is it possible to mechanically trim resistors? - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: August 10, 2018, 11:48:19 am
content: Hello forum!I wonder if it is possible to increase/trim value of a resistor by mechanically removing part of conductive layer. What do you think about this? I keep in mind two cases: 1) through-hole (or melf) round resistors 2) square smd package. The latter can be particularly difficult.Application scenarios:1) sometimes I need an "odd" value (like 50 Ohms, while only 47 or 51 are available)2) To better match resistors. Matched resistors / networks can cost quite a bit.I'd love to give it a try, but don't have access to the lab at the moment. Asking now because this questions bothers me since a long time ago.

author: Brumby
author_rank: Unknown
timestamp: August 10, 2018, 12:05:32 pm
content: Sure, you could do it - but getting an accurate result would be a challenge.Even if you were able to 'dial in' a value, there's the question of stability. Mechanical abrasion may cause weakness in parts of the component that may crack or fail. Add to that age, thermal cycling and moisture ingress and drift and/or failure will be higher than a regular component.Then there's the issue of uneven heating for a resistor that's dissipating a significant amount of power.Personally, I'm a big fan of series/parallel connection of multiple, intact resistors to achieve a similar result.

author: Gyro
author_rank: Super Contributor
timestamp: August 10, 2018, 12:13:58 pm
content: I agree, your carefully trimmed resistor is unlikely to stay that way. Over-coating with lacquer or epoxy might help an little, but not when you get down to smd sizes.As Brumby says, resistor combinations are the way to go. Two parallel 100R for 50R, 2x150R for 75R etc. You get twice the power rating and half the inductance (some applications even use 4x200R for these reasons).There are programs linked on this forum somewhere that make calculation of Exx resistor combinations really easy.

author: DannyTheGhost
author_rank: Contributor
timestamp: August 10, 2018, 12:21:05 pm
content: All standard resistor values were designed in such a way, so you can easily use them for typical applications like voltage divider, or get any other possible resistor value, so this type of trimming is unnecessary

author: Muxr
author_rank: Super Contributor
timestamp: August 10, 2018, 02:23:04 pm
content: You could make your own wirewound resistors to your own spec. There are tutorials on how to do it on youtube as well.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: August 10, 2018, 02:32:21 pm
content: There are 49.9R resistors and there are precisely 50 ohm microwave resistors. So what's the problem? If you are still designing with a method that depends on matching resistor values, may I suggest setting the time machine to 2018?PS: The word is "hobbyist".

author: AndyC_772
author_rank: Super Contributor
timestamp: August 10, 2018, 02:59:48 pm
content: You've nothing to lose by having a play. Perhaps start with solid carbon type resistors, so you're removing bulk material rather than trying to make tiny changes to a thin layer on a hard ceramic substrate?It's been suggested before:https://youtu.be/gVxtleOMAOo?t=489Any changes you make this way will never be particularly accurate or stable, but for a bit of fun as a learning exercise, go right ahead.

author: exe
author_rank: Unknown
timestamp: August 10, 2018, 04:45:20 pm
content: Quote from: In Vacuo Veritas on August 10, 2018, 02:32:21 pmNeeded values are not available on my distributor's site (tme). Often values that I'd expect to be very popular. Like 50 Ohm, 9kOhm, etc. Or not available in the package I'd like (e.g. 0603).

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 10, 2018, 04:57:09 pm
content: Many moons ago, when the most common resistors were thru-hole, 1/2 watt carbon composition types, one could use a triangular file to slightly increase its value.You would start with a 47 ohm device and measure it with a multimeter and slowly file away until you got the desired resistance value. Of course the power rating would be diminished by the reduced cross section, and the resistance value could drift......Would drift. Carbon comps were not stable to begin with.Nowadays SMT manufacturers use the same technique to trim resistors, utilizing a laser to perform an L-cut.Google "resistor L-cut". There are hundreds of images.

