Chinese Metal Film Resistors - How are they on noise specs? - Page 1
Forum section: electronics / projects

author: jaunty
author_rank: Regular Contributor
timestamp: January 03, 2016, 06:26:15 am
content: Hi - I bought a huge mess of chinese metal film resistors in basic values off the ebay - and did some basic testing of their stability (checking resistance while holding them to a blowtorch) and was pretty impressed - but after having now soldered up a few hundred of these - I was wondering (since resistance stability isn't the only spec that matters) how they fare noise-wise. HAS ANYONE TESTED THESE THINGS? Also - just curious if anyone has the inside track on how resistor noise determinations are made (just a signal source with a square wave and an oscope with a x1 probe? or...?)thanks

author: TiN
author_rank: Super Contributor
timestamp: January 03, 2016, 09:29:37 am
content: Which ones we talking about?You can measure tempco with digital temperature monitor, heater box (metal box with 20-50ohm high-power resistors connected to PSU would do)..Lower tempco usually often indicate for better stability as well, but really only periodic tests in same conditions will tell you for sure.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2016, 10:19:03 am
content: There are two types of resistor noise. The usually larger one is the normal thermal noise, that is the same for every resistor, only depending on value and temperature.The other noise is the excess noise that appears if current is passed through a resistor. This is often much smaller and thus not that reasy to measure. So you likely need more than just a scope.I would use a low noise amplifier (suitable for the resistor value, e.g. OPA140 could be a good choice for 100 K ... 1 M) and a low noise ADC, e.g. a sound card, maybe a high qualitiy AC voltmeter. Then use 4 equal resitors in a bridge circuit and measure the noise in one diagonal. The other diagonal is either powered by batteries to give a resonable power to the resistors (what is later used, e.g. 9 or 18 V for 100 K resistors) or shorted with a resistor equal to the internal resistance of the batteries.One can than compare the noise of the powered and unpowered resistor bridge. As there is not much change (maybe 1%) expected with good resitors one may have to repeat it a few times and keep every thing very constant and quiet.Anway it's an interesting thing to test - though likely not easy.

author: tggzzz
author_rank: Super Contributor
timestamp: January 03, 2016, 10:46:52 am
content: First understand all the sources of noise. Second understand the magnitude of the noise, particularly as a function of frequency. Thirdly, ensure your measurement tools and techniques are less noisy (or, more practically, have the same noise). Finally measure the noise.The first two are easy: just google. The third and fourth are less easy and require good theoretical and practical skills.You will find that the answer to your question will depend on the circumstances in which noise will be a problem to you. For example, DVMs, audio, RF all have different requirements.

author: dannyf
author_rank: Super Contributor
timestamp: January 03, 2016, 02:09:50 pm
content: QuoteIf you care about noise specs, you probably shouldn't have bought it off ebay. Kind of like walking into a Chevy dealership and expecting a Ferrari.

author: SeanB
author_rank: Super Contributor
timestamp: January 03, 2016, 02:38:22 pm
content: To be fair to the GM dealership down the road they have taken in many luxury cars on trade in. The Ferrari's end up across the road at the Mosque. Been a few there recently as well.

author: onlooker
author_rank: Frequent Contributor
timestamp: January 03, 2016, 03:00:31 pm
content: For eBay resistors, you need to make sure you actually got metal film ones. There are many on ebay that are actually carbon film in metal film's clothing (blue). A tempco test can be a guide to separate the two (e.g. +50ppm/C vs -300ppm/C).

author: TimFox
author_rank: Super Contributor
timestamp: January 03, 2016, 03:16:04 pm
content: The important thing about "excess" or "flicker" or "1/f" or "pink" noise in resistors is that it happens when current flows through the resistor.See any of the standard textbooks about noise, or this pdf:https://dcc-dev.ligo.org/public/0002/T0900200/001/current_noise.pdfWith metal film resistors, bad noise usually results from crummy terminations between the end of the film and the wires or SMT termination. Other types (such as carbon composition or carbon film) are inherently noisy due to tortuous conduction paths through the film.

author: Pjotr
author_rank: Frequent Contributor
timestamp: January 03, 2016, 05:48:05 pm
content: OP didn't mention what kind of resistors. Not only noise plays a role. At high frequencies inductance comes in also. My experience with MELF is that Chinese (Yageo) can be 10 times worse than Vishay/Beyschlag

author: tggzzz
author_rank: Super Contributor
timestamp: January 03, 2016, 06:49:37 pm
content: Quote from: Pjotr on January 03, 2016, 05:48:05 pmTrue.But Vishay produces many many types of resistor, and unfortunately don't provide a good and comprehensive guide as to theirrelativeadvantages. If anyone has spotted such a guide, I'd like to see it

author: jaunty
author_rank: Regular Contributor
timestamp: January 04, 2016, 04:05:46 am
content: Hi - thanks (!) for replies thus far....It seems a number of folk are asking me for more specifics on the resistors - i'm not sure if you mean 'which seller' or power rating - or something else... they are (as mentioned) the 'usual' blue (half watt in this case) MFRs... I assumed that - having a good thermal stability - they would be good to go... but I bought them instead of carbons for the stability (which didn't disappoint) but it didn't occur to me at the time that maybe noise might not be consistent with the usual lower values of other western manufacturers...I'm simply wondering if anyone else has had experiences with these types and been disappointed (apparently at least one person has had a bad experience with 'dolled up' carbon resistors!)...thanks (will get on with figuring out the noise testing business later if I get time - but for now I'm just hoping i don't have to pull all my resistors... I'd LIKE to think there are some DECENT cheapo resistors out there)

