Author Topic: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?  (Read 5064 times)

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Offline bdunham7

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #50 on: September 03, 2020, 01:48:35 pm »
I understand benchtop DMM's are fragile. Cat. II 300V is 2,500V/12R impulse. Will a BBQ lighter kill it?

They aren't that fragile, at least properly made ones.  Your CAT II/300 test requires clamping 100+ amps, the BBQ lighter is just a bit of ESD.  The two are not similar events in any way.

Quote
"Keithley products are designed for use with electrical signals that are measurement, control, and data I/O connections, with low transient overvoltages, and must not be directly connected to mains voltage or to voltage sources with high transient overvoltages. Measurement Category II (as referenced in IEC 60664) connections require protection for high transient overvoltages often associated with local AC mains connections. Certain Keithley measuring instruments may be connected to mains. These instruments will be marked as category II or higher."

That's a lot of words to simply say you need a CAT II rating to measure non-isolated mains-derived sources.  Keithley DMMs will all meet at least CAT II AFAIK, but there are scanner cards and so on that do not.
« Last Edit: September 03, 2020, 07:05:48 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #51 on: September 03, 2020, 06:13:06 pm »

Obtaining the equipment to verify its performance will cost more than replacing it.

Makes perfect sense. I'd totally do that. Because I would learn something! And, GAS.

Offline bson

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #52 on: September 03, 2020, 09:57:44 pm »
I wouldn't pay extra for any cat ratings whatsoever, for a bench instrument whose purpose is things like measuring the linearity of a high resolution DAC or ADC.

A dog rating however, that I'd pay for! :-DD
 

Offline floobydust

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #53 on: September 03, 2020, 11:35:47 pm »
They aren't that fragile, at least properly made ones.  Your CAT II/300 test requires clamping 100+ amps, the BBQ lighter is just a bit of ESD.  The two are not similar events in any way.

It's that I want hard proof this very expensive DMM is impulse-tested for differential and common-mode transients to some value say at least 1,500V. Not someone's "opinion", not "designed to" (engineers with good intentions) or "meets" (who says?) or "CE" sticker or "spec'd to" etc.

An ongoing problem with multimeters is fake or misleading 61010 approvals.
"Cat. II" on the front panel means nothing - was it evaluated to 61010-1, 61010-2-033 ? And even then, a certification agency can make mistakes. Remember the Keysight 600V fiasco it was just terrible.

For a bench DMM it's not safety approvals so much as a guarantee it can take a BBQ lighter so I'm not looking at a massive repair cost due to normal use.
i.e. DMM6500 has Intertek approval ETL marking for 61010 but Keithley 2000, 2001, 2002 has nothing- which is probably why they frequently get damaged.
 

Online joeqsmith

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #54 on: September 04, 2020, 03:11:14 am »
They aren't that fragile, at least properly made ones.  Your CAT II/300 test requires clamping 100+ amps, the BBQ lighter is just a bit of ESD.  The two are not similar events in any way.

It's that I want hard proof this very expensive DMM is impulse-tested for differential and common-mode transients to some value say at least 1,500V. Not someone's "opinion", not "designed to" (engineers with good intentions) or "meets" (who says?) or "CE" sticker or "spec'd to" etc.

An ongoing problem with multimeters is fake or misleading 61010 approvals.
"Cat. II" on the front panel means nothing - was it evaluated to 61010-1, 61010-2-033 ? And even then, a certification agency can make mistakes. Remember the Keysight 600V fiasco it was just terrible.

For a bench DMM it's not safety approvals so much as a guarantee it can take a BBQ lighter so I'm not looking at a massive repair cost due to normal use.
i.e. DMM6500 has Intertek approval ETL marking for 61010 but Keithley 2000, 2001, 2002 has nothing- which is probably why they frequently get damaged.

61010-1 is "for electrical equipment for measurement, control, and laboratory use –"

-2-033 is for "Particular requirements for hand-held multimeters and other hand-held meters, for domestic and professional use, capable of measuring mains voltage"

"It's that I want hard proof ..."
I doubt you would ever find it.    BBQ starters will very a LOT.   I have characterized the one I use, presented the data for it and have shown how it compares with IEC waveform.   I would expect completely different results if I went to the hardware store and bought a new one.   BBQ starters are going to output several KV, far beyond the 1.5KV you mention.  Their rise times can be well under 1ns and pulse width is going to be <100ns.    I think the one I used can put out close to 5A peak, far from what the standards call for.   

