EEVBlog Rebranded Brymen BM235 & Backlight noise research - Page 1
Forum section: electronics / testgear

author: Scottjd
author_rank: Frequent Contributor
timestamp: February 21, 2016, 04:02:59 pm
content: So I've had a couple of nights without sleep, or minimal sleep at most.But every time I woke up one question was on my mind "What is that noise with the BM235 back light?"Oh, and the fact that Dave's Manual said it would NOT void the warranty. That alone was a big tease and I just had to take it apart.So I know this video is not the best quality. And heck I might even be a little goofyfrom lack of sleep, I just needed to open that meterand look around to see what made that highpitch noise.So I did not originally intend to do this as a video, it was more of a after thought and this is why the video starts with the meter back already pulled off. But I explain the things you need to watch out for and any surprises you may run into.But I did record putting it back together and the follow up testing to make sure it was still as accurate just as it was when I did the review. Since I have the numbers from my the review I can say the tear down did not affect the meter at all.I also ruled out some possibilities for the noise, and think I may have narrowed down whats causing it.Oh, and I know the lighting is bad. I tend to forget things without sleep like turning on the lights....Its time for me to get some more, I hope...... Enjoy the video.......If your interested in the review this is the thread:https://www.eevblog.com/forum/testgear/eevblog-branded-brymen-bm235-in-depth-feature-review-and-thoughts/https://youtu.be/mcz8Ovm5NmQ

author: idpromnut
author_rank: Unknown
timestamp: February 22, 2016, 02:58:24 am
content: I watched your video, nice workAlthough I was yelling at the screen a bit when you were saying that the whining was coming from the LEDsAfter a bit of hunting, it looks like if you remove C44 (smoothing cap on the backlight charge-pump output) the whine will be completely gone (except if you can hear into the 32kHz range, then you're screwed as that is the base frequency that the charge pump is running at). I don't see any long term issues with it running like that, and there are no parts to add.C44 is a ~ 10uF ceramic.A few current measurements (using a uCurrent + Fluke 27):With C44 in place:- No backlight on: 2.2mA- With backlight on: 26.2mAWith C44 removed:- No backlight on: 2.2mA- With backlight on: 27.3mA

author: idpromnut
author_rank: Unknown
timestamp: February 22, 2016, 03:00:01 am
content: I also shot a couple of videos of me bumbling around trying to find the cause:And finally the fix:

author: Brumby
author_rank: Unknown
timestamp: February 22, 2016, 04:14:00 am
content: I wonder if we can get an 'official' thumbs up or thumbs down on this - or an alternative?

author: MarvinTheMartian
author_rank: Frequent Contributor
timestamp: February 22, 2016, 06:43:15 am
content: Probably a very,verystupid question (but hey, I'm new)...Would some snot glue on the cap dampen down the vibration at all?(I'm assuming it is a physical vibration of the cap.)I probably don't have a clue what I'm talking about - right? Doh!

author: EEVblog
author_rank: Unknown
timestamp: February 22, 2016, 07:23:22 am
content: Quote from: Brumby on February 22, 2016, 04:14:00 amBrymen are looking into it.Changing the cap to another brand, model or size etc will likely make a difference.MLCC capacitor microphonics are very manufacturing process specific.

author: wraper
author_rank: Unknown
timestamp: February 22, 2016, 07:51:17 am
content: If the original cap is Z5U or similar, changing it to X5R, X7R type most likely will kill the noise.

author: ebastler
author_rank: Super Contributor
timestamp: February 22, 2016, 11:20:52 am
content: Quote from: EEVblog on February 22, 2016, 07:23:22 amIs there an accepted term for the opposite of "microphonic" behavior, as present in this case? "Speakerphonic"?Anyway, it's not an uncommon effect. Murata are even promoting "low noise capacitors" to fix it:http://www.murata.com/products/capacitor/mlcc/solution/Naki

author: EEVblog
author_rank: Unknown
timestamp: February 22, 2016, 11:25:54 am
content: Quote from: ebastler on February 22, 2016, 11:20:52 amReverse piezoelectric effect among others.I mentioned it back in episode 33

author: Salas
author_rank: Frequent Contributor
timestamp: February 22, 2016, 03:09:41 pm
content: Quote from: ebastler on February 22, 2016, 11:20:52 amThe "Microphonic" old term comes from the reasoning that a component picks up a frequency mechanically. More accurate would be "resonant" in the audio band.

