Voltage rating of resistors and what the failure looks like

[SergeiF]

Hello,

This is my first post here, so please bear with me .
I thought eevblog forum (I am subscriber to Dave's channel) would be the best place to discuss the issue below.

As the topic title suggest I am here to discuss the Voltage ratings of a fixed resistor (specifically 1/4W through-hole).

I have got into an argument with a friend of mine regarding importance of the voltage rating of resistors where the mains (240V) voltages are concerned. He states it is nonsense and that the only way the resistor would fail if it would overheat (ie exceed its power rating). His argument is that the distance between two end caps on through hole resistor is too great for voltage to ark through.

I would like to actually see the images of the failed resistors due to over voltage if they do exist. If these images do not exists I am happy to jerry rig a low voltage rated mega-ohm range resistor to "blow up".

I am located in New Zealand and normally source my components from RS (or jaycar if I am desperate). I am having difficulty locating a ~50V rated through hole resistor so I could supply mains voltage and let the magic smoke out on camera.

The actual experiment will involve a low voltage rated 1MOhm resistor being simply connected to mains voltage (the power dissipation will be around 0.05W at 240V, unless I made a mistake).
That is if I can source low voltage rated resistors. So far the lowest I could find were 200V, which is not enough for them to fail reliably ().

[james_s]

240V can arc over small package resistors. All it takes is a bit of condensation or other contaminant on the resistor body, especially something that is just conductive enough to cook into carbon. If the element breaks internally then an arc will form between the two sections and quickly burn up the whole resistor, once an arc is started it can be drawn out much further. With a high value resistor it's possible to get a lot of voltage across it before exceeding the power rating.

When a failure like this happens there isn't much left to the resistor, I've encountered some that were nothing but a charred spot on the PCB. Whether it was due to inadequate voltage rating of the part or something else I have no way of knowing.

Look at it this way, resistor manufactures don't provide voltage ratings just for decoration.

[SergeiF]

Tonight I will grab >500kOhm random resistors I have to see if any of them fail within a few hours at mains voltage (I suspect the experiment will fail).

Due to age difference between me and my friend I need extraordinary proof.

[james_s]

The ratings tend to be pretty conservative, they may not fail in a few hours, although if you were to try say 0402 resistors with 240V they likely will fail instantly. The thing is, when you design a product that is mains powered, it needs to operate for many years, and most importantly do so *safely*. Is it really sensible to operate a component outside of its ratings when the consequence of failure could be a house fire? Maybe there is only a 0.0001% chance that it will flash over and catch fire, but what if it does?

It's like storing a can of gasoline (petrol) next to a gas fired water heater, it's probably not going to blow up but would you take that chance?

[Paul Moir]

Google image "carbon film resistor construction".  The spiral cut in these resistors' film only leaves a very tiny gap compared to old carbon composition resistors.  It's fairly obvious once you see the construction.

[MagicSmoker]

Paul Moir beat me to it on the spiral construction, but I was also going to point out that most resistive materials have a voltage coefficient of resistance (usually negative, and with film types being the worst), so that is another factor that tends to limit the voltage a resistor can be used at beyond what Ohm's law would claim.

[Benta]

Short answer:

There is a reason for the existence of high voltage resistors, eg, Vishay VR25, VR37, VR68. I use these exclusively when designing mains connected networks.

Normal metal-film resistors are spiral-cut (for trimming) and susceptible to flash-over between the cuts. One voltage spike is enough.

[grouchobyte]

One word....google it....ELECTROMIGRATION

@grouchobyte

[Tomorokoshi]

Let's say the resistor is in series with a circuit that is at a high voltage, but the current through the resistor is low.

Now let's say a PCB track runs underneath the resistor that is at ground. This gives two more failure modes:

1. If it's a high-value resistor with one lead connected to ground, the arc distance is reduced because the track running underneath is closer to the high-voltage lead.

2. Even with no current through the resistor, the potential on the resistor could make the insulation on it break down and current would flow to the track underneath. Once that current flows the resistor could get damaged very quickly.

Consider that the PCB mask gets thin at the edge of the track, the resistor insulation could get damaged during assembly, they could be cheap low-voltage resistors compared to the circuit they are on, etc.

[SergeiF]

Awesome guys .

Thank you.

