Wanted precision resistors values 2M 1M 20k 200 20ohm - Page 1
Forum section: electronics / general

author: muvideo
author_rank: Frequent Contributor
timestamp: June 19, 2012, 11:08:08 am
content: Hello, I'm repairing a decade resistor box, I need some precision resistors,the original are wirewound but any good quality low drift resistor is wellcome.I think the drift (edit: and TCR) is preferred over the initial accuracy, I can trim the values.The ideal ones would be values slightly less than specified (I'm thinking tofluke 19.991K I've seen in some instruments), but I can also combine seriesor parallels if needed.I can buy some good 20k and lower values form some reseller at good price,but precision 1 & 2Meg are difficult to find.In order of priority:- 4 units 2Meg- 1 unit 1Meg- 1 each 20k,220200, 20if anybody has some resistors, or some old broken equipment containing them cancontact me here or in PM.Thank you,Fabio.Edit: the patient is a DB877 in awful conditions, teardown will follow

author: SeanB
author_rank: Super Contributor
timestamp: June 19, 2012, 04:01:20 pm
content: Buy 10 1M resistors, they will be cheaper in multiples. That way you only have to source one value, just put 4 in series and 2 in series. As you are replacing a WW unit there will be enough space for sure. With the low value resistors get the highest wattage you can find, they often get overloaded. In this position a 0.5% or 1% resistor will be good enough, you can make them from 10R resistors.

author: saturation
author_rank: Super Contributor
timestamp: June 19, 2012, 04:09:17 pm
content: Here's a maker of precision wire wounds, there are expensive but they may have some of your values in their overstock:http://www.precisionresistor.com/

author: muvideo
author_rank: Frequent Contributor
timestamp: June 19, 2012, 06:17:44 pm
content: Here is the teardown:https://www.eevblog.com/forum/product-reviews-photos-and-discussion/db877-teardown/For 20k and lower here there are some good precision resistors:http://www.rhopointcomponents.comserie 8E16 and 8G16The price seem good.Quote from: SeanB on June 19, 2012, 04:01:20 pmAre you sure? Seem that the original ones have high specs.Fabio.

author: SeanB
author_rank: Super Contributor
timestamp: June 19, 2012, 08:05:38 pm
content: Compared to the originals they will be poor, but as part of the overall range they will do, unless you want to spend to get the original precision back, in which case you will have to replace all the resistors with 0.05% types ( at a price) and get low TC types as well for all of them.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 19, 2012, 08:25:38 pm
content: My bad, now I understand you intend 200 and 20 ohm.As for the 1M I still need very good ones.Fabio.

author: SeanB
author_rank: Super Contributor
timestamp: June 19, 2012, 08:43:13 pm
content: Then you will be buying 0.1% 1M resistors. Just make the 2M ones with 2 in series, and replace all the resistors in the decade with new ones at the same time, so they will track TC and voltage wise. For the lower values you will have to use 2W or higher rated units, as they will be the most likely overloaded ones.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 20, 2012, 02:09:28 am
content: Note that there can be a difference between "precision" & "high stability"resistors.Although one usually implies the other,it is not always the case.A "high stability" resistor may not be exactly its marked value,but in 20 years time,it will not have shifted markedly from its true value.As I said "high stab" is usually implied,but unless it is spelled out in the specs,you may have a resistor which is "spot on" value now,but with no guarantee as to whether it will change value over time.

author: T4P
author_rank: Super Contributor
timestamp: June 20, 2012, 06:58:18 am
content: Basically, high stability is all you need, no point having precision resistors when they are going to drift away eventuallyLook at the HP power supply and meter teardowns, the precision ain't great but they are really high stability parts

author: codeboy2k
author_rank: Super Contributor
timestamp: June 20, 2012, 08:45:34 am
content: Quote from: DaveXRQ on June 20, 2012, 06:58:18 am... and I'll add it's because HP engineers know that precision can be trimmed, stability cannot be.If it's not precise, so what about that? no one cares. It's better that it stays imprecisely wrong for the next 10 years.The calibration routines can fix that imprecision. They have more trouble tracking the drift (if they even attempt to)tldnr; buy low tempco, high stability devices

author: muvideo
author_rank: Frequent Contributor
timestamp: June 20, 2012, 11:34:04 am
content: vk6zgoDaveXRQcodeboy2kThank you all for the advice, I wrote the subject wrong, but I thought that "precision" wouldmean "high quality" devices in general.In the catalogues I find the category "Precision" and in this category usually there are variousclasses of "low TCR", "0.01%" etc.In the original post the main requirement was stability:Quote from: muvideo on June 19, 2012, 11:08:08 amFabio.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 21, 2012, 09:29:41 am
content: I think I saw an interesting post by Bored@Work about precision and accuracy,(I think there was also an answer by him to the DB877 teardown)was I dreaming or the posts disappeared?Bored@Work, if there was a reason to delete it or the posts weren't from you,sorry for highlighting it, in the case I'll delete also this post.Fabio.