If say you pick some magic number (1.5K) as your criteria.  Let's not focus on how you came up with it but assume you know this is what you require.   What does the waveform look like?  What's the voltage rise time into and open circuit?  What's it's decay look like?  What's the current look like into a short? ....     These are the questions I was pondering when trying to come up with a way to benchmark handheld meter.   Being unable to locate any sort of existing standards, my proposal was to somewhat follow the surge open circuit voltage waveform, greatly limit the energy available and not superimpose it onto the line.   

If I applied this type of waveform at 8KV peak to your favorite bench meter and it survived, it may give you a great feeling of comfort but it will not mean you can connect the same meter to 2KVDC or a MOT.    Hard proof requires hard requirements.  From there it's possible you could have these tests performed at some outside lab.  The cost  may far exceed the cost of the meter but you would have your hard proof.   

A beginner may suggest if a meter survives +/-1.5KV DC, it would handle every transient up to that.  At one time I made an attempt to provide some insight on how to harden a cheap UNI-T UT-61E and show how a fast edge can be a problem.  Sadly, most of the people who watched the video didn't seem to care about the approach to solving this sort of problem.   The wiki experts just need to know how to mod their cheap meters. 

I just finished the chapter on colleges.   An excellent book.
https://www.amazon.com/Death-Expertise-Campaign-Established-Knowledge/dp/0190469412
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline Doctorandus_P

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #55 on: September 04, 2020, 03:40:30 am »
I've thought long and hard about buying a benctop DMM, and decided not to.
The deciding factor was that the boxes are too big. They are some 25 cm deep.
If they had the same depth as a digital scope I very likely would have bought one.

I had a look at the Siglent ...3055. and though about sawing the box in half, maybe relocate the transformer.
The box is almost empty, with just the front panel PCB and a PCB at the bottom which is about half the size of the box. But Also decided against that. I'm not going to spend EUR500 on a DMM to cut he box in two.

Maybe, some day I'll design my own custom benchtop DMM.
 

Online Fungus

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #56 on: September 04, 2020, 03:53:32 am »
I've thought long and hard about buying a benctop DMM, and decided not to.
The deciding factor was that the boxes are too big. They are some 25 cm deep.
If they had the same depth as a digital scope I very likely would have bought one.

I had a look at the Siglent ...3055. and though about sawing the box in half, maybe relocate the transformer.
The box is almost empty, with just the front panel PCB and a PCB at the bottom which is about half the size of the box. But Also decided against that. I'm not going to spend EUR500 on a DMM to cut he box in two.

https://www.aliexpress.com/item/4000343031770.html

 

Offline bdunham7

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #57 on: September 04, 2020, 03:55:56 am »

"It's that I want hard proof ..."
I doubt you would ever find it.    BBQ starters will very a LOT.   I have characterized the one I use, presented the data for it and have shown how it compares with IEC waveform.   I would expect completely different results if I went to the hardware store and bought a new one.   BBQ starters are going to output several KV, far beyond the 1.5KV you mention.  Their rise times can be well under 1ns and pulse width is going to be <100ns.    I think the one I used can put out close to 5A peak, far from what the standards call for.   

A beginner may suggest if a meter survives +/-1.5KV DC, it would handle every transient up to that.  At one time I made an attempt to provide some insight on how to harden a cheap UNI-T UT-61E and show how a fast edge can be a problem.  Sadly, most of the people who watched the video didn't seem to care about the approach to solving this sort of problem.   The wiki experts just need to know how to mod their cheap meters. 

Meters (well made and well specified) typically have a dV/dt limitation of a few million V*Hz, so your BBQ lighter will greatly exceed that, of course. However, I'd consider it poor design if that limitation was anything other than a thermal limitation on whatever absorbs that high frequency energy. 

I'll drag out my old fried 8842A again, here's a picture of the damaged unit with the incinerated parts removed.  Do you see anything in the picture that might help with that fast edge?  Do you think there's room in a hand-held DMM for this type of protection?  Is there something else that can do the same thing in less space?

[attachimg=1]

« Last Edit: September 04, 2020, 05:20:13 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Offline bdunham7

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #58 on: September 04, 2020, 03:59:40 am »
https://www.aliexpress.com/item/4000343031770.html

Did you look at the comparison photos where they are demonstrating it with the probes into a power strip and then connected to a 9V battery.  Perfect for the target market!
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #59 on: September 04, 2020, 04:13:59 am »
I've thought long and hard about buying a benctop DMM, and decided not to.
The deciding factor was that the boxes are too big. They are some 25 cm deep.
If they had the same depth as a digital scope I very likely would have bought one.