author: jancumps
author_rank: Unknown
timestamp: August 10, 2018, 05:16:19 pm
content: ... one of the reasons why your desired resistor isn’t there is because of the tolerance. That’s why the Exx ranges of resesistors are there. They are covering the whole range, taking tolerance in account.

author: Fred27
author_rank: Unknown
timestamp: August 10, 2018, 05:24:08 pm
content: https://hackaday.com/2017/04/10/hackaday-trims-its-own-resistors/

author: jeroen79
author_rank: Frequent Contributor
timestamp: August 10, 2018, 05:44:25 pm
content: Quote from: jancumps on August 10, 2018, 05:16:19 pmTrue.You could just buy 100 resistors close to the desired value and then pick the one that is closest.

author: exe
author_rank: Unknown
timestamp: August 10, 2018, 05:58:11 pm
content: Quote from: jeroen79 on August 10, 2018, 05:44:25 pmThat's what I often do.Still another issue exists: the desired value is just not on sale and there is no value that would be close. Say, I need 17kOhm, it's out of stock. I could buy a ton of 15 or 18 kOhm 5% resistors, but that's just waste of resistor and tedious (and not guaranteed that I'll get the value I want).Why I sometimes need precise value? Most often is to simplify calculations and verification. While I can correct/adjust values in software, while debugging / prototyping it's good to have, say, amplifier with ~10x amplification and not 8x or 12x.Or I want save some boards space, so don't want to parallel resistor.But I got the message. SMD resistors are not easy to trim. Looks like the best solution is just to pick distributor that has required values in stock.

author: Gyro
author_rank: Super Contributor
timestamp: August 10, 2018, 06:06:09 pm
content: Quote from: exe on August 10, 2018, 04:45:20 pmRemember that it's quite easy to stack smd resistors to tweak values without extra footprints... e.g. A 1k 0603 with a 1M padding resistor soldered on top will give you 999R. You can get very fine adjustments using common values.P.S. I previously mentioned resistor calculators. I use Rescalchttp://www.pmillett.com/rescalc.htmIt's a good one because you can specify the Exx range and limits for the resistors that you are using - so it doesn't suggest silly values.

author: jancumps
author_rank: Unknown
timestamp: August 10, 2018, 06:23:20 pm
content: Quote from: jeroen79 on August 10, 2018, 05:44:25 pmIf you have to do that, you're probably buying a wrong tolerance series?

author: KL27x
author_rank: Super Contributor
timestamp: August 10, 2018, 09:17:54 pm
content: QuoteAssuming this is true where you live (or that you don't want to buy and wait for a resistor for a one off):It might be easier to trim the resistor with other resistors. Use the 51 and whip out the calculator to find the value you want to solder in parallel to it. 2.55K. In this case a 2.5K in parallel would get you to around 49.98R. If you need bang on, you will be trimming anyway.Say all you got is a 3K. Add that in, recalculate, and you need to add 17K in parallel to that.Repeat with the closest higher value you got until you get as close as you want. Or just tack on 1 meggers until you get as many zero's behind that decimal point as your heart desires.Gyro beat me to it. But you just need a calculator that has a X^-1 button. You gots a scientific calculator, right?I have more than once made low ohm power resistors/shunts with a pile of 1206's and a piece of copper clad.

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2018, 10:48:11 pm
content: I use two resistors in series or parallel and the site linked below, to calculate the resistor values, if I need an odd value resistor.http://jansson.us/resistors.htmlAnother way to trim is to choose a resistor with a higher value, than needed and place many much higher value resistors in parallel with it, the PCB track can then be cut, to remove one of the higher value resistors, until the desired value is reached. This method can be used for trimming the output of a voltage regulator of offset nulling.

author: jeroen79
author_rank: Frequent Contributor
timestamp: August 11, 2018, 09:42:51 am
content: Quote from: jancumps on August 10, 2018, 06:23:20 pmProbably, but what if even the E192 series does not have the exact value you need?

author: Gyro
author_rank: Super Contributor
timestamp: August 11, 2018, 10:32:42 am
content: I've just remembered another resistor combination calculator from this very forum - Resistor Fun:https://www.eevblog.com/forum/projects/fun-with-resistors/Much more featured - All resistor series, selectable number of resistors, voltage dividers with multiple taps, analysis of the effect of TC etc.

