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1-10 of 74 messages
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resistors as fuses
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by W8JI on November 1, 2009
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Let's set a myth aside. Resistors do not make good fuses. Fuses themselves are notoriously slow for minor overloads, some fast blow fuses taking a minute or more at a 200% overload.
Resistors are horrible fuses.
I connected 4 ten-ohm resistors in parallel and ran one amp through them from a current limited supply, and they are at 5 ohms resistance and sinking!
That was initially a 5x power overload on them. Instead of opening, they are slowly shorting.
Carbon is a semiconductor. How many diodes do you find that fail open from overload currents or excessive heat?
Where does this stuff about resistors being good fuses or a good form of minor overload protection come from? When did we quit reasoning through simple problems with things like resistors?
Are we getting so advanced we are forgetting how a basic simple resistor actually behaves, or forgot how to connect a resistor to a power supply and do a simple experiment?
Tom
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RE: resistors as fuses
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by W5DXP on November 1, 2009
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OTOH, fused-resistors have been readily available as far back as 1968 when I was specifying them for the external digital input time constant on SCADA systems. Here are some product examples:
http://www.alibaba.com/showroom/Resistor_Fuse/--------------------3------.html
http://www.vishay.com/docs/30232/wwf.pdf
http://www.alibaba.com/product-gs/255703592/Temperature_Fuse_Ceramic_Resistors.html
--
73, Cecil, w5dxp.com
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RE: resistors as fuses
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by W8JI on November 1, 2009
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But fused resistors, like fuses, are still notoriously slow and have variable failure times.
A fusable resistor not a regular resistor, and behaves more like a regular fuse with a series resistance. With a few hundred percent current overload you might have 30 seconds to a few minutes for it to open. A regular carbon resistor often would never open.
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RE: resistors as fuses
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by W5DXP on November 1, 2009
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> W8JI wrote: But fused resistors, like fuses, are still notoriously slow and have variable failure times. <
The fused resistor market is large with many successful products for many kinds of applications. Those products meet their technical specifications.
I'm just adding additional information to what you posted - not looking for an argument. Between resistors and fuses, there is a middle-ground product called a "fused resistor" with which some of the readers may not be familiar.
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73, Cecil, w5dxp.com
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RE: resistors as fuses
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by K4DPK on November 1, 2009
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A good example of a resistor used as a fuse is in the B+ of an L4B amplifier.
There you have a .8 ohm resistor of maybe 3 watts, which will fail if the B+, in the neighborhood of 3 kv, goes to ground.
In that instance, the resistor is subjected to waaay in excess of 3000 watts, over a thousand times its rating.
Fuses have to act quickly to protect anything. In my test on another thread and in Tom's, both simulating a hypothetical use in a cathode circuit, none of the resistors failed, even under the extreme conditions of the tests.
Phil C. Sr.
k4dpk
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RE: resistors as fuses
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by K1BXI on November 1, 2009
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Phil, is that .8 ohm resistor carbon or wire wound?
My question is, if a resistor is used as a protective device, would a wire wound work better as a fuse than a carbon composition resistor?
Although I still like regular fuses and circuit breakers.
John
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RE: resistors as fuses
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by K4DPK on November 1, 2009
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The whole issue here,I think, is that resistors can be used reliably as fuses ONLY when the expectation is whatever catastrophe may occur will be absolutely and immediately destructive in terms of the resistor ratings.
This would be wattage many, many times (>100X, 1000X, ?) the resistor rating.
I'm not sure what that .8 ohm resistor is, maybe a small ww.
Tom may know.
Phil C. Sr.
k4dpk
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Urban Legends & Grandma's Brisket
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by N2EY on November 1, 2009
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W8JI very rightly asks:
"Where does this stuff about resistors being good fuses or a good form of minor overload protection come from? When did we quit reasoning through simple problems with things like resistors?"
Perhaps this story will explain how some urban legends are born...
---
There was once an extended family that had a big dinner gathering almost every Sunday, as well as all holidays. The various family members took turns hosting these dinners, and all were good cooks.
But the best of the all was Grandma, whose best dish was a beef brisket roasted her special way. Others in the family tried, but none could compare with Grandma's.
Grandma's brisket wasn't just good food or great food. It was close-your-eyes-curl-your-toes-better-be-sitting-down-because-you'll-go-weak-in-the-knees food. Grandma, however, considered it only fair-to-middlin', and didn't understand what all the fuss was about.
And you knew it was Grandma's because she cut off a two or three inch piece of the narrower end and roasted it on its side, next to the main part.
As time went on, various family members became concerned that the secrets of Grandma's brisket would go with her to the Big Kitchen Upstairs someday, and that they'd better document it while they could.
But Grandma was the kind of cook who didn't use written recipes, and whose measurements consisted of a handful of this, a pinch of that, a scoop of something else, cook till done, etc. Asking her to write anything down was pointless, she just said "it's not that good, not worth the time to write down".
Thus was born The Secret Brisket Project. Various family members would find out when Grandma was making her brisket and would arrive early, to "help out". Others would take her shopping for groceries, even though Grandma had no problems driving or shopping on her own. Of course what they were really doing was secretly observing and documenting the whole brisket process, a step at a time.
The various relatives compared notes gathered over more than a year and figured out the process start to finish. Grandma was fussy about some things, such as only going to a particular butcher for the meat, and only using a certain brand of spices, but much less fussy about other things ("salt is salt", as Grandma would say).
The one thing they couldn't figure out was why Grandma cut off that two or three inch piece of the narrower end and roasted it on its side, next to the main part.
Various theories were proposed, and the one that gained the most credence was the idea that the narrow end cooked faster, and had more fat, and by cooking it on its side the moisture and flavor would not be lost. The family was convinced that this was Grandma's secret.
And indeed the family members succeeded in duplicating Grandma's brisket in test runs by following the secretly-obtained directions exactly.
For Grandma's birthday, a special dinner was held, with the brisket as the main course, end piece cut off and all. And as a memento, the family presented Grandma with a framed copy of the recipe, in calligraphy.
At her birthday dinner, the family presented the memento to Grandma as they brought out the main courses. The first instruction, after the list of ingredients, read:
"Cut off a two to three inch piece from the narrow end and place on its side in the roasting pan".
Grandma blinked once, then twice, her eyes wet. Speechless, her hand holding the framed recipe began to shake.
It was an intense moment - until people realized that she was speechless because she was laughing so hard she couldn't get a word out.
Finally she regained her composure enough to say:
"But, dear, I cut off the end piece because the roasting pan I have is a little too short."
---
It's Sunday. Have a nice dinner.
73 de Jim, N2EY
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RE: resistors as fuses
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by K2OOG on November 1, 2009
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Tom,
what you say makes really excellent sense!
In designing circuits an engineer should never use resistors instead of fuses, when the design clearly calls for appropriate fuses.
It terms of resistors, would not a lower power rated resistor be more prone to burnout under overload than a higher rated resistor, or have you found that the power ratings of resistors are totally meaningless?
In designing, how do we determine which power rating to specify?
David
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