Will this 2V signal damage my bench multimeter over time?

[Amaruk]

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).

[langwadt]

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 cycles

you'd have to look up the specific relay.

[Amaruk]

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?

[commongrounder]

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.

[Fungus]

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?

Yes, 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 for 1500 insertions.

Best to enjoy it while you still can!

[Amaruk]

I’m thinking that is the DC input impedance relay changing back and forth from 10G-ohm to 10M-ohm.

Great 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 probe any DC voltages above 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!

[Fungus]

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!

That's not great. Just the click would annoy me, never mind the wear.

[Amaruk]

Yes, obviously it will.

Best to enjoy it while you still can!

Very 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...

[Amaruk]

That's not great. Just the click would annoy me, never mind the wear.

Agreed, 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...

[tautech]

That's not great. Just the click would annoy me, never mind the wear.

Agreed, the clicking is a bit annoying. Especially since there is no way to avoid it.

That'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 memory

However you contradict this with this earlier statement:

You can't probe any DC voltages above 2.2V without activating this relay.

Can 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.

[Amaruk]

Can you check again to clear any doubts please.

Yes, 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 is 2.2V 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.

[AVGresponding]

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...   

[Kleinstein]

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).

[Circlotron]

Some say that relays have a life-span of 100,000 activations.

Maybe 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.

[Martin Miranda]

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.

[Kleinstein]

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.

[BeBuLamar]

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.

[Martin Miranda]

... amaruk was still right...

WHY!? hahaha why there is a relay clicking!? 

[Amaruk]

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!

[tautech]

Some suspicions I have about this:
https://www.eevblog.com/forum/testgear/siglent-sdm3045x-enough-is-enough/msg3908639/#msg3908639

[Amaruk]

It might be useful to know exactly which relay is switching?

Yes, 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...

[Amaruk]

Some suspicions I have about this:
https://www.eevblog.com/forum/testgear/siglent-sdm3045x-enough-is-enough/msg3908639/#msg3908639

WOW - what a good find! Thanks for sharing!! Very interesting...

[Amaruk]

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.

[Amaruk]

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 has the exact same issue and 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!

[eloso]

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 are very dodgy) 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 2.2V

Incidentally 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