author: Twoflower
author_rank: Frequent Contributor
timestamp: August 11, 2018, 11:09:55 am
content: Does any parallel or series circuit actually make any sense at all? As the tolerances are so wide that a E24 100Ohm resistor can be anywhere between 104.5Ohm and 115.5Ohm. The next higher E24 resistor (120Ohm) spans from 114Ohm to 126Ohm. As you see they actually even overlap. So doing any magic with E24 resistors to get a 115Ohm resistor (as example) would fail at the tolerances. Unless you select them. In that case you might consider going to an higher E series.At least that's my understanding.Series or parallel circuits might make sense for example to reduce the number of different resistor types in your BOM or increase the allowed voltage or power ratings.

author: Gyro
author_rank: Super Contributor
timestamp: August 11, 2018, 11:34:05 am
content: That very much depends on the situation - whether the E24 resistors are 5% or 1% for instance. It's very easy to go overboard but instances that come to mind:1. 50R termination resistors in E242. Fine trimming things like voltages references where you are padding a precision resistor with a much higher value, less critical resistor to bring it to spec without a pot.3. Production trimming of voltage regulators where you cut out resistors to achieve trim (there's a NS app note about that somewhere).4. One-off home designs where you want to fine trim something.5. Gone blank... but there are others.As I said, it's easy to go overboard using low quality resistors to try to achieve close precision, but it is a useful tool.

author: jancumps
author_rank: Unknown
timestamp: August 11, 2018, 02:56:51 pm
content: In the end, when using resistors with the same tolerance, isn't this exercise virtually moot?edit: this message wasn't a reply to the last one - It's about trying to improve resistor precision within a given tolerance range: does scratching up a 47 Ohm 5% resistor so that it measures 50 Ohm improve anything?

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2018, 07:52:17 pm
content: With the coating removed, thin film and metal foil resistors can be trimmed by rubbing them with a pencil eraser but they tend to continue drifting with their passivation layer removed.

author: mzzj
author_rank: Super Contributor
timestamp: August 11, 2018, 09:44:52 pm
content: Quote from: schmitt trigger on August 10, 2018, 04:57:09 pmMany moons ago there were also vertical resistors that you could cut shorter to trim the resistance value. Snapsistors or something like that.Never seen them in use, just in old advertisements.

author: JS
author_rank: Frequent Contributor
timestamp: August 11, 2018, 10:02:13 pm
content: I've done it for fun with a few metal films, just trim a tiny bit, like selecting some 999Ω from the 1k batch to trim down to 1.000k few years after I read exactly the same, but only 3 zeros, I've just received the 121GW so I could try something with 4 zeros.I used some small files and went slowly, turning the thing as I went so I extracted as little material from a single place as possible and then coated them with some epoxi. I guess higher values would be harder as the strip is much thinner so there's a better chance of extracting too much material easily.The hard part is that you are heating and getting conductive dirt onto the conductive layer as you file, so you need to clean, wait temp to stabilize and then read, that make the thing tedious and slooow. For the final trim you could go slow, blow some ambient temp air and read faster, once they settle make a good clean, put some epoxy and they might be good, age them for a bit before trusting them.Or just buy better resistors to start with. Some pretty usual way is to build a network with a trimmer in it, but the trimming range being pretty small, so the tempco of the trimmer doesn't affect much the tempco of the combined resistor.Carbon might be easier to carve but as they come in really horrible tolerances you could very well select a value from a batch somewhere off the nominal. If you need 10X divider,910 and 82820 and 91makes a pretty exact value, you just need to select the resistors from a batch and you will find some. X2 is easy, to equal ones, time 3, probably use 3 equal ones and do series of 3 (that makes self heating more even than parallel) or get some 1k and 2k. All standard values of E24 series and there might be a perfect combination for each ratio you want, just grab the calc and do the math... or an excel sheet.JS