author: wraper
author_rank: Unknown
timestamp: February 22, 2016, 03:35:00 pm
content: Quote from: Salas on February 22, 2016, 03:09:41 pmCompletely inaccurate.You can call it "sound emission", "resonant in audio band" means what it says (resonant, by the nature of the part, electrically resonant), nothing about vibration/emitting the sound.http://www.keysight.com/upload/cmc_upload/All/exp79.pdf

author: ebastler
author_rank: Super Contributor
timestamp: February 22, 2016, 03:52:03 pm
content: Quote from: EEVblog on February 22, 2016, 11:25:54 amYep, I realize it's mostly a piezoelectric effect in capacitors. Other effects might inadvertently translate electrical signals into sound as well (in other components): Electromagnetic force of course is a common source of transformer hum. Electrostatic forces, maybe, in some types of capacitors? Acoustical noise from modulated light pressure optical data transmission, anyone? (Very high ultrasonic pitch, hence people don't often complain about it)Just meant to say that "microphonics" did not seem the right term here, and that I was surprised to realize that the inverse effect does not seem to have a general name -- at least not one that is commonly used.

author: Salas
author_rank: Frequent Contributor
timestamp: February 22, 2016, 04:25:39 pm
content: Quote from: wraper on February 22, 2016, 03:35:00 pmWhy only electrically resonant, when it can also mean mechanically resonant or piezoelectrically resonant just as well?

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: February 22, 2016, 04:51:24 pm
content: Quote from: EEVblog on February 22, 2016, 07:23:22 amIs it physically the cap making the noise? Because I remember that Bigclive said that ceramic caps tend to have piezoelectric qualities that cause them to make slight noises when they get charged/discharged

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2016, 05:24:17 pm
content: The light pressure is quite low, so you usually will not hear this. Another source of sound is due to thermal effects - AC current causes modulation in temperature and thus through thermal expansion a mechanical vibration. This works with both thermal expansion in the solids and air. It's usually not very loud unless amplifies by a resonance, but it is measurable .At university I have measured this effect on solar cells when passing AC current through them - this even allows to distinguish where exactly the diode gets hot. It funny where exactly a diode / solar cell gets hot and cold.The piezo effect with capacitors can vary a lot, as this is a second order effect and needs some asymmetry in polarity to have a piezo effect. There is a small quadratic electrostriction effect that is present in essentially all materials, not just the high capacitance MLCC.

author: marshallh
author_rank: Unknown
timestamp: February 22, 2016, 06:41:52 pm
content: I will swap out the cap and see what happens.This is also happened to me on a switching reg with MLCCs.

author: wraper
author_rank: Unknown
timestamp: February 22, 2016, 06:41:56 pm
content: Quote from: Salas on February 22, 2016, 04:25:39 pmBut it is not mechanically resonant at the frequency you are hearing the whine.

author: markone
author_rank: Frequent Contributor
timestamp: February 22, 2016, 07:13:28 pm
content: Quote from: wraper on February 22, 2016, 06:41:56 pmHow can you exclude that ?It could be electrical, mechanical or a combination of both.I guess that capacitor is there to low pass and level the LEDv PWM driver's output, in par with the inductance of course, so it would be interesting to know L & C values a put a scope's probe to see if the resonace is actually nearby the square wave's fundamental or is from edges overshoots.

author: MarvinTheMartian
author_rank: Frequent Contributor
timestamp: February 22, 2016, 10:44:10 pm
content: Wow!This sounds a whole lot more complicated than I had assumed.I thought it would have to be a purely physical vibration to produce sound.Must say I didn't notice the whine in my EEVblog meter until read this topic.I have to hold the meter about 12cm from my ear before I hear the whine (and I have almost perfect hearing according to my hearing test yesterday).Maybe I was lucky with the meter Dave sent to me - whoo-hoo!Edit: Just found out I have a slight dip in my hearing around 4Khz (still in normal range though) so that might explain my not easily hearing the the whine at 5KHz.

author: wraper
author_rank: Unknown
timestamp: February 22, 2016, 11:02:27 pm
content: Quote from: markone on February 22, 2016, 07:13:28 pmIt's whining at the electric ripple frequency. Good luck to get it to match exactly with a capacitor mechanical resonance frequency. So no, you cannot call the capacitor whine being resonance. Think about speaker or piezo buzzer, yes they have resonance frequency(ies) but they do emit the sound in much wider frequency range.

author: Salas
author_rank: Frequent Contributor
timestamp: February 22, 2016, 11:10:50 pm
content: It can be excited and sing in whatever mechanical frequency it has. When you hit a bell even once you do not hit with its resonant frequency, you just excite it.

author: wraper
author_rank: Unknown
timestamp: February 22, 2016, 11:30:27 pm
content: Quote from: Salas on February 22, 2016, 11:10:50 pmThen please demonstrate how to excite the piezoelectric transducer by applying some different frequency signal to it in a way that it will emit the sound with it's own resonant frequency.