I had a look at the Siglent ...3055. and though about sawing the box in half, maybe relocate the transformer.
The box is almost empty, with just the front panel PCB and a PCB at the bottom which is about half the size of the box. But Also decided against that. I'm not going to spend EUR500 on a DMM to cut he box in two.

Maybe, some day I'll design my own custom benchtop DMM.
The SDM3065X is deeper again !
They can't be too compact without compromising safety clearances or the size for TH components used to withstand the voltages they are rated for.
As for being almost empty cases, I can assure you when the 16ch scanner card is installed there isn't much free space left.
Instruments like SDM's are made to be stacked so while you desire something smaller this also compromises stability of anything stacked on top of it.
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Online Fungus

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #60 on: September 04, 2020, 04:35:06 am »
https://www.aliexpress.com/item/4000343031770.html

Did you look at the comparison photos where they are demonstrating it with the probes into a power strip and then connected to a 9V battery.  Perfect for the target market!

I don't own one myself but all the threads on it have concluded that it's quite a good meter if you can get past the silly photos and alarm clock part.

Plus you can take it with you on a picnic and listen to music.
 

Offline floobydust

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #61 on: September 04, 2020, 05:20:58 am »
Most models have a 500Vpk CM voltage rating and are using slow gas-tubes for protection of that.
If you have a 750VDC source that is earth-grounded, what happens when you first connect one DMM probe? You are trying to fast lift the entire in-guard section (capacitance) to 750VDC.
Compare that with nicely connecting both leads to the (off) HV power supply under test, where the DMM (-) gets 0V potential and upon PSU power-on there is no overload beyond the auto-ranging having gone to the mV range and having to clank relays up-ranging.
A common-mode choke at the input does help against a fast transient - but they don't use those anymore. Very rare to see one, it's extra manual labour. Nowadays, it's tiny smt inductors with low breakdown voltage, 10 miles after the input jacks, pic related.
My position is these multimeters need to have been well tested, that they really withstand ESD and fast transients at their rated voltage because repairs are super expensive.
 

Offline bdunham7

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #62 on: September 04, 2020, 02:37:43 pm »
Quote from: floobydust
"Cat. II" on the front panel means nothing - was it evaluated to 61010-1, 61010-2-033 ? And even then, a certification agency can make mistakes. Remember the Keysight 600V fiasco it was just terrible.

"Nothing" is a bit harsh.  Sure, certifications aren't infallible, but what else can you do unless you do your own test?  The Keysight deal was indeed a shitshow and I'm not sure what I would do if I had one of the implicated devices.  A bit more transparency would be reassuring.

Quote
Most models have a 500Vpk CM voltage rating and are using slow gas-tubes for protection of that.

Which ones?  I'm not seeing that on mine except on a very old 8505A, which is a reference meter not really intended for bench service use--it has the 500V CM limitation.  Everything else I have is at least 1000VDC/700VAC to ground, and the 8846A has to be at least 1500V common and differential because it allows 1000Vrms inputs--and doesn't go into overload even slightly over that.  I haven't disassembled it to see what it looks like.

Quote
My position is these multimeters need to have been well tested, that they really withstand ESD and fast transients at their rated voltage because repairs are super expensive.

I'd prefer repairability, but that ship sailed long ago.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #63 on: September 04, 2020, 03:27:34 pm »
I'd prefer repairability, but that ship sailed long ago.
Hardly any surprise why in this day and age.
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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #64 on: September 04, 2020, 03:35:02 pm »
Bench meter voltage on the front:

Keysight 34461A/65A/70A: CAT II 300V, 500Vpk to gnd
Keithley 6500, 7510: CAT II 300V, 500Vpk to gnd
Fluke 8846: CAT II 600V, CAT I 1000V
Rigol 3068: CAT II 300V, CAT I 1000V
Siglent 3045X: CAT II 300V, CAT I 1000V, 500Vpk to gnd
East Tester ET3240: CAT II 300V, 500Vpk to gnd

The "to gnd" is from the black terminal.

The CAT II 300 is nearly universal, only Fluke will handle more.

 

Offline bdunham7

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #65 on: September 04, 2020, 04:14:52 pm »
The CAT II 300 is nearly universal, only Fluke will handle more.

