Exceeding a resistors rated voltage - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: June 04, 2016, 09:40:22 am
content: I have something bugging me with surface mount resistors in this case. Obviously the maximum working voltage of a resistor is actually defined by its wattage and resistance as it will be the voltage at which the maximum power dissipation you are happy with or that the device can sustain has been reached. But what about transients and spikes?I ask because I quite often design very low power devices for automotive and I do include a TVS diode but in order to make spiky world of automotive electrics a bit easier I tend to put a resistor in series with the power input as even a couple of hundred ohms is acceptable. Obviously at 12 or 24 V this is not a problem but what happens if I get a 250 V spike coming into a resistor that is spec for 150 V? And of course I have to assume that the host vehicle might not be perfect and might be worse than that and have higher voltage spikes. Will the resistor be damaged or does the maximum working voltage mean that beyond this the current would just arc between the caps of the resistor? If arcing occurs then frankly I'm not too concerned as there is a TVS diode right after it to take the voltage causing that arc and clamp it. I also tend to pot my products which I'm assuming would prevent voltages arcing through the air from one end of the resistor to the other.

author: Rerouter
author_rank: Super Contributor
timestamp: June 04, 2016, 09:48:26 am
content: It comes down to the package size your using, an 0402 and your out of luck, a generic 1/4W through hole and you should be fine, based on most guidelines a separation distance of almost 2 mm is required for "functional isolation" for 250V,Now i do think the resistor will drift from these encounters, but it would take a fair few to start causing worry. (large negative spikes at low duty cycle greatly reduced the value on some foil type resistors i have used)Again that drift will depend on the materials chosen, and if they spec for low duty cycle currents,

author: wraper
author_rank: Unknown
timestamp: June 04, 2016, 10:10:24 am
content: Thick film resistors will reduce their resistance if the arching occurs within them.http://www.rohm.com/web/in/r_what10QuoteDoes your potting flow under the part?

author: Simon
author_rank: Unknown
timestamp: June 04, 2016, 10:17:55 am
content: Quote from: wraper on June 04, 2016, 10:10:24 amWell at least the resistor would continue to conduct which is better than filing open circuit. No I could not guarantee that the potting compound flows under the resistor.I have to look into resistors with bigger voltage ratings. Or just put a small but higher voltage TVS diode right at the input.

author: coppice
author_rank: Super Contributor
timestamp: June 04, 2016, 11:09:12 am
content: The specifications for SMD resistors usually specify the maximum working voltage, and its quite low for the smaller ones. It has little to do with resistance and power. If you applied a large pulsed voltage you would stay within the power rating of the resistor, but go way over the rated working voltage. The voltage rating is mostly about breakdown.

author: coppice
author_rank: Super Contributor
timestamp: June 04, 2016, 11:20:43 am
content: Quote from: blueskull on June 04, 2016, 11:10:49 amIf you pot properly, in a vacuum, that might be true (I'm not sure. There might still be issues with the case of the resistor for all I know). I you pot the crude way most electronics is potted there will be so much gas present that you still have discharge issues.

author: Simon
author_rank: Unknown
timestamp: June 04, 2016, 11:21:07 am
content: Quote from: coppice on June 04, 2016, 11:09:12 amyes it is the break down voltage I am lookng at

author: Simon
author_rank: Unknown
timestamp: June 04, 2016, 11:38:40 am
content: the potting is not the issue, the resistor itself will break down anyway.

author: KJDS
author_rank: Super Contributor
timestamp: June 04, 2016, 11:41:14 am
content: I've seen a few SMD resistor data sheets with detailed voltage spike characteristics specified.

author: Zero999
author_rank: Super Contributor
timestamp: June 04, 2016, 11:53:31 am
content: Unless the overvoltage is extreme, the resistor will not normally breakdown and become a conductor. What will happen is the resistance will drop below the nominal value. Whether it's reversible or not depends on the extent of the surge. I suspect twice the rated voltage, for short periods, is unlikely to do any real damage.

author: Simon
author_rank: Unknown
timestamp: June 04, 2016, 11:56:00 am
content: judjing from the link posted earlier it causes damage to gthe material and starts to make links that should not be there gradually reducing the resistance.