author: markone
author_rank: Frequent Contributor
timestamp: February 22, 2016, 11:31:06 pm
content: Quote from: wraper on February 22, 2016, 11:02:27 pmWith XTALs what you call ripple frequency is in perfect phase with the physical device's mechanical resonance tone, also capacitors with piezo effects have mechanical resonant frequencies.Let's put things in this way : the whining tone could be the result of step response of a resonant system with a mechanical component, with have a square wave so lot of edgesI'm not saying is like that for sure, i'm saying it could be, without any data we can argue for days without converging.So it's time to put a scope's probe on the darn thing and post the waveform

author: markone
author_rank: Frequent Contributor
timestamp: February 22, 2016, 11:36:27 pm
content: Quote from: wraper on February 22, 2016, 11:30:27 pmIt's called resonant system step responce, if the piezo transducer has a one or more resonant points (and everything has at least one), applying a low freq square wave, maybe under its freq responce, you could obtain high freq tones on par with resonant points.

author: markone
author_rank: Frequent Contributor
timestamp: February 22, 2016, 11:40:44 pm
content: Just one question to the OP: the whining noise's level is so high to be a deal breaker ?

author: wraper
author_rank: Unknown
timestamp: February 22, 2016, 11:48:22 pm
content: http://www.kemet.com/Lists/TechnicalArticles/Attachments/62/2007%20CARTS%20-%20Reduced%20Microphonics%20and%20Sound%20Emissions.pdfnot a single word about resonance.

author: markone
author_rank: Frequent Contributor
timestamp: February 22, 2016, 11:59:44 pm
content: Quote from: wraper on February 22, 2016, 11:48:22 pmSo those capacitors are flat response wideband speakers ?

author: wraper
author_rank: Unknown
timestamp: February 23, 2016, 12:08:14 am
content: Quote from: markone on February 22, 2016, 11:59:44 pmseems like that

author: markone
author_rank: Frequent Contributor
timestamp: February 23, 2016, 12:23:01 am
content: Quote from: wraper on February 23, 2016, 12:08:14 amIMHO i do not think so, i'll bet they sound only in high freqs due to small mechanical dimensions.Anyway, that's not the point, let's wait for some waveforms screens.

author: wraper
author_rank: Unknown
timestamp: February 23, 2016, 12:28:46 am
content: Quote from: markone on February 23, 2016, 12:23:01 amcheck out the graphs on the second last page.

author: Brumby
author_rank: Unknown
timestamp: February 23, 2016, 12:57:36 am
content: Quote from: markone on February 23, 2016, 12:23:01 amI agree.Resonance isn't necessarily relevant here and the explanation for the phenomenon does not require it - so I don't think we need to spend any more time jumping around in that blow-up castle.

author: idpromnut
author_rank: Unknown
timestamp: February 23, 2016, 01:29:43 am
content: Not sure how important it is to the discussion, but I captured a couple of (crappy) waveforms at both C44 and C43 showing both the fundamental ~32kHz drive for the charge-pump (on C43), and the ripple on the output (C44).

author: markone
author_rank: Frequent Contributor
timestamp: February 23, 2016, 02:43:14 pm
content: Quote from: idpromnut on February 23, 2016, 01:29:43 am32Khz are well above human hearing capabilities, i guess you are listening to the tone deriving from square wave burst repetition period, if read well your screen should be around 4Khz.Also my Oregon Scientific alarm clock emits whining noise with backlight turned on, but i can live with

author: idpromnut
author_rank: Unknown
timestamp: February 23, 2016, 04:51:55 pm
content: Quote from: markone on February 23, 2016, 02:43:14 pmWithout C44, there is no 4-5kHz repetition period; I'm not sure if it is the 32kHz itself I am hearing or some harmonic of it, but it is very very high pitch, right at the limit of my hearing. It is similar to the whine that I heard with older CRT-based TVs. Not sure if that is in the same frequency range, but that is the closest that I can describe it. Either way, that high frequency noise is very attenuated so I'm not bothered by it, especially with the case closed.

author: markone
author_rank: Frequent Contributor
timestamp: February 23, 2016, 05:26:41 pm
content: Quote from: idpromnut on February 23, 2016, 04:51:55 pmIt's highly unlikely, i would say almost impossible, that you are able to hear a 32Khz tone or its harmonics (that would be even higher), CRT noise's pitch freq. was on par with horizontal scan rate, around 15-16Khz depending on TV standard (PAL, SECAM, NTSC).Anyway considering the amount of time that normally one spend with DMM backlight turned on, this does not seem a trouble at all.

