Is 1 mOhms = 1 milliOhms?? - Page 1
Forum section: electronics / projects

author: luiHS
author_rank: Frequent Contributor
timestamp: September 12, 2022, 08:58:16 am
content: HiI had to design a circuit with two MAX25208 switching regulators, in the schematic, among other components, a 2 milliohm resistor and a 1 milliohm resistor are used.I ordered components from Mouser, these are the references:https://www.mouser.es/ProductDetail/603-AF0402JR-071MLhttps://www.mouser.es/ProductDetail/603-RV1206JR-072MLI assemble the board and it doesn't work, there is no output voltage on either regulator. I have spent hours reviewing everything, looking at my schematic, the datasheet, another schematic of an evaluation board, apparently everything was well designed in my circuit, but it did not work.Finally I was lucky, because one of the resistors is 1206, all the others are 0402. The 1206 resistors still have the value printed, so in one of the microscope reviews I read that this resistor has printed 205, that I interpret as 20 and 5 zeros, that is 2 Mega Ohm, instead of 2 milli ohm. I remove from the circuit both the resistor that is supposed to be 2miliohm and also the one that is supposed to be 1miliohm, I measure them and they are 2 Megaohm and 1 Megaohm.So either Mouser has misclassified these resistors or I'm wrong in interpreting 1mohm as 1milliohm and 2mohm as 2 milliom. The lowercase m would be for Milliohm and the uppercase M would be for Megaohm. It is right ?.

author: Psi
author_rank: Super Contributor
timestamp: September 12, 2022, 09:05:13 am
content: Mouser has misclassified them.If you check the datasheet, those resistor part numbers don't do milli-ohm values.Probably some new staff member who doesn't understand m vs M was assigned classification duty.Just let mouser know you got the wrong part due to website error and give them correct part numbers you wanted.They will ship you the correct ones for free.(Make sure you explain the issue so they know its the website being wrong and not just a picker person sending you the wrong one, otherwise they might send you the same part numbers again)

author: wraper
author_rank: Unknown
timestamp: September 12, 2022, 09:19:38 am
content: If you search for part number it becomes obvious that Mouser screwed up with stated resistance.https://www.digikey.com/en/products/detail/yageo/RV1206JR-072ML/5948372Actually I checked the category, and it seems all or at least most of single digit milliohm resistors are screwed up. If I select 50mOhm, I get correct results like thishttps://www.mouser.es/ProductDetail/Royalohm/1206W4J050LT5E?qs=e8oIoAS2J1T6NcwNDN%252Bd%2FA%3D%3D

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 12, 2022, 10:19:23 am
content: Lower value resistors are placed in their own category, "Current sense resistors"https://eu.mouser.com/c/passive-components/resistors/current-sense-resistors/This leaves mostly mislabeled parts in the lower ranges of the "Film Resistors" category. Curiously the current sense resistor category also has a lot of resistors mislabeled as megaohms instead of milliohms.

author: JohanH
author_rank: Frequent Contributor
timestamp: September 12, 2022, 01:15:10 pm
content: Quote from: luiHS on September 12, 2022, 08:58:16 amPet peeve of mine. If everyone stuck to the standard, life would be easier. It's not "Mega Ohm", "milli ohm", "Milliohm" and neither "Megaohm". All of these quotes are wrong.Prefix names (like milli or mega) are always written with lower case (except in the beginning of a sentence of course).Prefix symbols differ in case:m milliM megaUnit names are also always written with lower case. Whereas symbols might differ in case (e.g. farad/F, lumen/lm).Milliohm is written mΩ and megaohm MΩ. For the sake of simplicity "mohm" and "Mohm" could be accepted where the Ω symbol isn't easily written (even if it is mixing prefix symbol and unit name). You don't usually write "kgram" either, it is "kg" or "kilogram".Units are neither expressed in plural. It is 2 ohm, not 2 ohms. 2 km, not 2 kms (s stands for second; km times second would be something else!).That component suppliers sometimes have wrong values on their web page isn't uncommon. Always check the data sheet!

