Will this 2V signal damage my bench multimeter over time? - Page 1
Forum section: electronics / testgear

author: Amaruk
author_rank: Contributor
timestamp: December 30, 2021, 04:08:46 pm
content: I noticed something strange with my Siglent SDM3045X bench multimeter. It has an internal relay that is activated even when the meter is in manual range… The funny thing is that it happens at a voltage level that is not even a range limit and since there is no way (I know of) to prevent this relay from triggering will some signals damage the meter in the long run? Some say that relays have a life-span of 100,000 activations. If you measure a signal that crosses the 2V level four times per minute you will reach 100,000 activations in about 400 hours. Quite a lot of time for most users but if you have this meter in a production environment it might be a concern.Demo: Here is a video in which I have a very slow sine wave signal (25mHz) with a voltage that goes from 1.9V to 2.3V (2.1V offset, 0.4Vpp, -90 degrees phase). I have my Siglent SDM3045-X in manual range (6V DC) so there should not be any range changes. However, as you see in this video, when the voltage passes 2.2V on the way up or 2.0V on the way down the internal relay is activated. Normal range changes when the meter is in auto-range are silent (600mV, 6V etc).

author: langwadt
author_rank: Super Contributor
timestamp: December 30, 2021, 04:22:06 pm
content: Relay lifetime varies a lot, there's how long the contact will last switching the rated load, and then there's how many times the relay can mechanically switch. Contact wear is probably not a big issue for a meter and some relays are mechanically rated for 10s of millions of cyclesyou'd have to look up the specific relay.

author: Amaruk
author_rank: Contributor
timestamp: December 30, 2021, 04:26:48 pm
content: Good point. I just picked that 100K number as a wild guess. I am just curious, what is that relay doing? Why does it need to activate in the first place? Is it a firmware bug? I don't recall working with a DMM that has a relay like this in a manual range. Very strange... Anyone with ideas as to what is going on?

author: commongrounder
author_rank: Frequent Contributor
timestamp: December 30, 2021, 04:38:07 pm
content: I’m thinking that is the DC input impedance relay changing back and forth from 10G-ohm to 10M-ohm. 2-volts is at a range change point for this meter, I believe, and 10G-ohm can’t be used above that. There should be a menu setting to force the meter to stay at 10M-ohm all the time, which might stop the relay actuations.

author: Fungus
author_rank: Super Contributor
timestamp: December 30, 2021, 05:03:51 pm
content: Quote from: Amaruk on December 30, 2021, 04:08:46 pmYes, obviously it will.(for some definition of "long")The input jacks will also wear out as you plug in and remove the cables. And the USB socket - those type A sockets are only rated for1500 insertions.Best to enjoy it while you still can!

author: Amaruk
author_rank: Contributor
timestamp: December 30, 2021, 05:05:14 pm
content: Quote from: commongrounder on December 30, 2021, 04:38:07 pmGreat idea! I went into the manual and read about it as I thought you had nailed it! However, the default impedance is 10MOhm for DC measurments but at the manual 600mV range (only) you can set it to 10GOhm. At all other ranges (including the 6V manual) you can't change the input impedance... Thanks for the suggestion though as it made a lot of sense to me!I should also mention that this is not related to slow AC signals. This is really a DC issue. You can't probeany DC voltagesabove 2.2V without activating this relay. Imagine you want to measure a 5V DC rail. The probes will see 0V before you contact the rail so it crosses that magic 2.2V boundary on the way up when you make contact with the probes. If you then lift the probes, the probes will see 0V again and you will cross the magic 2.0V boundary on the way down and the relay will activate... Huh!

author: Fungus
author_rank: Super Contributor
timestamp: December 30, 2021, 05:07:55 pm
content: Quote from: Amaruk on December 30, 2021, 05:05:14 pmThat's not great. Just the click would annoy me, never mind the wear.

