Does the voltage rating for a resistor matter? - Page 1
Forum section: electronics / beginners

author: lazarusr
author_rank: Regular Contributor
timestamp: May 29, 2022, 04:44:28 pm
content: I am working on a project where size is at a premium. So, 0402s is very much the way to go. I am using a 7M5 resistor in a voltage divider where the voltage across that resistor is 200V. So the power dissipated by that resistor should be (by Ohm's law) 5.3mW. No difficulty finding an 0402 resistor based on power. But most resistor data sheets also give a voltage rating and for an 0402 package that is usually 50V. With such a low power dissipation, is the high voltage likely to impact on the accuracy or lifetime of the resistor? Any advice very much welcomed.

author: wraper
author_rank: Unknown
timestamp: May 29, 2022, 04:48:54 pm
content: You should not exceed the voltage rating or resistor may arc over. Applying 200V to something as small as 0402 is a no go.

author: TimFox
author_rank: Super Contributor
timestamp: May 29, 2022, 04:55:38 pm
content: In a proper datasheet for a particular construction of resistor, there is often a specified resistance value above which the maximum voltage for the package is the operating limit, but below which the power dissipation is the operating limit.If you are really constrained for volume with such a high voltage, you probably have to consider encapsulation, which is a discipline by itself.

author: bdunham7
author_rank: Super Contributor
timestamp: May 29, 2022, 05:03:42 pm
content: Quote from: lazarusr on May 29, 2022, 04:44:28 pmYes, exceeding the rated voltage by 4X may be suboptimal. And if you have a circuit where there is substantial energy involved, you want a significant safety margin as well. You can get your resistor in 0603 7M5 200V, but if the source is something like rectified mains, you're pushing your luck. 3 2M5 in series would be better, although obviously chewing up space.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: May 29, 2022, 05:12:37 pm
content: Years ago a board I had to deal with, but which was designed by another person, placed line to a power triac underneath the resistor that goes between the trigger and a low-impedance load.The resistor was a basic through-hole 1/4W carbon type maybe rated for 50V.The failure mode was between the resistor body and line. A transient would then trigger the triac, and keep it on, running the load at full power until something else broke.The solution was to raise the resistors and conformal coat the board.

author: coppice
author_rank: Super Contributor
timestamp: May 29, 2022, 05:21:19 pm
content: Quote from: TimFox on May 29, 2022, 04:55:38 pmThat is a poor way to specify a resistor. It only makes sense when the applied voltage is continuous. If its rapidly pulsed with a small duty cycle you might see very little power dissipation when a high voltage is applied to a low value resistor.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 29, 2022, 05:40:49 pm
content: Its a bad idea. You wont meet creapage and clearance for traces with solder resist which has to be greater than 0.4mm for 200V DC Don't gamble that an overvoltage won't occur, it will.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 29, 2022, 05:48:02 pm
content: All those ratings are made by stupid idiots and they are just sucked out their thumbs to fill paperwork that nobody in their right mind reads anyway./sarcasm.

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2022, 06:05:46 pm
content: At the very least, if the resistor isn't damaged, its value might shift from the specified one, either temporarily or permanently.

author: tooki
author_rank: Super Contributor
timestamp: May 29, 2022, 06:07:11 pm
content: Quote from: coppice on May 29, 2022, 05:21:19 pmThat’s obviously not the only specification in the datasheet, and that particular bit of information is more of a convenient FYI than a core spec.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 29, 2022, 06:13:18 pm
content: If you are 100% sure there will never be any transients, you can use a 0603 part if you can find one rated for 200V.High voltages and miniaturization do not play along easily. It's possible but requires special procedures.If this is voltage divider off something mains-derived, use resistors rated for 600-1000V or so. It's very typical to use two or three through hole parts, each rated for 300V, in series. This is because mains will have transients, and you don't want arc-overs, each time materials will carbonize and slowly start conducting.

author: bdunham7
author_rank: Super Contributor
timestamp: May 29, 2022, 07:15:19 pm
content: Quote from: Siwastaja on May 29, 2022, 06:13:18 pmIf he can use TH, these deserve a mention. I use them when repairing things where the designers previous choice of resistor voltage hasn't worked out well over time. It's pretty much guaranteed that something else will give before these resistors do:https://www.mouser.ca/ProductDetail/Vishay-BC-Components/HVR3700007504FR500?qs=7DG%2Fna1Fp6u4fJjDlBJ%2FNw%3D%3D

author: lazarusr
author_rank: Regular Contributor
timestamp: May 29, 2022, 07:26:48 pm
content: Quote from: bdunham7 on May 29, 2022, 05:03:42 pmI am not using mains voltage. This high voltage, low current (approx 2mA) generated by a step-up convertor from a small LiPo. I am aware of the 0603, high voltage, package and will stick with that. This project is all about compromises battling between space, voltage rating and thermal dissipation. The good thing is I am learning loads in the process.