author: Scottjd
author_rank: Frequent Contributor
timestamp: February 24, 2016, 04:17:43 am
content: Quote from: markone on February 23, 2016, 05:26:41 pmI guess it depends on your application. For videos I tend to use the fluke because I can adjust the backlight to be on up to 30 minutes. This isn't because it's dark, but the backlight help the display he captured over the studio lights. So the 10 minute backlight is a nice feature for me to have when I need two meters measuring separate parts.Dave has mentioned that this is intermittent and not all meters are as loud as mine or idpromnut's meter. It just happens that we released the first reviews on the meter. He also said that Brymen is looking into it.I was way to tired and still recovering from a hostilely procedure to do any probing or take any measurements, thanks for idpromnut for taking those readings. I may go back to that if I have time and take some measurements. The irony is the 235 was very acurate for taking mA readings, but I think my fluke and hp bench should be able to take the same.If I had to guess the only reason these additional component are inline with the LED back light is to support the flashing on and off with the continuity feature. Most of my other cheaper meters don't have any filtering before the lighting like this one has. But they also don't flash on and off, or habe a backlight that stays in for 10 minutes. They are more 10 second lights.I think I'll wait it out and see what Brymen comes up with after they finish researching. If they need one that is loader then normal I can always ship mine in for an exchanged unit. I'm always willing to work with companies that listen to their customers and are willing to spend time to improve their products.

author: idpromnut
author_rank: Unknown
timestamp: February 24, 2016, 12:55:03 pm
content: Quote from: Scottjd on February 24, 2016, 04:17:43 amI think this was more a case of following the application notes for the LCD driver chip (which I mistakenly thought was the DMM frontend). You can see here in the datasheet on page 14 (http://www.hycontek.com/attachments/LCD/DS-HY2613_TC.pdf) that there is an output filtering cap (4.7uF). So nothing to do with the flashing backlight (indeed, the backlight "flashes" at a very low frequency, say about 10Hz max, so it should not be affected by a 32kHz ripple at all).

author: mos6502
author_rank: Frequent Contributor
timestamp: February 24, 2016, 03:07:43 pm
content: Strange decision to use a switch mode LED driver. Doesn't the electrical noise affect the measurement? On the UT139C, the LEDs are simply connected to battery voltage via a resistor and switched to GND with an NPN transistor.

author: ve7xen
author_rank: Super Contributor
timestamp: February 25, 2016, 12:16:07 am
content: Quote from: mos6502 on February 24, 2016, 03:07:43 pmMeter is powered by 2xAA batteries. Nominal voltage is 3V but should expect it to work down to 2.2V or less. Most white LEDs have Vf greater than this, and there may be several in series. A switcher is needed. It should not affect measurement if designed properly.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 25, 2016, 01:46:13 am
content: Quote from: ve7xen on February 25, 2016, 12:16:07 amThe UT139C is powered by two AA batteries as well. Funnily, the manual says that the backlight may stop working as the batteries discharge.Still, this could easily be fixed by replacing the LEDs with amber ones (Vf=1.8V). Looks better anyway, no unnecessary complexity and no acoustic noise. But apart from that, the switcherdoeshave an effect. Even if it doesn't interfere with the multimeter's measurement itself, it could interfere with a sensitive circuit that is being measured. I mean, if you hold an AM radio next to the meter, I'm sure you're going to hear the noise from the switcher.

author: EEVblog
author_rank: Unknown
timestamp: February 25, 2016, 05:06:06 am
content: Quote from: ve7xen on February 25, 2016, 12:16:07 amYes. And this is why some older meters have green backlights, lower voltage drop so can direct power from 2 1.5v cells

author: vsboost
author_rank: Contributor
timestamp: February 25, 2016, 10:19:47 pm
content: Mine is barley audible, literally have to put my ear to the meter to hear it, and yes my hearing is perfect.

author: boggis the cat
author_rank: Regular Contributor
timestamp: February 25, 2016, 11:36:04 pm
content: Quote from: vsboost on February 25, 2016, 10:19:47 pmThe one I got seems to be fairly quiet, too. Seems quite a high frequency, so possibly my aging hearing doesn't perceive it well.Edit:Used the iPad to record the noise. It is a reasonable spike, at a bit over 10kHz.

author: EEVblog
author_rank: Unknown
timestamp: February 26, 2016, 01:01:36 am
content: Just shot a video on this, editing now.