author: wraper
author_rank: Unknown
timestamp: June 04, 2016, 12:42:09 pm
content: Quote from: blueskull on June 04, 2016, 11:59:03 amWhy do you think there is any carbon involved?

author: wraper
author_rank: Unknown
timestamp: June 04, 2016, 12:48:15 pm
content: Quote

author: Jeroen3
author_rank: Super Contributor
timestamp: June 04, 2016, 03:04:58 pm
content: Use MELF resistors. They have better surge capability. You're going to need the larger power anyway for a load dump scenario.

author: madires
author_rank: Super Contributor
timestamp: June 04, 2016, 03:22:28 pm
content: Multiple resistors in series to match the expected voltage spike?

author: coppice
author_rank: Super Contributor
timestamp: June 04, 2016, 04:13:32 pm
content: Quote from: madires on June 04, 2016, 03:22:28 pmThat's a very common and effective technique, but its prohibited by regulations in a few applications, so take care.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2016, 06:34:57 pm
content: Quote from: coppice on June 04, 2016, 04:13:32 pmHuh, got any cites?Tim

author: MK14
author_rank: Super Contributor
timestamp: June 04, 2016, 08:03:51 pm
content: Quote from: madires on June 04, 2016, 03:22:28 pmThe problem with that method, is that if the voltage spikes, are very high frequency, it is more the stray capacitance (and inductance etc), and specific PCB layout etc, which effect how the voltage spike is transiently split amongst the resistors.So very briefly, one (or more) of the resistors, may see MOST of the voltage transient, EXCEEDING the datasheet specification, and hence potentially gradually breaking down the insulating properties of the resistor.There are some very good article(s) about this, on the internet. Offhand, I've forgotten where a really good one was, that I read many years ago.tl;drBest to use a properly voltage rated resistor, rather than try to share the voltage amongst multiple resistors which may be unreliable or even unsafe in the longer term.EDIT:For similar reasons. This is why on some relatively high voltage PCB circuits, slots are cut out in the PCB, in appropriate places. To stop the high voltages/transients gradually breaking down the insulating properties of the PCB.

author: Neilm
author_rank: Super Contributor
timestamp: June 04, 2016, 09:08:13 pm
content: Quote from: Jeroen3 on June 04, 2016, 03:04:58 pmI have used Vishay cmb0207 MELF resistors in pulse applications. Normally good for 1W rated for almost 500W pulses at voltages up to 10kV.

author: Simon
author_rank: Unknown
timestamp: June 04, 2016, 09:52:31 pm
content: I would have thought that if you are pushing resistors in series to increase their voltage rating would need to make sure they are placed in a straight line end to end increasing the distance between 2 points where the potential is applied. If for example you put the resistor's head to tail next to each other this could mean a potential spike would jump from one resistor to another. I don't think a suitable layout is too difficult to do it just requires a little bit of common sense and the acceptance that you cannot place components any old how to save space and make things the way you want them. Dave has often pointed to examples of resistors in series to increase the rated voltage in his videos. In my case I am not trying to make up massive voltage ratings with multiple smaller resistors. Officially a 24 V system should not have a spike higher than 250 V which is what a standard 1206 resistor package is rated for so it would be prudent to put 2 of the 1206 packages in series just to ensure I have 500 V for safety sake bearing in mind that the system should only see half of that which is what each resistor is rated for anyway. I also would not see a problem putting to 0805 packages in series which are rated to 150 V each as this would get you 300 V total.Looking on Farnell I would have to buy at least 150 resistors and a resistor rated for 500 V costs between 13 and 30p versus the 1 or 2p of a standard 250 V resistor and these resistors are much larger packages than a 1206 so they are not exactly anything special they are just bigger. I'm going to guess that if I get a high voltage rated 1206 package it will cost the earth whereas there is not really anything against me putting to 1206 resistors end to end to give myself plenty of margin.