author: lazarusr
author_rank: Regular Contributor
timestamp: May 29, 2022, 07:29:49 pm
content: And it is just brief pulses of high voltage. Most of the time, the whole device is running on 3.3V with about 3mA of current draw.

author: lazarusr
author_rank: Regular Contributor
timestamp: May 29, 2022, 07:33:53 pm
content: Thanks to everyone for all the feedback and comments. It is of great help.

author: coppice
author_rank: Super Contributor
timestamp: May 29, 2022, 08:47:01 pm
content: Quote from: tooki on May 29, 2022, 06:07:11 pmIt is, however, a weakness in resistor specs. A few times I've been pulsing power into a resistor, and had trouble finding what would be an acceptable short term power burst for most resistors.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 29, 2022, 09:18:26 pm
content: The maximum voltage ratings are usually due to the chance to get some arcing or fast increasing leakage currents. So even a short excursion above the rating can damage the resistor. Sometimes MELF resistors are used for a relatively good voltage rating for the size - though usually more comparable to 1206.

author: bdunham7
author_rank: Super Contributor
timestamp: May 29, 2022, 10:11:32 pm
content: Quote from: coppice on May 29, 2022, 08:47:01 pmActually it isn't problematic at all. The maximum voltage is just that for every resistor in the line and you typically shouldn't exceed it ever--unless it has a specific separate pulse voltage like the Captain Overkill model that I posted. The lower values arealso, not alternatively, limited by maximum power dissipation. Whether they could be subject to short-term surges up to the max voltage spec has been debated here before and I'd say most of the time you probably can if the total energy involved is <<(Power X 1 sec) or in some cases (Power X 5 sec). But YMMV and a recent discussion convinced me that heat-sinked resistors might be limited to <<<< (...)

author: exmadscientist
author_rank: Frequent Contributor
timestamp: May 30, 2022, 12:48:43 am
content: The RohmKTR series of surface-mount partshas proven pretty dependable for us in medium voltage service. 350V in 0603, 500V in 1206, and they're cheap cheapcheap. But they can't surviveeverything...Quote from: bdunham7 on May 29, 2022, 10:11:32 pm...so we had a somewhat challenging application that was killing a lot of fancy resistors. Resistors that, according to their datasheets, really looked like they should have survived. Now, we knew this was a truly nasty application (something to do with the minimum output impedance in a half-kilowatt Genuinely High Voltage circuit), so we weren't really surprised, more just disappointed. Cue General Overkill:The Vishay Draloric Z300-C Surge Rated series. They're a decent complement to Captain Overkill in the lower ohms range. And plenty hard to kill, especially with that 12 kV surge rating, which we may or may not have exposed one to (depending on whether that firmware bug got fixed...).Haven't had a failure yet! Well, at least notthere... plenty of failureselsewhere....

author: wizard69
author_rank: Super Contributor
timestamp: May 30, 2022, 06:56:15 pm
content: I'm surprised nobody mentioned the issue of dust causing failures on such parts. If yo are already on the edge of failure outside factors can make the failure happen even faster.

author: TimFox
author_rank: Super Contributor
timestamp: May 30, 2022, 06:57:49 pm
content: Yes, and dust particles can be attracted by the electrostatic field between two closely-spaced electrodes.

author: coppice
author_rank: Super Contributor
timestamp: May 30, 2022, 07:08:04 pm
content: Quote from: wizard69 on May 30, 2022, 06:56:15 pmDust, surface dirt and other things can cause breakdown at quite low voltages with small SMD parts on PCBs, but that isn't an issue of the actual part's rating, which was the basis of the original question. A thoroughly cleaned PCB sealed in a dessicated chamber won't suffer those issues.

author: Gyro
author_rank: Super Contributor
timestamp: May 30, 2022, 07:17:18 pm
content: It may be simplistic, but at the end of the day, you don't use components outside their ratings, be it dissipation or package voltage rating, as applicable.