author: Scottjd
author_rank: Frequent Contributor
timestamp: February 26, 2016, 11:14:52 pm
content: Quote from: EEVblog on February 26, 2016, 01:01:36 amThanks for the video, I was debating on swapping the cap after idpromnut confirmed that C44 as the noise maker.I didn't want to probe or trace anything since I didn't want to affect the acuracy or warranty of the meter. I know continuity feeds a voltage, and I feared that voiding the warranty. I know not all of them are as loud as mine, I didn't have to turn up the gain on my mic and it probably from different manufactured capacitors like you said. But I think I'll forget about the warranty and replace the cap. I purchased a microscope with a camera a last month and I've been wanting to try it and see how good it records video, so I think this cap replacement will make a good test for the microscope camera. After all, it was originally going to be a gift so if I screw it up I still have two other meters I fully trust.Unfortunately I don't have any new ceramic 10uF caps, I only have up to 1uF. So I'll probably pull one from a scrap board. I do have some 1206 tantalum caps but not any ceramic.I also had the idea of just re-flowing the cap it came with. I started to wonder if the vibration could be amplified from a weaker solder joint, and if that's the difference between meters also?Unlike yours in the video, I can't get my sound to change by touching the meter and it doesn't sound shakey. Mine seems to be a constant steady frequency. So when I saw your sound can change by touching the meter I wondered if the sound is being amplified off the board, and maybe re-flowing it would turn the volume down?I was more couriose of the LED, that's why I decided to take it apart. I've built 30+ LED flashlights with different LEDs and drivers, all with different programmable features and power outputs and never heard a noise that louad with LEDs.I'm going to stay with review videos and not fix videos. I know I'm new at the fixing and learning the electronics side. I try not to do tear down videos despite that I've torn down a lot since I was a kid, but I don't have the reverse engineering skills or understanding of electronics to explain it like you do. That's the part I just started teaching myself in my spare time as a new hobby. Building a soldering with batteries and RC stuff I've been doing since a kid, but that doesn't mean I fully understood how the electronics worked.

author: idpromnut
author_rank: Unknown
timestamp: February 26, 2016, 11:59:25 pm
content: Quote from: Scottjd on February 26, 2016, 11:14:52 pmWe're talking about Dave here... he sells T-Shirts with the logo "Warranty Void if Not Removed!" Also, regarding the continuity tester: if you're worried about the voltage it feeds into the circuit under test, then THAT is an /excellent/ video to shoot as to whether it will or will not matter! Hint: it probably will be just fine, now your turn to figure out why!

author: Scottjd
author_rank: Frequent Contributor
timestamp: February 27, 2016, 12:58:41 am
content: Quote from: idpromnut on February 26, 2016, 11:59:25 pmAhh yes, this I know. His manual even says he encourages you to take it apart. But taking it apart and probing around, well I view that differently. As for diagnosing and probing I'll leave that to your and Daves channel. I have 3 things on my bench to review still from vendors and 6 personal items I bought that I want to review. So I'm a little backed up on the reviewing side. My health has backed me up in the schedule for reviews and life in general so unfortunately this takes away from the learning electronics hobby time slots.But if I get around to it I'll be sure to report back. Thanks for proposing the challenge.I probably could have traced the inductor, but didn't think about it. Oh well, to late.

author: Scottjd
author_rank: Frequent Contributor
timestamp: February 28, 2016, 09:50:25 am
content: Quote from: Scottjd on February 26, 2016, 11:14:52 pmSo the curiosity about this question I asked myself got the better of me and I decided to do some experimentation and find out the answer. Besides I was wanting to try out my new microscope anyway. This is not what I should be doing, I should be reviewing some other items but sometimes I get stuck on curiosity with one item and need to answer that one question before I can move one.So I guess I answered my own question....... and now I can move on......https://youtu.be/TfNuTmqtiD4

author: NilByMouth
author_rank: Regular Contributor
timestamp: February 28, 2016, 10:13:03 am
content: Got to thank Brymen for designing a meter with an inherent design fault. So much more fun than if they'd designed a perfect meter without any problems.

author: Scottjd
author_rank: Frequent Contributor
timestamp: February 28, 2016, 10:26:06 am
content: Quote from: NilByMouth on February 28, 2016, 10:13:03 amIt's still a great meter. I don't have anything as acurate as this under $400 US, and I think this was a little over $100. Hands down, can't beat it even if you get one with a loud backlight. I would still recommend it.But now maybe Brymen and stop looking into why some are louder then others and just change the design to have a bigger capacitor.

author: idpromnut
author_rank: Unknown
timestamp: February 28, 2016, 03:35:39 pm
content: Quote from: Scottjd on February 28, 2016, 10:26:06 amUgh, either your measurement is off (itshouldbe closer to 10uF), the tolerance on that part is horrendous, or it was from a fairly crap batch of capacitors

author: Scottjd
author_rank: Frequent Contributor
timestamp: February 28, 2016, 06:37:02 pm
content: Quote from: idpromnut on February 28, 2016, 03:35:39 pmOk Idpromnut, you have to stop proving me wrong, lol.I just checked four ceramic caps and one tantalum. My fluke is constantly reading the ceramics way to high. Why is the Brymen making my fluke look like crap now? Oh, and the old extech also. I guess this is why people say you should own multiple meters.I need to pull out the manual for my fluke and see what the tolerance is on capatamce readings, and if it's out of spec I guess it goes in for service and a free calibration. I did recently pull the batteries and when I replaced them I received the "clock and date not set" message and it's supposed to hold this information with a super cap. Maybe it sprung a leak?If you take the average that the fluke is reading the caps high, then that would make the cap in the Brymen about 9.1uF. The other 10uF I found if I ended up replacing the cap also just showed close to 14uF on the fluke. So I would say the cap was a 10uF on the Brymen. By bad, or should I say my flukes bad?