[T3sl4co1l]

On a moment-to-moment basis, absolutely nothing.  Probably.

Do you want that resistor exploding with full mains force, on the off chance it should fail?

Or, that it fails quietly and stops doing whatever important thing it was supposed to be doing.

Mains is a dirty environment.  Over several years time, expect to get several surges over 1000V.  The average case (240V RMS) is not the worst case.  Statistics rules all!

Tim

[joeqsmith]

CC on my AC line tester can be seen in first few seconds of this video.

[grouchobyte]

On a moment-to-moment basis, absolutely nothing.  Probably.

Do you want that resistor exploding with full mains force, on the off chance it should fail?

Or, that it fails quietly and stops doing whatever important thing it was supposed to be doing.

Mains is a dirty environment.  Over several years time, expect to get several surges over 1000V.  The average case (240V RMS) is not the worst case.  Statistics rules all!

Tim

Ya, mains is certainly a good training ground for what not to do.....thanks for reminding me Tim!
At least 25 years ago, i had designed an autoranging power supply to meet nternational power requirements (90-264 V Rms,  50/60 hz). A sensing resistor (carbon comp, i recall) was specd in by yours truly) no worries, right?

Well, in the midst of a bunch of critical path deadlines and swamped with stuff to do,  an ECO was hand delivered to me at my desk with an urgent request to buy an alternative sensing resistor because the one I had specd in was not available from the distributor. I just scribbled my initials on th ECO without thinking twice about it. Big mistake.

A year later, I learned that two systems with my autoranging supply just stopped working. One in India and the other in Peru. When we got the systems back we found the sensing resistor a complete open circuit, but no physical damage, burn marks etc....cosmetically perfect. SEM ANALYSIS CONCLUSION: failure due to electromigration......and it was a metal film resistor, not the carbon comp I specd in.

Argggg........

[basinstreetdesign]

Here is another over-voltage failure example.  These 10 Ohm/2W resistors were for a one-off home project.  They were surge limiting resistors connected between the bridge rectifier of a high-voltage power supply (about 500V) to the first filter cap.  They were to carry about 1-1.5A Pk current pulses, about .3A avg(max) and dissipate about 1 watt max.  I don't know who made them, I just bought them at a local hobby shop known for importing a LOT of stuff from China, because they were available.

The first time I turned it on, nothing untoward happened for about 10 min.  Then a BANG and a small cloud of smoke told me they were toast. Huh?!

They developed a small hairline crack which allowed the 3V across it to change to several 100's of volts.  This, of course jumped across the crack and arced loudly until the crack widened to be larger than what would allow the spark to persist.

Three of them did this.  I replaced them with name-brand resistors later and all was well.

[T3sl4co1l]

On the other hand, I once had a similar application with a good resistor: 10 ohm, metal oxide film, flameproof.  It was exposed to several hundred volts when the switching device above it failed shorted.  There was a little spark of material ejected from the resistor, and it failed open circuit, just like that.  No smoke (well, a tiny puff with the spark), no smell, no flame, and no more current flow!

Flameproof, fusible resistors are commonly used in small power supplies (under 10W), because they're cheaper than fuses(!), and double as inrush protection (saves a separate NTC resistor).  The quiescent loss (because, it doesn't stop being a resistor under normal operation!) is a small enough fraction of total power dissipation that it's easily ignored.

Tim

[DimitriP]

I am having difficulty locating a ~50V rated through hole resistor so I could supply mains voltage and let the magic smoke out on camera.

So...what does that tell you ?

[SergeiF]

If you didn't pick this up from original post, yes I fully agree that the voltage rating of a resistor is very important.

I just need to convince an old-schooler that knows better than me. The only way to convince him is via some practical examples.
The pictures provided here should suffice, as they are different from overheated diode, having specific points of failure consistent with tracking.

This makes the neon screwdrivers somewhat dangerous.

[Brumby]

The difficulty you are likely to have with convincing an "old-timer" is that it is likely they have seen many, many resistor failures over time - and that those failures have been the result of excessive power dissipation.  This is (in my experience) by far the most common failure mode - so much so that the first instinct in assessing the cause is to assume this.

Once you have this bias in the forefront of your thinking, then some failures that were not from that root cause will still look like they could have been.