author: MK14
author_rank: Super Contributor
timestamp: June 04, 2016, 09:58:39 pm
content: Quote from: Simon on June 04, 2016, 09:52:31 pmIn short the voltage ratings DON'T add up like that. Since there is NO mechanism to ensure perfect voltage split between the components. Hence datasheet violations and potential eventual component failure/danger.EDIT:BUT in one of the cases you are describing, the single resistor was already rated for the maximum rating it would encounter. So the second/extra resistor, was to just hopefully increase the safety margin. That does NOT sound too bad. Although even in that case, it would be better to use a single, higher voltage rating part.Although you are giving examples of stuff Dave has opened up. NOT everything which he can show, has been designed 100% correctly, as regards everyone's opinion/experience.tl;drNot everything you see in manufactured electronics equipment, would be a good idea to copy. There are many cases of poor design, unfortunately.

author: Cerebus
author_rank: Super Contributor
timestamp: June 04, 2016, 11:31:34 pm
content: Quote from: MK14 on June 04, 2016, 09:58:39 pmThe place in Dave's videos you frequently see the string of resistors in series to reach a voltage rating are the input protection on bench multimeters, in particular HP/Aglient/Keysight's ones. The thing that doesn't get pointed out at the same time are the associated inductors and capacitors that make the edges of any input pulses slow enough or capacitors that feed edges forward around resistors to give the whole string time to balance out. - this is poorly put, but you know what I mean; it's late and I'm not up for offering a full detailed explanation at the moment. Plus they use higher voltage rated parts too.In the HP34401A input chain I can see a series of eight 2512 500V rated resistors, 3 capacitors, two inductors, a gas discharge tube and a MOV. All this for a working rating of Cat II 300V which equates to an overvoltage rating of 6000V pulse.

author: MK14
author_rank: Super Contributor
timestamp: June 04, 2016, 11:41:23 pm
content: Quote from: Cerebus on June 04, 2016, 11:31:34 pmI agree.If PROPERLY designed and implemented, it CAN be done. (I.e. a mechanism to reliably ensure necessary even voltage sharing is created, via suitably placed passive etc components, even during very fast transients).But the thing is that you CAN'T just connect a string of resistors in series and declare the new maximum (transient especially) maximum safe voltage to be the sum of the resistors individual ratings.My understanding is that a lot of complicated design/development/test work takes place, in order to safely come up with such designs.

author: Simon
author_rank: Unknown
timestamp: June 05, 2016, 07:07:50 am
content: Quote from: MK14 on June 04, 2016, 09:58:39 pmThat is also true I have seen some pretty horrific things myself some of them designed for safety critical systems. Yes as I said the standard resistor voltage ratings are pretty much what is required but I'd like to be a little bit more resilient. As the maximum voltage I expect is 250 V a 250 V part on paper is fine but as we all know what on paper does not always describe real-life so I think I will be fair in saying that if I put 2 resistors in series no single resistor would see more than two thirds of the maximum applied voltage if I lay my board out properly which would give me a reasonable derating of the component. I suppose you could use an inductor but wouldn't the windings need to be rated for voltage? Surely if you have an inductor the voltage will simply jump from winding to winding rather than go round the winding.I usually follow my input resistors with a capacitor or and a TVS diode I'm still trying to get my head around how much voltage and current you would actually get through the resistor considering the actual amount of power in a spike which at 250 V would not be enough to produce enough current through the resistor and or a 250 V voltage drop.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2016, 07:21:56 am
content: Automotive transients are generally quite a lot easier to deal with than ESD, EFT and surge in mains-connected equipment. Don't worry about it.Tim

author: coppice
author_rank: Super Contributor
timestamp: June 05, 2016, 07:23:28 am
content: Quote from: T3sl4co1l on June 04, 2016, 06:34:57 pmI haven't hit this directly, but I have several instances of customers looking for alternatives to a chain of resistors, because they had problems with approvals. Two examples are German smart meters (energy meters in most countries use a chain of several SMD resistors), and a power quality meter that had issues with UL approvals when it used a chain of resistors.

author: Simon
author_rank: Unknown
timestamp: June 05, 2016, 07:24:19 am
content: need a what ?

author: coppice
author_rank: Super Contributor
timestamp: June 05, 2016, 07:26:33 am
content: Quote from: Simon on June 04, 2016, 09:52:31 pmThey don't need to be all in a straight line, but you do see some very dumb designs, like a zig-zag layout which brings every second connection close together.

author: Simon
author_rank: Unknown
timestamp: June 05, 2016, 07:28:18 am
content: Quote from: coppice on June 05, 2016, 07:26:33 amyes that is what I meant by head to tail I high voltages would just jump across the board from top to the bottom of the ladder or across sections. After all a a higher rated resistor is nothing more than the same device in a bigger package which increases the distance between the entry points so I can't see a lot against replicating that with multiple parts particularly where you are just adding a little safety margin and peace of mind for another 2p.