author: bson
author_rank: Unknown
timestamp: February 28, 2016, 10:19:41 pm
content: Quote from: NilByMouth on February 28, 2016, 10:13:03 amWell, clearly they know their audience!Can't wait for the next batch so I can have one, too!

author: idpromnut
author_rank: Unknown
timestamp: February 29, 2016, 01:10:27 pm
content: Quote from: Scottjd on February 28, 2016, 06:37:02 pmI didn't prove you wrong; we identified a problem with your measurement set up, and now it's fixed! So the next capacitance measurement(s) you do will be spot-on. Publishing, review and adjustment in action!

author: Robomeds
author_rank: Frequent Contributor
timestamp: February 29, 2016, 05:24:23 pm
content: So I have a Greenlee version of this basic meter family (DM-200A). The Greenlee, in addition to being green, has an amber backlight. As far as I can tell there is no noise. Perhaps the lower voltage of the amber LEDs avoids the need for a charge pump. Not that it maters much but I actually kind of like the amber better.

author: Scottjd
author_rank: Frequent Contributor
timestamp: February 29, 2016, 07:45:37 pm
content: Quote from: Robomeds on February 29, 2016, 05:24:23 pmWithout seeing the schematic of your meter is hard to validate. But some older meters that switched to LED backlighting was done by using a simple resistor to ensure the LED didn't receive to much current and was powered within its limits. This is an old way or running LEDs and usually restricted the time the LEDs could be turned on or powered this way before the LED had a thermal runaway and was damaged. This is also why some older meters used LEDs for backlighting was still restricted to the light being on for 10 to 30 seconds to ensure the LEDs didn't overheat and fail.This was efficient for short backlight times and worked, but was not optimal from a power consumption point and still waisted energy using the resistor, and wasn't optimal for the lifetime health of the LED.The BM235 has a backlight that can stay on for 10 minutes at a time because it's using a power efficient LED driver to properly run the LEDs at the optimal rated current and voltage for the LEDs. Using a low energy LED driver is not only more energy efficient but also optimal for the LEDs life time and keep them away from thermal runaway failures without the need to worry about the LEDs being powered by to much current or the need for additional thermal heat sinks.Although LED drivers have become the best way to run LEDs some drivers must also act as a boost circuit to allow the driver to run the LED even when the power source (AAA Bateries, or 9V battery) becomes low. Part of this boost circuit uses other components such as inductors and capacitors to help maintain and regulate the specified current and voltage to the LEDs despite the batteries continually draining over time by using the meter. An example pointed out on the thread was a UNi-T meter, the manual stated the LEDs light may go out as the batteries drained, but the meter would continue to run properly. This UNi-T meter either uses an older LEDs driver that's not efficient enough for the task or uses the LEDs with a resistor.Boost circuits are mostly not an efficient way of controlling power. So as LED drivers continue to get better, cheaper and more efficient were some don't need boost circuits and use what's called a charge pump buit into the LED driver. This is what's used in the BM235 allowing the LEDs to be run at the optimal current, voltage and temperature and also allowing the backlight to be on for 10 minutes while not effecting the battery lifetime of the unit. This charge pump required/used an external ceramic capacitor and this was the capacitor that was traced to be the source of the noise.The explanation on how some ceramic capacitor vibrate/resonate at and audible frequency that can be heard by the human ear is explained in Dave Jones EEVBlog latest video and some of his older videos. Most ceramic capacitors have become bigger with more layers allowing them to be used for higher voltage applications and because of the additional mass they may still resonate, but not at a frequency that can be heard by the human ear. It's not a common issue these days, or I haven't noticed it that often myself. In fact even the capacitor for the BM235 charge pump built into the LED driver isn't that loud, but some meters like the one I received had just enough room or maybe not the same amount of solder others had to cause the sound to amplify in some way making the noise a lot more noticeable in the case of my meter.