author: thm_w
author_rank: Super Contributor
timestamp: September 12, 2022, 10:03:40 pm
content: Sounds like a failure of the metric system for having two prefixes with the same letter.Although when the doctor prescribes you 10Mg of warfarin, they wouldn't call for a dump truck.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 13, 2022, 04:32:17 am
content: Quote from: thm_w on September 12, 2022, 10:03:40 pmThere are about 20 prefixes and about 30 official units (base and derived), plus a number of "obsolete" metric based units still in wide use for which it is advisable to avoid conflict if possible. Plus there are letters that are easily mistaken for each other. One way or another the system is going to have some issues: you either have to live with some near ambiguities, or eliminate common derived units -- which also would cause confusion and mistakes as common everyday measurements would have to be expressed as combinations of several units with powers.With the higher powers, they even made it a bit of a convention: Z and Y are inverses of z and y, although their use is rare and the situations where they could be confused are negligible.I'm sure there are better ways that what we have, but I'm also sure that no matter what you picked people would find creative ways to screw it up.

author: gbaddeley
author_rank: Regular Contributor
timestamp: September 13, 2022, 07:09:57 am
content: The resistor values and Ohm prefixes have nothing to do with the metric system. It is scientific / engineering notation that needs to be clear on resistor values. Big M is mega. Little m is milli. There should be no doubt when these are used. If used wrongly, call it out.As for small drug quantities, all upper / lower case combinations of m and g are surely referring to milli Grams. The same with k and g to refer to kilo Grams. Also u (strictly its the Greek letter mu) and g for micro Grams. Vastly simpler overall than imperial units.

author: thm_w
author_rank: Super Contributor
timestamp: September 13, 2022, 09:13:04 pm
content: Quote from: gbaddeley on September 13, 2022, 07:09:57 amSurely engineering notation is directly using the metric prefix system, no?https://en.wikipedia.org/wiki/Metric_prefixHandwriting can be quite ambiguous, small or big M might look the same to someone not familiar with your writing style.Anyway, as I noted, this is probably only really ever an issue in electronics. Not many other fields will you be able to confuse a milli and mega amount being used in the same place.

author: TimFox
author_rank: Super Contributor
timestamp: September 13, 2022, 09:28:33 pm
content: Quote from: gbaddeley on September 13, 2022, 07:09:57 amIt is common with drugs to use "mcg" to mean microgram. "gram" is never capitalized.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 14, 2022, 02:19:51 am
content: Yeah well. Mistakes do happen. What's interesting is all the part with us trying to justify the ones we make.Oh and to avoid any possible confusion on schematics and BOMs, it's usual to use the "Meg" prefix for M.But alternatively, what I have otherwise done was to mark mega as M and milli as mR. It's also good practice to add "R" to the number for a resistance value without multiplier prefix. So by extension, mR for milliohm values looks OK and relatively consistent. If you will.Like 1M = 10^6 ohm10R = 10 ohm2mR = 2.10^-3 ohmAnd of course distributors do pretty much what they like or can and you need to double check when in doubt.

author: TimFox
author_rank: Super Contributor
timestamp: September 14, 2022, 03:08:32 am
content: Also, in Spice (which started off with case-insensitive Hollerith cards), 106is always "MEG", and "M" is always 10-3(still case-insensitive).

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 14, 2022, 04:10:56 am
content: Quote from: TimFox on September 14, 2022, 03:08:32 amYes. Let's keep in mind that when Spice was invented, case-insensitive languages were still the norm and case-sensitive ones were a curiosity.Looks like 50+ years later, we still have a hard time with case-sensitive stuff.

author: JohanH
author_rank: Frequent Contributor
timestamp: September 14, 2022, 06:56:53 am
content: Quote from: SiliconWizard on September 14, 2022, 02:19:51 amWith modern computers, there isn't anything preventing from using the correct Ω symbol instead of "R" when marking component values. It even prints correctly on circuit boards as long as you use the greek letter Ω and not the dedicated and rarely used unicode ohm symbol that looks the same. "R" is the symbol for quantity (resistance). It has always seemed very odd to me to use it instead of the symbol for the unit (ohm). "10R" = "ten resistance".

author: thm_w
author_rank: Super Contributor
timestamp: September 14, 2022, 10:17:57 pm
content: Quote from: JohanH on September 14, 2022, 06:56:53 amSure nothing prevents it, but unless you setup a custom keyboard, you don't have an ohm key.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 14, 2022, 10:43:13 pm
content: Quote from: thm_w on September 14, 2022, 10:17:57 pmThat and, UTF-8 is also still far from being used everywhere either. If you are not 100% sure all software you use is using UTF-8, and same for everyone else involved including your contractors and suppliers,you're screwed.Plus, remember that those conventions we are talking about are there to make things easier and less ambiguous in all kinds of situations and setups. 'R' is easy to type, easy to read and hard to confuse with anything else. The omega letter, depending on fonts, displays, printers, and eyesight may look like something else. For the same reason, it's best to put the prefix in infix position on schematics and BOMs. (Like: 2.2k => 2k2, etc. For 2.2ohm that would be 2R2.) "Dots" are typically too slippery there.