The other thing is, if that person has not been involved with design - especially component selection - then they may well not be even considering the voltage ratings of resistors in products that they have seen.

The big problem is that as long as the power rating is respected, the ability for a resistor to survive voltages greater than it was designed for is fairly high - so you can get situations where components in such situations work fine for a time.  Sometimes, for a very long time.  People who do not respect all elements of a datasheet will end up developing so much of a familiarity with this observed 'successful' usage that they end up dismissing voltage specifications - or at least ignoring them.

This robustness of the humble resistor is also likely to make any demonstrations of problems from exceeding voltage specifications frustratingly inconclusive - especially in the short term.  (Obviously that excludes voltages capable of arcing and excessive power dissipation.)

The best source for evidence is to do what you have done - come here.  We may not be able to give you an experimental setup for demonstration, but there are some experiences that other members can share (and have).  Unfortunately, I am not one of them.

[mmagin]

Paul Moir beat me to it on the spiral construction, but I was also going to point out that most resistive materials have a voltage coefficient of resistance (usually negative, and with film types being the worst), so that is another factor that tends to limit the voltage a resistor can be used at beyond what Ohm's law would claim.

Definitely this.  I've mis-used quarter-watt carbon film resistors as a improvised voltage divider where they probably saw 1 kv across each one, and without any discernible failure, the effective resistance as indicated by circuit behavior was much much lower than spec.

[james_s]

I once blew a 1k(?) resistor in a Fluke 77 monitoring the current out of a microwave oven transformer driving something else. It was a roughly 2kV transformer but the meter was completely isolated from ground, sitting in series between the transformer and load. I'm still not entirely sure how that happened but I replaced the resistor and it fixed the meter.

[Mechatrommer]

I have got into an argument with a friend of mine regarding importance of the voltage rating of resistors where the mains (240V) voltages are concerned. He states it is nonsense and that the only way the resistor would fail if it would overheat (ie exceed its power rating). His argument is that the distance between two end caps on through hole resistor is too great for voltage to ark through.

I would like to actually see the images of the failed resistors due to over voltage if they do exist.

they do exist, no picture since i had no time for a selfie long years ago and the arc is like sub millisecond how can you possibly shoot that im prom tu?, when i was repairing my newly bought strobe light. it turned out a TH resistor in the line of flash light ignitor is arcing like everytime i push the button. but we are talking like tens or hundreds of KV across. the solution was to put a chunk of 3M tape in the path of the arc across the resistor's leads. but the strobe's twin had no problem, same resistor. maybe due to longer geometry during hand assembly or lower operating ignitor voltage. otoh in the sub KV range like 240V mains, i've never seen one, so yes i guess your friend is right at that lower range, as far as the arcing is concerned, so long the resistor is operating at its rated wattage.

[tecman]

The standard method used for high voltage is to use multiple equal value resistors in series to divide the voltage across an individual resistor.  Look at what a manufacturer recommends as a guide.

paul

[mikeselectricstuff]

Resistor voltage ratings really are a thing, and you should not exceed them.
They can degrade over long periods (years) if exceeded, increasing in value or going open-circuit.
I've seen failed resistors in switchmode PSUs, where the startup supply was fed via a single resistor instead of multiple parts.
 

[SeanB]

A lot of SMPS startup issues are the startup resistor going high value or open. As there typically is a 100k 0.25W carbon film or higher value resistor there, and it might take 2 years before the resistor fails from the 300VDC applied across it, you get away for a long time with this, but these will fail eventually.  Solution is 2 resistors in series, and if you look at the better SMPS manufacturers then you see they typically use 2 leaded resistors in series, or in SM format often 4 or more resistors, even though the current through the series string is low enough that one will do, they will fail in use otherwise.

Simplest way to demo this is to take a 1M 0402 SMD resitor, and connect it to a pair of wire leads, and place this across the rectified and smoothed mains ( bridge rectifier, 10uF 450V capacitor and a very good  3A ceramic fuse in the supply line, 3A because this is a common plugtop fuse in the UK, so cheap) for a few minutes. The 1M resistor will generally either blow up, or go open circuit, the fuse being there to break the arc fault when it does flash over.

Another thing to ask him is that electrolytic capacitors are typically formed at 1.5 times operating voltage, so why are they not run at this voltage in use.