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 01, 2016, 04:18:18 am
content: Scott,Our forum friend MrModemhead has some great pictures of the inside of the same meter I've gothttp://mrmodemhead.com/blog/gallery/dm200a/It looks like the main boards have significant differences (not surprising given the differences in features). The DM-200A backlight is limited to something like 2 minutes. I've never timed so it might be only 30 seconds or perhaps more like 4-5 minutes. I just know it's short enough that when I've used it I've often had to turn it back on.I do understand the current limiting resistor bit as well as the rest of the discussion. I'm not at all questioning why the BM235 is making the noise, just noting that a similar meter doesn't but it also doesn't use white LEDs. Not sure what the forward drop is across the white vs amber LED but I would assume the amber is less.

author: sangvikh
author_rank: Contributor
timestamp: March 01, 2016, 01:52:09 pm
content: Quote from: Scottjd on February 29, 2016, 07:45:37 pmThat sounds wierd! Are you sure about the thermal runaway issue?I know resistors in series with LEDs have been used in a lot of circuits for decades. And the LED's have been running for decades too.

author: Scottjd
author_rank: Frequent Contributor
timestamp: March 01, 2016, 01:54:18 pm
content: Quote from: Robomeds on March 01, 2016, 04:18:18 amI'm not sure either if their is a different forward drop accosts the white or amber. I've never been asked to build an amber color LED flash light 😀I looked at his pictures and it is different, after a closer look i saw that the main PCB on that green one is marked as a 251 model with the 25X riser board,for,the 251,252,255,257 meters. According to the manual only the 257 and 255 models have a backlight that turns off in 32 seconds. So your is probably a rebranded 255 or 257 Brymen meter.

author: Scottjd
author_rank: Frequent Contributor
timestamp: March 01, 2016, 02:26:52 pm
content: Quote from: sangvikh on March 01, 2016, 01:52:09 pmWell I didn't design any of these meters so I can't say for sure if the short times are for partial thermal or over all battery life deciding factors, or both.I wasn't saying that the LEDs on the meter would have immediate failures from thermal runaway, but the fact that when their ran at a higher current and the power is not being regulated by a driver this causes the LED to heat up out of spec and shortens the life span.Since most LEDs have a claim to fame with 50,000+ hours average life span so people have been running them slightly out of spec for years. Why pay for a driver and extra IC draining power when LEDs in series and a resistor still give the LEDs a long run time, even if it cut the lifespan in half of the specs runtime this usually is still a very long life time.Even I have built flashlights that when they are on high output the LED throws more light and I know it's out of spec. Some people just want brighter, and as long as I manage the extra heat correctly to not effect the Li-Ion battery putting the user in danger at that point, then I still build it. I even bought a few (before I stated building them) that deliberately push the LED for the extra light. And the flashlights are using a driver.It's the factor of more light on the flashlight for 24,000 hours runtime vs 50,000 runtime. It's been a common trade off in flashlights, so with backlights this is a feature that is probably used the least on a DMM (unless you work in the dark) and a shorter life span is still a long time. 50,000 hours is little over 6 years if the LED was constantly on. But now days the drivers have become more efficient, smaller, less heat, and cheaper it so I expect to see light being in longer times and the leads should run their full expected life time.I think with the progression speed of technology advancements most wouldn't ever see the LED burn out from its normal use in its rated lifetime, and most likely something else on on the device will fail fist or people would just want to upgrade to a new versions of that device.

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 01, 2016, 03:02:17 pm
content: Quote from: Scottjd on March 01, 2016, 01:54:18 pmI've looked into it before and figured it didn't have an exact Brymen counterpart. When looking at the 251, 252, 255 and 257 none are really an exact match. The 252 and 251 have no backlight yet they are the only ones with the 4 button (vs 6) layout of the 200A. It's possible the 200A is a 251 with the backlight enabled. Since these things are microprocessor driven I suspect all Brymen has to do is make a general PCB, populate the parts needed for a feature set, then program the correct code in the micro controller.The table here shows amber LEDs with a 2.0-2.1 forward drop while blue is 2.5-3.7. I'm assuming blue for white since what I understand is most white is blue + phosphorus coating that responds to the blue wavelengths. Anyway, given the system runs on ~3V an amber LED should be find with a current limiting resistor while a blue/white may need a charge pump.http://www.radio-electronics.com/info/data/semicond/leds-light-emitting-diodes/characteristics.php

author: retiredcaps
author_rank: Super Contributor
timestamp: March 01, 2016, 06:22:02 pm
content: Quote from: Robomeds on March 01, 2016, 04:18:18 amI measured the DM-200A at 33 seconds, but manual states 30 seconds as per screenshot below.