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2016, 07:42:35 am
content: Quote from: Simon on June 05, 2016, 07:07:50 am2/3rds is a good rule of thumb.

author: MK14
author_rank: Super Contributor
timestamp: June 05, 2016, 08:06:34 am
content: Quote from: Simon on June 05, 2016, 07:28:18 amWhere it is just the second resistor, and the first already meets the intended maximum voltage rating. I agree the 2p adds some extra protection and peace of mind.Where resistor stacks somewhat badly fall to pieces, is when E.g.It is a 10,000 Volt supply/transient and there are 10, 1000 Volt rated resistors.Ignoring the PCB/inductance and PCB capacitance (to simplify the explanation). The variations between the different stray capacitances of the different resistors, can mean that the 1000 Volts x 10, are NOT evenly split during very rapid transients and/or switch on/off events.Hence (I might be exaggerating a little bit), one of the resistors may get a disproportionately high portion of the voltage. So one might get 2000 Volts (they are ONLY rated for 1000V each), because it happens to have the lowest stray capacitance value. So in time these 2000V (I accept I may be way off here. But anything over 1000V is potentially bad datasheet wise anyway. Ultimately it depends on the stray capacitance variations and other stuff) transients, can break down the resistors insulation.If you exceed the maximum voltage of a resistor, it can concentrate most of the voltage on a VERY small part of the insulator, and break it down. In time the broken part of the insulation (which now potentially conducts), GROWS. Eventually compromising the entire resistor e.g. making it somewhat conduct. Hence the other 9 resistors will get almost all the 10 KV, which in time will blow another of resistors. So in time all/most of the resistors can/will fail.Because you are specifically talking about having only two resistors, and hopefully keeping well below the (incorrect) 2 x maximum working voltage. You should NOT have this problem.

author: Simon
author_rank: Unknown
timestamp: June 05, 2016, 08:22:57 am
content: Yes I agree I would not take the maximum voltage applied and divide it by the maximum voltage the resistor could take to get the amount of resistors in the network. If you are talking 10 resistors I would probably go with 2 to 3 times expected voltage for some safety margin. If you are starting to need so many resistors it is probably time to just use a higher rate resistor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2016, 09:24:13 am
content: Quote from: coppice on June 05, 2016, 07:23:28 amAs I recall, IEC 60384-1 says cumulative gaps are okay, so long as the single-point-failure total is equivalent. That is, if one resistor has a rating and gap suitable for 250V, then you need N = 1 + Vnom / (250V) number of resistors. So for 500V, you'd use 3 or more.As others noted above, you need to make sure that's true at all frequencies of interest, either by filtering (attenuation) or bypassing (C divider in parallel).And,Quote from: coppice on June 05, 2016, 07:26:33 amif they're doing dumb stuff like this, it's their own fault reallyTim

author: coppice
author_rank: Super Contributor
timestamp: June 05, 2016, 01:16:10 pm
content: Quote from: T3sl4co1l on June 05, 2016, 09:24:13 amThat's the rule most people use. A lost of approvals test programs require the resistors to be shorted one by one to demonstrate the design really supports that SPOF requirement.

author: Cerebus
author_rank: Super Contributor
timestamp: June 05, 2016, 02:39:35 pm
content: Quote from: T3sl4co1l on June 05, 2016, 09:24:13 amWhat people tend to forget is what I might call the "frequencies of non-interest". If someone is working on a low frequency circuit they tend to put all issues associated with higher frequencies out of mind. So you bumble around designing your circuit ignoring small parasitics of a few pF here, 20 nH there and so on because "It's only audio frequencies". Then your circuit encounters a 500V/us transient coming in via the mains lead...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2016, 09:48:09 pm
content: Heck, from what I've seen, people have a hard enough time making things work atDC...Tim