author: jaunty
author_rank: Regular Contributor
timestamp: January 04, 2016, 04:16:19 am
content: Quote from: Pjotr on January 03, 2016, 05:48:05 pmthanks - brand wasn't given - it's probably these ones - this was the same seller.... and same item description here:http://www.sun-pec.com/home/metalfilm_121

author: jaunty
author_rank: Regular Contributor
timestamp: January 04, 2016, 04:17:47 am
content: gosh - i thought that was only with coiled wire resistors - the inductance thing... but I guess the 'spiral cutting' and metal film deposition on the spiral would lead to inductive behaviour... i just never heard that before...Quote from: Pjotr on January 03, 2016, 05:48:05 pm

author: jaunty
author_rank: Regular Contributor
timestamp: January 04, 2016, 04:22:58 am
content: I guess it wouldn't really be a disaster for my purposes if they turned out to be 'lipstick on a pig' carbon resistors since it's for a programmer board for an old school analogue synthesizer (the music kind)... and I think there are no audio circuits on it - but it's good to know for the future...

author: Pjotr
author_rank: Frequent Contributor
timestamp: January 04, 2016, 09:33:36 am
content: Resistor inductance of that kind of resistors only comes into play above say 10MHz. For regular non critical (digital) stuff and audio gear I wouldn't bother.

author: tggzzz
author_rank: Super Contributor
timestamp: January 04, 2016, 10:50:13 am
content: Quote from: jaunty on January 04, 2016, 04:17:47 amIt is worse than that. As a rule of thumb, 1mm of wire (of whatever resistance) has an inductance of around 1nH.Do the calculations for an 0603 resistor, or for a 6"/150mm scope probe ground lead witha 15pF scope tip capacitance.

author: djQUAN
author_rank: Frequent Contributor
timestamp: January 04, 2016, 11:40:09 am
content: Slightly going off topic but just wanted to ask here...Are today's SMD resistors mostly metal film or similar or are there still carbon film SMD parts especially chinese ones? From what I've seen, they are all metal film (general purpose types, low R shunts are bulk metal) but not 100% sure on that.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2016, 02:03:11 pm
content: Low cost SMT resistors are usally thick film resistors. Carbon is rare in SMD.So they are a little better than carbon film, but not that much. Only the better ones are thin film metal film.

author: Pjotr
author_rank: Frequent Contributor
timestamp: January 04, 2016, 04:29:22 pm
content: Quote from: Kleinstein on January 04, 2016, 02:03:11 pmThick film SMD resistors are usually metal oxide/metal powder embedded in a glass carrier.https://dcc-dev.ligo.org/public/0002/T0900200/001/current_noise.pdf

author: SharpEars
author_rank: Unknown
timestamp: January 04, 2016, 04:36:33 pm
content: Never mind the noise specs, most 1% tolerance resistors from China are +/- 20% on tempco, just touching the thing with your finger (no need for a blowtorch). So unless you are running 1 nA through them, don't expect them to stay within 1%.If your blowtorch test indicated a good tempco you got really REALLY lucky.

author: TimFox
author_rank: Super Contributor
timestamp: January 04, 2016, 09:21:10 pm
content: Another important problem with SMT resistors in general can happen with very high resistance values, say above 10 megohm.With small package size (0603 or 0805), the resistance path must be very short in dimension, so the voltage gradient is correspondingly large even at only 10 V. With thick-film resistive materials, this can cause a noticeable voltage co-efficient of resistance (VCR) from 0 to 10 V. Good resistors may have this VCR quoted (at least as a typical), but this is rare. At the physical level, the VCR is a function of the voltage gradient (V/mm) in the material, which is obviously much higher in an 0603 part than in a full-size RN65 leaded part. In my previous day job, we had to search carefully for SMT resistors (often 50 megohm) where this non-linearity was important. We were able to find premium parts in 1206 size, but the same vendor's parts in 0805 were noticeably worse.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 02:34:01 am
content: I wanted to add my findings since the last post:many years later - but i wanted to add to the findings.A test trial of ten random resistors from a pack of 10Ks = meter is Tek TX1 (calibrated 2012)09.91K 0.90%10.00K 0.00%10.01K 0.01%10.02K 0.02%09.86K 1.40%10.05K 0.50%10.01K 0.10%10.01K 0.10%10.02K 0.20%09.98K 0.20%I thought that was pretty great overall - what they don't give in precision the make up for in accuracy. (precision being consistency and accuracy being 'hitting the target')Passed the heat test with FLYING colours (as previously mentioned - about two sec at the tip of a small blowtorch - definitely WAY too hot to touch for about a minute after that - not at all precise a test but enough to win me over)On the down side: the same resistors appear to have a higher than usual 'parasitic' capacitance - I measured them out at 400nF (or 0.4µF) regardless of value - ±0.02µF)

author: onlooker
author_rank: Frequent Contributor
timestamp: February 10, 2016, 03:15:45 am
content: QuoteIt is highly unusual and should be due to error in measurement. In this case, I think the parallel mode should be used. Some meters will start on the series mode. The user needs do the mode switching. Also the inductance may cause additional errors.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 04:51:22 am
content: thanks for the contribution. Yes - and no. it really doesn't matter WHAT the absolute 'readout' on these things in terms of capacitance (or any other quality or value you want to pick out) - the question is - there is obviously something about the construction of these things that leads to a VERY DIFFERENT result than we are getting from the control resistors (carbon, metal film, etc from various sources) - that result is totally consistent to about as highly repeatable as you can get. That's what matters about the 'scientific method'...so i'd kind of like to know what they put in these things - perhaps that's got to do with the 'trick' they are using to make them thermally non-reactive...Quote from: onlooker on February 10, 2016, 03:15:45 am