OK, didn't realize that those meters all had specific 500V-pk to ground ratings as I don't have any of them.  But CAT II/300 doesn't imply that--my modest BK Precision 2831E (by TongHui) is CAT II/300 and yet 1kV max to ground.  Of course it is also CAT I/1000V, so I suppose those meters that omit the CAT I/1000 rating really are fragile in that you can't measure 500VDC or 300VAC without being sure of how it might be ground referenced--so even floating or presumed floating above those limits would be off-limits.  That does seem weak tea.

And then there's the Siglent.  Not sure how you can be CAT I/1000 and still have the 500V-pk to ground limitation.  Perhaps someone with knowledge of the CAT standards could clarify?

« Last Edit: September 04, 2020, 04:18:47 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Online tautech

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #66 on: September 04, 2020, 04:25:12 pm »
And then there's the Siglent.  Not sure how you can be CAT I/1000 and still have the 500V-pk to ground limitation.  Perhaps someone with knowledge of the CAT standards could clarify?
Specifically this is how it's rated:
Measuring Method and other Characteristics
DC Voltage
CMRR 120 dB ( For the 1 KΩ unbalanced resistance in LO lead, max ± 500 VDC )
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Offline bdunham7

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #67 on: September 04, 2020, 05:06:06 pm »
Specifically this is how it's rated:
Measuring Method and other Characteristics
DC Voltage
CMRR 120 dB ( For the 1 KΩ unbalanced resistance in LO lead, max ± 500 VDC )

I'm not clear on exactly what that means, but does that mean that there is some other non-protection related reason that the 500V-pk limitation is posted?  I suppose you could design a meter that passes CAT x, but then you otherwise greatly limit its maximums for other reasons.  What confuses me is that although I can't find a specific cite, the CAT voltages are V-rms, so 1000VAC => 1.4kV-pk.  Unless the  IEC procedures allow you to specify the the black lead must be ground referenced--no floating or split supplies-- the math doesn't work.
« Last Edit: September 04, 2020, 05:09:58 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Online SilverSolder

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #68 on: September 04, 2020, 05:15:40 pm »

The 3 biggest disdvantages of a benchtop DMM are Size, Size, and Size.

But, that is also their advantage...  Large displays that can be read from across the room in some cases, plenty of knobs and buttons brought out on the front panel for convenience, many options for connectors front and rear, including remote control...   Room inside for better / more stable electronics with more features.  They are heavy enough not to slide around on the table in use...



 

Online tautech

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #69 on: September 04, 2020, 05:21:08 pm »
Specifically this is how it's rated:
Measuring Method and other Characteristics
DC Voltage
CMRR 120 dB ( For the 1 KΩ unbalanced resistance in LO lead, max ± 500 VDC )

I'm not clear on exactly what that means, but does that mean that there is some other non-protection related reason that the 500V-pk limitation is posted?  I suppose you could design a meter that passes CAT x, but then you otherwise greatly limit its maximums for other reasons.  What confuses me is that although I can't find a specific cite, the CAT voltages are V-rms, so 1000VAC => 1.4kV-pk.  Unless the  IEC procedures allow you to specify the the black lead must be ground referenced--no floating or split supplies, the math doesn't work.
I take it to mean the LO (negative) lead has a max +500VDC rating WRT mains ground.
Which implies for HV or floating measurements with a 500+VDC offset relative to mains ground connect the positive (Hi) lead first.
CAT ratings will be in addition to this.

I suspect it's due to internal clearances primarily as a question I sent to Siglent's tech support recently about series ganging their PSU's had the same 500V limitation in that you can series several units together as long as the 500V max was observed.....which is also specified in their PSU datasheets.

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Online HKJ

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #70 on: September 04, 2020, 05:25:19 pm »
The 3 biggest disdvantages of a benchtop DMM are Size, Size, and Size.

That depends, if you also have power supplies, arbs, electronic loads, etc. they all stack nicely.

I have 15 devices stacked in a couple of stacks, I would never have had space for them if they had each had their own shape.
 

Offline Electro Fan

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #71 on: September 04, 2020, 07:13:28 pm »
It's only a matter of time until either the budget or the space is maxed out.
 

Offline Electro Fan

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #72 on: September 04, 2020, 07:24:06 pm »

The 3 biggest disdvantages of a benchtop DMM are Size, Size, and Size.