author: Amaruk
author_rank: Contributor
timestamp: December 30, 2021, 05:10:20 pm
content: Quote from: Fungus on December 30, 2021, 05:03:51 pmVery true! But curious minds want to know what is going on here... What is that relay doing in the first place?Also, which I did not mention in my original post, every time you cross these magic 2.2/2.0V levels the DMM pauses for a second or so (see video). Sometimes you need a fast response from a DMM, especially in an automated setting, and this relay slows things down when it activates...

author: Amaruk
author_rank: Contributor
timestamp: December 30, 2021, 05:18:15 pm
content: Quote from: Fungus on December 30, 2021, 05:07:55 pmAgreed, the clicking is a bit annoying. Especially since there is no way to avoid it. I should also say that you can speed up the wear quite a bit by presenting a faster signal to it. Imagine a 1Hz 5V square wave signal... I don't want to try that as I don't want to stress my DMM just for fun. But it theory such a fast changing signal activates the relay twice per second (if it can keep up), that means that you reach 100K activations in about 14 hours or 1M activations in 6 days...

author: tautech
author_rank: Super Contributor
timestamp: December 30, 2021, 08:45:28 pm
content: Quote from: Amaruk on December 30, 2021, 05:18:15 pmThat's not how I interpret the manual:DC input impedance selection:While the DC input impedance is selected as 10MΩ, the input impedance of all measurement range is 10MΩ. While the DC input impedance is selected to 10GΩ, the input impedance for 600mV is 10GΩ; for 2V, 20V, 200V and 1000V measurement range is kept at 10MΩ. The default value of DC input impedance is 10MΩ; settings of DC input impedance are stored in the nonvolatile memoryHowever you contradict this with this earlier statement:Quote from: Amaruk on December 30, 2021, 05:05:14 pmCan you check again to clear any doubts please.FYI in Dual measurement mode relays click each second but these are at signal level without any meaningful current therefore the mechanical operational spec of the relay is what's important as it's not switching any current.The 5.5 and 6.5 digit models that use the scanner card also switch every second and I very much doubt Siglent will have used some POS relay with a low lifetime spec.

author: Amaruk
author_rank: Contributor
timestamp: December 30, 2021, 10:28:39 pm
content: Quote from: tautech on December 30, 2021, 08:45:28 pmYes, I think the manual is a bit confusing here and my statements above did perhaps not clear things up. Let me try and explain better.The only way to set the input impedance is to first set the DMM to the manual 600mV range. The default value is 10MOhm which is what my DMM was set to in this range. But you can set it to 10GOhm if you want. For all other ranges there is no option to control the input impedance (menu option goes away) as it is using 10Mohm all the time. Anyway, I set it to the 600mV manual range, set the impedance to 10GOhm, went back to the 6V manual and re-ran my test. Same outcome. Relay clicking at 2.2/2.0V.Thus, anytime you plan on measuring a voltage that is2.2V or above(for which you can't use the 600mV manual range), the impedance will be at 10MOhm and the relay will activate as soon as it detects the voltage. For example, every-time you want to probe a 5V rail the relay will activate twice - first when you make contact with 5V rail and then again when you lift the probes away from the rail. If you do this often you will have a lot of relay action going on. And you can't avoid it... I will probably send an email to tech support and see what they say as there might be an option to turn this off that I have not found yet.

author: AVGresponding
author_rank: Super Contributor
timestamp: December 30, 2021, 11:10:47 pm
content: It might be useful to know exactly which relay is switching? Presumably these units have range relays as well as the impedance switching relay. It would be odd for either of these to be switching while in a manual range...

