Resistors - do they short or do they open? - Page 1
Forum section: electronics / beginners

author: kalel
author_rank: Frequent Contributor
timestamp: July 28, 2017, 02:09:10 pm
content: Once a resistor becomes faulty, perhaps through excessive current and heating, does it generally open, or does it sometimes short, or does it mostly short, or is it random?This is speaking of standard resistors, I understand that some resistors intended to be used as fuses exist too.Related, but a different question would be also can a notable permanent change in resistance occur if the resistor is 'overdriven' but not to the point of "breaking", i.e. shorting or opening? I understand that resistance changes with temperature, but don't know if a permanent change is also possible.In case there are videos about these things that I've missed (I watched some that show resistance change with temperature temporarily, but that's all), I would love to check them out as well.Update: I forgot to do the search first. It might turn out that this has been asked millions of times.

author: helius
author_rank: Super Contributor
timestamp: July 28, 2017, 02:26:26 pm
content: A resistor seldom fails short. If it is getting hot enough to fail, a short would typically dissipate even more power and cause the material to ignite or explode.That being said, there are some resistor constructions, like the bifilar wire wound (used for low-capacitance and low-inductance special resistors) that could short out internally. In high-energy circuits, it's important to either use a fuse in line, or specify a fusible resistor, which is tested to always become open under fault currents.About permanent changes in resistance: these are more likely in older-style carbon composition resistors. They are made of a mix (dried slurry) of clay and carbon particles. If heated near their melting point, some of the carbon atoms can migrate and change the bulk resistivity of the mix. The metal film resistors that are overwhelmingly used now don't suffer this effect but they can ignite if hot enough.

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2017, 03:35:17 pm
content: Generally speaking resistors fail open circuit, but a high value resistor, with a voltage across it exceeding its maximum voltage rating, can fail short by arcing over, which can drastically reduce the resistance value.

author: grumpydoc
author_rank: Super Contributor
timestamp: July 28, 2017, 03:40:21 pm
content: Quote from: Hero999 on July 28, 2017, 03:35:17 pmI think Hero999 means generally open circuit.More often they just become higher resistance rather than becoming completely open.But, as they say, anything is possible.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: July 28, 2017, 04:18:16 pm
content: Also consider the application. What is the voltage rating of the resistor? What is the spacing to other circuits? How high does the ambient temperature get under normal operation? Under fault operation? What type of mechanical stress are they under, such as vibration and shipping?What happens to downstream circuits if the resistor fails to any arbitrary worst-case condition?Some design standards evaluate all these and more when it comes to resistors.

author: Brumby
author_rank: Unknown
timestamp: July 28, 2017, 04:19:58 pm
content: Quote from: grumpydoc on July 28, 2017, 03:40:21 pmThis ^^^I have yet to personally find a resistor fail as a short (or low value). They mostly go high for me - how high depends on how well cooked they get.

author: free_electron
author_rank: Super Contributor
timestamp: July 28, 2017, 04:25:47 pm
content: resistors fail by changing value.some go up , some go down , some arc over, some bodies fall from between the pins ( in which case the air resistance remains which is heavily dependent on local pressure and humidity )some special fusible resistors will open in most cases. but, surpass the voltage rating and break current and they still may arc and catch fire ...

author: Brumby
author_rank: Unknown
timestamp: July 28, 2017, 04:27:08 pm
content: The other thing is ... when a resistor is cooked, the protective coating is very often compromised. This means that you can get external environmental influences (such as moisture) affecting the performance of the resistor.While such changes are pretty well guaranteed to be permanent, you shouldnotexpect them to be stable.

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2017, 06:16:26 pm
content: Quote from: grumpydoc on July 28, 2017, 03:40:21 pmYes, you're correct. I've amended the post. Resistors usually fail open but high values can fail short circuit, due to insulation failure caused by over voltage.