But, that is also their advantage...  Large displays that can be read from across the room in some cases, plenty of knobs and buttons brought out on the front panel for convenience, many options for connectors front and rear, including remote control...   Room inside for better / more stable electronics with more features.  They are heavy enough not to slide around on the table in use...

+1 on advantages...

After years with only handheld DMMs the bench DMM has become the go to DMM.  It takes up more space than the handhelds but it wasn't space I was using, and I never have to look to see where it might be; and it holds still (hands-free) - just grab the leads and measure (although you still have to change out leads based on the connectors needed). 

And without a doubt a large and multifaceted (graphical) display is way better than the relatively small and dim handheld DMM.  Both have their uses - probably should have at least one of each, but unless you have no handheld DMMs it's worth seriously considering a bench DMM.  If you already have a handheld DMM, I don't see any disadvantages in a bench DMM.
 

Online joeqsmith

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #73 on: September 04, 2020, 07:48:31 pm »

"It's that I want hard proof ..."
....
Meters (well made and well specified) typically have a dV/dt limitation of a few million V*Hz, so your BBQ lighter will greatly exceed that, of course. However, I'd consider it poor design if that limitation was anything other than a thermal limitation on whatever absorbs that high frequency energy. 

I'll drag out my old fried 8842A again, here's a picture of the damaged unit with the incinerated parts removed. 

Quote
Do you see anything in the picture that might help with that fast edge? 
It's hard to say when it comes to something like ESD.  The edges are very fast and we are dealing with RF.   I addressed the weakness of the UNI-T UT181A by changing the layout.  It should be obvious that even wire will have some inductance and will effect the signal at these speeds.  Surely the large inductor on the front will play into it.   


Quote
Do you think there's room in a hand-held DMM for this type of protection?
Based on the number of hand-held meters I exposed to not only the BBQ lighter but also my home made gun which produces waveforms more in-line with the IEC standards,  I would say there are certainly ways to design the front ends to survive it. 

Quote
Is there something else that can do the same thing in less space?
Again, I went into detail the problems with the UT61E and one possible way to solve such a problem.


Most models have a 500Vpk CM voltage rating and are using slow gas-tubes for protection of that.
If you have a 750VDC source that is earth-grounded, what happens when you first connect one DMM probe? You are trying to fast lift the entire in-guard section (capacitance) to 750VDC.
Compare that with nicely connecting both leads to the (off) HV power supply under test, where the DMM (-) gets 0V potential and upon PSU power-on there is no overload beyond the auto-ranging having gone to the mV range and having to clank relays up-ranging.
A common-mode choke at the input does help against a fast transient - but they don't use those anymore. Very rare to see one, it's extra manual labour. Nowadays, it's tiny smt inductors with low breakdown voltage, 10 miles after the input jacks, pic related.
My position is these multimeters need to have been well tested, that they really withstand ESD and fast transients at their rated voltage because repairs are super expensive.

We have seen companies like Gossen successfully utilize GDTs in their meter.   I wouldn't suggest that a product that uses them would be any less robust.  The low capacitance and leakage would seem to make them a good fit for higher end products. 

I think my old HP 34401A uses a combination of GDTs, R,C and MOV for the clamp.  The nice thing about this technique over using the GDT alone is that when working with DC, once the transient dissipates and the voltage falls below the MOVs trip point, the circuit resets.   With the tests I run it doesn't really matter as the transients are never superimposed on another waveform.  They always start at 0V and and at  0V, allowing a GDT to clear.     


***
For got to say that your point about the power supply being active and clipping onto it is certainly a valid case and I wouldn't be surprised to hear of meters being damaged this way.   I look at my bench meters pretty much how I look at any other sensitive equipment.  I have learned to treat them with caution.    Now these new handhelds I have started using, not so much.  These things have seen some fair abuse in my hands.
« Last Edit: September 04, 2020, 07:57:47 pm by joeqsmith »
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline JimKnopf

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Re: What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
« Reply #74 on: September 16, 2020, 08:53:32 am »
In spring I bought a UT 8803e. I was very unhappy with it because it measures very, very inaccurately. In diode mode you can only measure down to 16 Ohm. Below that Err (short circuit) is displayed.

Today my Keithley DMM6500 arrived.
The first tests are very positive. Diode Mode I can measure very accurately down to below 1 Ohm. Continuity test is very fast. The only drawback is that capacitance is only up to 100µF or 20% above (all measurements 20% above max range), i.e. 120µF.
 


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