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2021, 09:48:21 am
content: If the high impendace mode is limited to the 600 mV range, it is a bit odd, that the realy switches at around 2 V and not at around 600 mV.Without a significant load to switch relays have quite a long service time and can operate some 100 million times (typical number, depends on relay type). So it makes some noise and causes some delay, but the relay still would not wear out before the probe tips or the display.One should be able to avoid the clicking noise by choosing a manual range (e.g. 6 V or 10 V depending on the meter).

author: Circlotron
author_rank: Super Contributor
timestamp: December 31, 2021, 11:00:52 am
content: Quote from: Amaruk on December 30, 2021, 04:08:46 pmMaybe 100,000 operations at full load.Some years ago I did a test on a small relay and had a micro driving it at about 3Hz and it checked that the contacts made and broke every cycle while powering a 10K pullup resistor as a load and if one cycle failed it would bring up a warning LED. The test got to 70 million cycles without a single switching failure and was still going fine.

author: Martin Miranda
author_rank: Regular Contributor
timestamp: December 31, 2021, 11:04:05 am
content: Maybe 100,000 operations at full load.same here i concur.bench multimeters are usually used in production lines. so i don't think there would be a problem.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2021, 11:11:56 am
content: Full load lifetime may be less than 100.000, but the no load life time is usually higher. Connecting the 10 M ohms divider is essentially no load and switching between taps is even less.The Panasonic Tx, Zettler AZ850 and ASM850 as typical small relays give 100 million cycles for the mechanical life time. That is about 3 years 1 switching cycle every second. So a few clicks from probing by hand should not be an issue.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 31, 2021, 12:50:45 pm
content: I don't see a good logic for making the impedance changes automatically. If only one impedance is possible for all ranges then just default to that and have and option to manually switch to high impedance when you need it.

author: Martin Miranda
author_rank: Regular Contributor
timestamp: December 31, 2021, 01:40:27 pm
content: ... amaruk was still right...WHY!? hahaha why there is a relay clicking!?

author: Amaruk
author_rank: Contributor
timestamp: December 31, 2021, 04:40:18 pm
content: Thanks for the heads-up in regards to quite long life-span of relays in this type of application. Sounds good to me.I just sent an email to the manufacturer to ask about this relay. Perhaps it is just a silly firmware bug.However, I am quite surprised that this relay issue has not been mentioned before. But perhaps other users like me, use the DMM in auto-range mode most of the time. And every time you probe a 3.3V or 5V signal and hear the relay activate you think it is just changing from the 600mV range to the 6V range as one would expect. But alas, that is not what is going on as shown in my video above. Real range-changes are completely silent.Fingers crossed that this is a silly firmware bug related to a relay for the input impedance or something like that. I will let you know what I find out.Don't get me wrong - I love this DMM. I use it all the time!Happy New Year!

author: tautech
author_rank: Super Contributor
timestamp: December 31, 2021, 04:44:25 pm
content: Some suspicions I have about this:https://www.eevblog.com/forum/testgear/siglent-sdm3045x-enough-is-enough/msg3908639/#msg3908639

author: Amaruk
author_rank: Contributor
timestamp: December 31, 2021, 04:47:26 pm
content: Quote from: AVGresponding on December 30, 2021, 11:10:47 pmYes, I fully agree. But I am not brave like Mr. Jones on here to open things up and take a closer look at the inside. I tend to handle all my instruments in white-glove fashion and opening fully working and calibrated instruments up is outside of my comfort zone...

author: Amaruk
author_rank: Contributor
timestamp: December 31, 2021, 04:48:59 pm
content: Quote from: tautech on December 31, 2021, 04:44:25 pmWOW - what a good find! Thanks for sharing!! Very interesting...

author: Amaruk
author_rank: Contributor
timestamp: December 31, 2021, 04:59:54 pm
content: Update: I just read the recent thread by Eloso that Tautech linked to above. And it mentioned that the relay click goes away at the 60V manual range. I tried it and yes, the click only happens on the 6V manual range! Measuring voltages 2.2V and above on the manual 60V, 600V, or 1000V ranges does not activate the relay. Cool. I never use those high ranges so I had not checked that. Just wanted to add it to this thread for completeness.

