the designer caving in to an argument that goes" That resistor dissipates half a watt, so a half-watt-rated part is sufficient. It will outlast the warranty period, and it will save $0.01 per unit. Don't worry, we'll still use a flameproof part."

I can't say that I know that it was that kind of reasoning accounted for design changes that I learned of in my manufacturing job. But I can say I observed some cost-cutting-motivated design changes that turned out to be problematic.
 
Oh, i just remembered another case, from another audio manufacturer that I think is deservedly well-respected, if a little obscure. Audio Analogue Maestros (1st gen, I think) have a resistor that is pretty much guaranteed to fail within a decade or so of use.

Again for this case, I can't know that it was a cost-cutting change. But it was another flagrant example where I believe a resistor rated for more than the nominal power dissipation should have been used.
 
I don't think under-rated parts are all that common, but what they don't take into account is time. You have to derate things even further if you want 20-50 year life. Boards start turning brown, plastic near lamps gets brittle and turns brown. Nylon gears and pulleys dry out and crack as they shrink around their brass inserts. Rubber parts and foams turn to goo. There are lubricants that don't gum up years down the road, but they cost a lot more than the grease that slowly turns into epoxy. Zinc-based flywheels develop "zinc pest" and crack or disintegrate. Designers only appreciate this stuff when they're near retirement.
 
ConradH said:
I don't think under-rated parts are all that common, but what they don't take into account is time. You have to derate things even further if you want 20-50 year life.
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They probably did consider time, just not that much, e.g. people would be stringing it along 30, 40, 50 or more years later. Sorta your point but a little different angle.
 
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I've been told that the job of an engineer is not to make the best thing possible, but to figure out the cheapest possible way to make the thing.
 
gadget73 said:
I've been told that the job of an engineer is not to make the best thing possible, but to figure out the cheapest possible way to make the thing.
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Where I work there is a margin improvement team that looks for cost reductions. The "normal" Engineers engineer to accomplish the design goals.

All margin improvement projects must also pass engineering sniff test, so to speak.
 
My wants vs what I expect someone else decided is totally unrelated. I just have to hope they didn't cut that safety margin too thin and it fails.
 
No mention of other components degrading causing an increase in the demands on resistors?

I believe I had that situation with a 2W resistor eventually dissipating just over the limit in an area where the heat couldn’t really escape easily. Board repaired, parts replaced, resistor upgraded to 3W and the amp sounds great now.
 
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