author: sangvikh
author_rank: Contributor
timestamp: March 02, 2016, 04:19:12 pm
content: Quote from: Scottjd on March 01, 2016, 02:26:52 pmThat may be the case with high power leds running at their design limits.There should be no problem selecting a resistor that limits the current to be within spec taking into account reduced Vforward with temperature etc.Never really gave this problem a lot of thought. You learn something every day, especially on EEVblog forums

author: hammy
author_rank: Unknown
timestamp: April 23, 2016, 11:39:46 am
content: I had success with this replacement cap for C44:Murata, GRM31CR71E106KA12L10µF, 10%, 25V, 1206, X7RCheershammy

author: fanOfeeDIY
author_rank: Unknown
timestamp: April 25, 2016, 09:27:28 am
content: Quote from: hammy on April 23, 2016, 11:39:46 amNice!It is also has information here too.http://www.murata.com/en-global/products/capacitor/mlcc/solution/naki?intcid5=com_xxx_xxx_cmn_hd_xxx

author: Scottjd
author_rank: Frequent Contributor
timestamp: April 25, 2016, 12:22:46 pm
content: Quote from: fanOfeeDIY on April 25, 2016, 09:27:28 amInteresting, putting the caps on little metal high rises they confirmed what had was theorizing. Thanks for that link to that site and a different explanation.To be fair making a thicker capacitor and increasing the thickness of the ceramic layers would have strengthened the capacitor also causing the same effect and results. I would think also adding more solder/stronger solder, or bigger solder pads for more solder would also prevent the contraction and expansion as I mentioned in my video. So I guess both explanations are correct between Dave and I, we just went different ways of looking at things when approaching the same issue for troubleshooting.But the name of the game in electronics seems to be how can we make things smaller, I wonder how small they can make those metal high rise pads?If we didn't have people look at things different ways then new ways and inventions would never be made, and things would never advance. It takes all kinds of people to make things better or different. That's why we brain storm and sometimes multiple ways are created to fix the same issue, it doesn't make either way wrong, they can both be correct.I miss the engineering brain storming, it's not about who's wrong or correct. It's about different views and new ideas. Maybe that's why I like this site and reading long threads, hearing different views from different people. One of the many things that makes EEVBlog and exiting platform to be part of.SIDE Note and take a vote:What do you think and what's your opinion. I wonder how Brymen decided to correct the buzz going forward.1. Making bigger solder pads and keeping the same capacitor they have in stock?2. Changing out the stock of capacitor and using a higher rated thicker capacitor?3. Just adding more solder?4. Using a different solder with more strength characteristics?I'm sure they probably fixed it or have taken these comments into consideration for the next DMM design. After all everyone's comment did a little of the work for them. But I also know from experience with big companies that the lower cost solution usually wins.What way do you think Brymen choose to fix the intermittent issue with some of these meters that are louder then others going forward?

author: Flipflop
author_rank: Contributor
timestamp: May 17, 2016, 10:12:13 pm
content: Have Brymen fixed this issue on the latest batch of meters?

author: thisguy
author_rank: Contributor
timestamp: May 18, 2016, 12:42:10 am
content: Quote from: Flipflop on May 17, 2016, 10:12:13 pmAs of today there are multiple reports that the sound is gone, Dave says it's fixed in the latest shipment, although one person says he can still hear it quietly and high pitched if the meter is held directly at his ear. (Mine makes no sound even when held directly to my ear.)

author: EEVblog
author_rank: Unknown
timestamp: May 18, 2016, 12:58:11 am
content: Quote from: Flipflop on May 17, 2016, 10:12:13 pmYes. The capacitor was changed from 10uF to 0.1uF. Sound if now gone. Although it seems one person here has reported a very feint sound.Obviously I can't open hundreds of meters and check them all.Only the first batch of 300 was affected (those in the coloured boxes).

author: deadlylover
author_rank: Frequent Contributor
timestamp: May 18, 2016, 03:36:38 am
content: Yep those guys are probably referring to me.I have to hold the meter right up to my ear like a mobile phone to hear it, it's inaudible 6"/15cm away from my ear. Even up close it's veeery faint and it sounds like it's in the ~13kHz region.It's a complete non issue IMO, it gets a thumbs up from me and keep in mind I'm in my mid 20s so my hearing is probably a bit better than most here.

author: Flipflop
author_rank: Contributor
timestamp: May 19, 2016, 07:09:02 pm
content: I went to collect my meter todayhad a quick play with it and I can hear a very slight noise when I hold it close to my ear but, nothing that is going to bother me.Overall very pleased with it.Worked out cheaper (including import taxes) than buying a BM257 from TME.

author: Jayeson
author_rank: Newbie
timestamp: June 25, 2016, 02:30:41 pm
content: I got mine this week and it is certainly audible to around 30cms or so. The level varies - not sure if it is the level from the meter changing or the orientation or just my ear tuning it out.[Edit: Removed off-topic comments on the display - I did not see that there was another thread on this]

author: imidis
author_rank: Frequent Contributor
timestamp: June 25, 2016, 08:02:52 pm
content: Mine only makes a very slight noise have to hold up to my ear to hear it, I agree some viewing angles aren't the best, however I have found that most of the ways I use it so far are at the better viewing angle. Quite happy with it all in all though, so far.