author: Amaruk
author_rank: Contributor
timestamp: December 31, 2021, 05:09:47 pm
content: Update 2:I just put my old Keithley 236 to use here to dig a bit deeper as I was curious about DC current. What I found was interesting so I wanted to share it here.It turns out that the DC current range hasthe exact same issueand the limits are 2.2mA (going up) and 2.0mA (going down). Putting the DMM in the 6mA manual range will not stop the relay for activating at these levels! However, if you use the 60mA, 600mA, or 6A manual ranges the relay will not activate. Very interesting!

author: eloso
author_rank: Regular Contributor
timestamp: December 31, 2021, 05:15:00 pm
content: I did a simple measurement that maybe someone with better secondary test equipment can confirm.I used the Siglent SDM3045X to probe my bench power supply, only in the positive lead I inserted a 1Mohm resistor.I used my VC8145 to measure the voltage across that resistor.With the Siglent in 6V Manual mode I started the PSU at 1V then gradually increased it.At the point of the relay clicking the voltage read by the Siglent was 2.2V and just prior to the click the mV reading on the VC8145 was 400mV. Bearing in mind there are some difficult high impedances interacting here including that of the VC8145 in paralell with the 1Mohm, this equates to a current just prior to relay clicking of 4x10-7amps. Since the Siglent was reading 2.19volts at the time then with 4x10-7 flowing through it the resistance of the Siglent is 5.5M ohms.Just after the relay clicked the voltage of the Siglent shot up to 2.89V but the voltage on the 1M resistor went down to 200mV. Current now therefore is 2x10-7 and input resistance of the Siglent, now measuring 2.89V, would appear to be 14.45 M ohm.Whatver that relay is doing, and whatever the real input DC resistances are (I accept these measurements areverydodgy) there seems little doubt that the DC input resistance jumps around somewhat on the manual 6V range. Personally I like a meter on a manual range to do no switching whilst the value measured is within its range. I can't actually provide a circumstance where its likely to mess up a reading for me, but if there are times when DUT loading by the instrument may be important to some, and I would have thought it unhelpful for a manual range to flip around like this.Incidentally, on going in the reverse order from higher volts downwards, the relay doesn't click until 2.00v, as measured on the Siglent, is reached so there is obviously some hysteresis in there - of course if there wasn't , the meter would go wild if measuring a voltage of exactly 2.2VIncidentally on the downward slope the Siglent dropped instantly from 2.0 to 1.87 volts. Again showing that since the 1Mohm loading resistor clearly didn't change, then something inside the Siglent must have done.Here's a challenge - does any other Bench Meter on the planet do this when set to a manual range (excepting when a range is exceeded of course, which isn't the case here) ?Eloso

author: Amaruk
author_rank: Contributor
timestamp: December 31, 2021, 05:55:19 pm
content: Quote from: eloso on December 31, 2021, 05:15:00 pmEloso, I did a measurement along the lines of your experiment above. In my case I used my Keithley 236 SMU as a voltage source as it can measure very low currents while putting out a voltage. I also set it to 4-wire sense and joined force and sense leads at the Siglent SDM3045X terminals (see image below). This is what I found out:For voltages just below 2.2V, and before the relay activates, the current drawn from the Keithley 236 power supply is about 100nA. For voltages just above 2.2V, and after the relay has activated, the current drawn from the Keithley power supply is about 250nA.Below is an image of my Siglent SDM3045X in action when I presented it with a 2.3V from the Keithley power supply and sensing at the input terminals. Impressive!And to answer your question: My Keysight 34461A DMM does not have this type of relay in the manual voltage range.Finally, I should just also add that the cabling from my Keithley include some in-expensive coax cables from the power supply to a breakout box with 4mm banana connectors so the isolation is not as good as it could be if I had used proper triax cables.