author: gbaddeley
author_rank: Regular Contributor
timestamp: September 15, 2022, 04:30:11 am
content: Printed Magazines used to avoid using dots because they can be hard to read. 2K2 is better than 2.2KReally old magazines (1950s & earlier) assumed meg Ohms, writing .05 for 50 K.

author: JohanH
author_rank: Frequent Contributor
timestamp: September 15, 2022, 11:47:06 am
content: Yes, you need to make sure your software supports special letters. And greek letters were available before unicode, so it's mostly an issue with fonts. If you can't print special characters, try another font.Before unicode, you could write Ω with Alt 234 (using the numerical keyboard) and e.g. µ with Alt 230. Nowadays with unicode, Alt+03A9 and Alt+03BC. This is in Windows. But I prefer to just copy and paste special characters, can't memorize them all.I see a point with not writing out dots, at least in the past. Nowadays with high resolution prints and microscopes I'm not so sure we have the same issues any more.I haven't worked in e.g. PCB industry, so please enlighten me if there really are standards or requirements for these kind of things.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 15, 2022, 12:51:49 pm
content: Quote from: JohanH on September 15, 2022, 11:47:06 amNot really, PCB industry from design to manufacture notoriously lacks any standardization, and all attempts to do so have failed by no one following them, so we just use whatever conventions we feel like using, and try to communicate the best we can and hope all the hidden assumptions are right. They often are when all parties are experienced.It's not the worst situation because this also allows us to usesaneconventions if we so wish - for example, I use the SI system and decimal points, so my BOM reads Res 4.7k and MLCC 100n.There have been some really weird conventions like using W to denote ohm. Think about the confusion because W is also used for watt!

author: luiHS
author_rank: Frequent Contributor
timestamp: September 15, 2022, 05:57:12 pm
content: I already contacted Mouser, and they confirm that they had those references wrongly assigned. But they tell me that they don't have resistors of those values (1mOhm and 2mOhm) and they will refund my money to my Paypal account.Searching Mouser with the component selector, it appears that all resistors below 10mOhm are miscoded, and are all MOhm and not mOhm. I made an order to TME, and they did send me the correct resistors of 1mOhm, 2mOhm and 10mOhm.Mouser always works quite well, when I have had a problem they have always solved it for me. I remember some problem with transistors that did not match the photo, description and data sheet, I complained and they sent me the correct ones. On another occasion, a component in a tube, which arrived with all the chips out of the tube, and since it was to be used in a pick and place machine, I complained and they sent them back to me at no cost.Mouser is a good supplier, I buy a lot from them, although for certain components they are somewhat expensive, so I buy from TME which is cheaper. But in microcontrollers and chips in general, Mouser offers the most variety and the best prices.

author: radiolistener
author_rank: Super Contributor
timestamp: September 15, 2022, 06:21:01 pm
content: yes1 mΩ = 1 milli Ω = 0.001 Ω1 MΩ = 1 mega Ω = 1000000 Ω

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 15, 2022, 06:56:09 pm
content: You can solve the whole problem by ordering your resistors using conductance (siemens). PO says, "Resistor, G = 1 millisiemen" You'll never receive a thing, even though the distributor has thousands of 1 kohm resistors on the shelf.

author: SL4P
author_rank: Super Contributor
timestamp: September 15, 2022, 09:40:31 pm
content: lower case == dividerUPPER case == multiplierNo decimals for the sake of printing lossesThank god for standards.

author: radiolistener
author_rank: Super Contributor
timestamp: September 15, 2022, 11:01:59 pm
content: Quote from: SL4P on September 15, 2022, 09:40:31 pm1 kΩ = 1 kilo Ω = 1000 Ω

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 22, 2022, 01:30:36 pm
content: Quote from: luiHS on September 15, 2022, 05:57:12 pmThey do have them. As I wrote in my previous post, they are in the "Current sense resistors" category. Here is one for example:https://eu.mouser.com/ProductDetail/YAGEO/PA1206FRE7W0R001Z?qs=eP2BKZSCXI53%252BE5Ff2Xr%252Bw%3D%3D