author: eloso
author_rank: Regular Contributor
timestamp: December 31, 2021, 06:17:42 pm
content: QuoteWell that should be a lot more accurate than anything I could do - thanks. Doing the sums seems to indicate that at less than 2.2V the DC input resistance of the Siglent is 9.5Mohm and above 2.2V it is 22Mohm.Surely, surely thats a bug ? I know we all have to be cogniscent at all times of the impact of our instruments on the DUT but it is particlarly unhelpful for this impact to change across a definite threshold. What if you were to ever measure voltage across a very high impedance sensor, tracking it against time ? Oh, that's a nice jump in my figures, just where it crosses 2.2VRegardsEloso

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2021, 06:30:12 pm
content: This really looks like something is totally messed up. A bit like getting wrong firmware (e.g. from the 3055) so that at 2 V another 10 M resistors / divider is added in parallel to 10 M already there below 2 V.Is the test with the 600 mV range impedance set to 10 M or high Z ? There is a chance this may make a difference.Given the relatively small price difference between the SDM3045 and SDM3055, the 3045 does not look that atractive anyway.

author: bdunham7
author_rank: Super Contributor
timestamp: December 31, 2021, 07:14:25 pm
content: Quote from: Amaruk on December 31, 2021, 05:55:19 pmThat's not kosher. It also appears not to meet the published specification for DC input impedance which is stated to be 10M +/- 2%. And is a non-trivial problem--I can think of multiple situations where this would make the meter completely unusable for certain tasks.

author: Amaruk
author_rank: Contributor
timestamp: December 31, 2021, 08:24:35 pm
content: Quote from: bdunham7 on December 31, 2021, 07:14:25 pmAnd you are absolutely right. My data was actually wrong. This time I hooked up my 34461A in series as a current meter and used my second Keithley 236 power supply as a voltage supply using its 2-wire sensing mode. And now it all checks out just fine-both before the relay activates (1.9V) and after (2.3V). As you can see in the statistics in the images below, the input impedance is steady at around 10Mohm both at 1.9V and at 2.3V. Thus, the relay does not seem to affect input impedance. That is good news!! Sorry for the confusion.I should also add that my Keithley's do not have valid calibrations but both my Siglent DMM and my Keysight DMM do. That is another reason this second set of data is better - the readings come from calibrated instruments. But I am still not sure what happened in my first set of measurements. A good example of why it is good to re-check measurements that do not make sense.

author: eloso
author_rank: Regular Contributor
timestamp: January 01, 2022, 11:01:51 am
content: QuoteThanks Amaruk. I too have revisited the measurement.Still using the voltage across a 1Mohm resistor to calculate current flow into the Siglent when measuring 1.8V and 3.6V. i.e. either side of the relay "click".What is different this time is that I am using a different millivoltmeter. I measured the input resistance of this millivoltmeter with the Siglent to be a suprisingly low 800Kohm which obviously loads the 1Mohm current sensing resistor quite a bit.Because I have learned never to trust my mathI created an LTSpice model of all of this and the results - both before the relay click and after the relay click, the LTSpice numbers all add up to 10Mohm input of the Siglent, matching the mV readings taken on the meter.So no impedance change on the Siglent when the relay clicks.I don't know what was going on with the VC8145 but on mV it was giving some very strange results. I did use a short twisted measuring probe set but maybe it was still some noise getting in.If my latest measurement was the only one, I would probably accept it with nagging doubts, but it ties in perfectly with results obtained by Amaruk which seem beyond doubt anyway.So definitely no funny impedance change problem.When that relay clicks, my VC8145 does give some totally different indication of impedance so whether it is a bug or not, it is definitely altering something. Based on our latest measurements I am at a loss to understand exactly what.The fact remains that on a manual range or auto range, if I want to measure a voltage between 2.2V and 6 volts , I have to put up with a relay click, and a delay every time I apply the probe to the DUT. Doesn't make me proud to own, nor happy to use, this meter.RegardsEloso

author: Amaruk
author_rank: Contributor
timestamp: January 01, 2022, 03:22:58 pm
content: Quote from: eloso on January 01, 2022, 11:01:51 amExactly!You can even make the case that there is no manual 6V or 6mA DC ranges on this multimeter...Sure, the spec sheet states that those ranges exist (see image below) and the meter allows you to select those ranges. But in reality they do not exist.The point of a manual range is to obtain extra measurement speed (see this answer from Keysight:https://edadocs.software.keysight.com/kkbopen/what-is-the-effect-of-choosing-auto-or-manual-range-580285257.html). Thus, when a manual range is selected the meter should measure and not go between ranges. That defeats the purpose...Hopefully it is just a firmware issue and it will get resolved quickly.For your reference, I experienced a "severe" firmware bug with a brand new Keysight power supply not that long ago. It took their support team less then 48 hours to acknowledge and release a new firmware that had a fix in it!! (Link to thread:https://www.eevblog.com/forum/testgear/keysight-edu36311a-current-reading-bug/msg3840200/#msg3840200)To have a firmware bug in a product is no big deal as it is very difficult to check for everything when you code. It all comes down to customer service AFTER the purchase. How quickly is something fixed? I hope Siglent can show us that their goal is to match and maybe even go beyond other companies in this industry.

author: eloso
author_rank: Regular Contributor
timestamp: January 01, 2022, 05:12:30 pm
content: Sensible - The 3045X's other little foibles I don't mind quite as much as this one so if they are able to fix it then fine.What is the best way to make a bug report that hopefully gets looked at? My experience with 99% of companies out there is the real guys that know about things are protected from customers behind a wall of people who don't. Unpalatable but for understandable reasons of course.Incidentally, do we know if other machines in the same range exhibit this kind of behaviour ? Awaiting the outcome of tautech's sporting activity with the 3055 firmware with bated breath !Eloso

author: Kleinstein
author_rank: Super Contributor
timestamp: January 01, 2022, 05:23:58 pm
content: The SDM3055 has a 2 V range as the limit for high Z mode and may thus switch ranges at 2 V in auto mode. I don't have very much hope the 3055 firmwire would fix the problem - chances are more that accidently some of 3055 firmware made is to the 3045 in the first place and triggers the switching.They seem to use the same base for the FW, but have still different hardware.The question is more if this problem was allredy there with much older FW.

author: tautech
author_rank: Super Contributor
timestamp: January 01, 2022, 07:17:36 pm
content: Quote from: eloso on January 01, 2022, 05:12:30 pmI'll report it when I have further info and write something up. I too strongly suspect a bug.Sporting activity status:Still doing due diligence on internal difference between models.Need to further investigate the China only SDM3055X-E datasheet for ranges and general specs as to how well they match SDM3045X.Have just 1x SDM3045X in stock that may have to become the workshop DMM......Hoping Notepad will suffice for FW code edits.UnknownsWestern SDM3055 use a Caddock divider whereas SDM3045X use passives but China SDM3055X-E not known.....highly suspect same as SDM3045X.

author: Amaruk
author_rank: Contributor
timestamp: January 10, 2022, 02:16:29 pm
content: I have no updates to report yet unfortunately. But I just realized that with instruments that are this good it is easy to forget that it is almost impossible to sell instruments for these consumer friendly prices and offer the same level of customer support as you get from brands that offer products that cost much more (2-3X). Somethings going to give. It is that simple. Having said that, I hope we get a fix for this but I think that is not very likely... I hope I am wrong.

author: audiotubes
author_rank: Regular Contributor
timestamp: December 18, 2023, 08:54:47 pm
content: Quote from: tautech on January 01, 2022, 07:17:36 pmSorry, didn't see an appropriate place for this, too many Siglent threads on the forumAnyway, can we please get some firmware that stops the horrible green flashing on the power button? I have the SDM3045X, the SDS2104X Plus scope, and the SDG 1032X signal generator, and only the DMM has this "helpful" annoyance.