EEVblog #929 - Designing A Better Multimeter

[EEVblog]

Dave looks at the current measurement front end of a typical multimeter and figures out how to redesign it for lower burden voltage.
It's not quite as easy as you might think, there are comprises and traps for young players.
And something you probably didn't know about HRC fuses.

µCurrent article: http://www.eevblog.com/files/uCurrentArticle.pdf

[MartinX]

Am I the only one that find the concept of burden voltage to be annoying? Why specify it in this way? Just tell me the shunt resistance, having it presented in mV/µA is much less intuitive to me and it does not make calculating the voltage drop or evaluating the effect on the circuit being measured any easier.

[NivagSwerdna]

What about using the fuse as the current detector?  (and then optically coupling later in the sensor circuit to maintain isolation?)

I imagine fuses do not have constant resistance with temperature but maybe you could compensate for that?

[Brumby]

What about using the fuse as the current detector?

I would SERIOUSLY doubt the accuracy of such an arrangement.  Precision resistance in a fuse is not required for it to provide it's prime function - especially when we are talking about a disposable, clip in field replaceable component.  Not to mention contact resistance between the fuse and holders at each end.

[nctnico]

Another thing that can bite you is the thermo electric effect if you are not careful with the PCB layout and one end of the shunt resistor has a different temperature than the other end. When dealing with signals in the uV range this is a serious thing to consider.

[EEVblog]

I imagine fuses do not have constant resistance with temperature but maybe you could compensate for that?

Massive variation, and yes it does change a huge amount with temperature. Simply not possible for a measurement instrument.

[EEVblog]

Another thing that can bite you is the thermo electric effect if you are not careful with the PCB layout and one end of the shunt resistor has a different temperature than the other end. When dealing with signals in the uV range this is a serious thing to consider.

Meters are essentially a sealed unit and the internals are pretty close to thermal equilibrium, so not an issue in practice.
But yes, the lower you go, the more this is an issue. A big issues in the likes of the 6.5/7.5 digit high end meters.

[nctnico]

Don't say that too quickly. If you run a decent amount of current through a shunt it will get warm and if the legs cool down unequal then you'll get phantom readings. I had some hands on experience with that recently and the ADC used was only 16 bit (2.5uV per LSB).

[digsys]

A possibility -
In past projects where I had similar issues, I made the "front-ends" direct connected and easily replaceable. I then put all the heavy protection after.
"Data' was opto-coupled across. It may be a bit excessive here, but you can achieve absolute accuracy. Replacement input stages could be either plug-in
or simple solder points. Just an idea to throw in the mix.

[Dr. Frank]

This is a very good explanatory video, about the difficulties of current measurements.

As you suggested, I'd like to add a better, more simple circuit, which may be found in mostly any higher grade DMM, in a similar way.
I only omitted the protection circuitry.

You don't need two MUX, and only a FET switch to change the gain.



Current measurements mostly suffer from offset voltages, as the crucial offset across the shunt resistor can't be removed frequently during the measurement, as it's not possible to switch off the current in between. That's a bit different compared to voltage measurements, where the DUT voltage can be switched off from the input amplifier, and an AUTO ZERO phase can be included.

Frank

[G7PSK]

Perhaps we need to go back 70 years and have separate meters for current and voltage at those very low levels, that way you need not worry about some of the compromises that have to be made. A case of the right tool for the right job.

[bktemp]

As you suggested, I'd like to add a better, more simple circuit, which may be found in mostly any higher grade DMM, in a similar way.
I only omitted the protection circuitry.

You don't need two MUX, and only a FET switch to change the gain.

Why is the FET switch better than using a MUX? I see more problems. For example the ON resistance of the FET affects the gain. Therefore you need to make the divider resistors rather high value to reduce the effect of the FET. Using a multiplexer, the ON resistance does not care, because it only switches the feedback voltage into the opamp either directly from the output (1x gain) or divided (10x gain).


The higher resistance of the multimeter fuses is probably because they are super fast acting (FF), therefore they need to dissipate more power to melt in a shorter time.

[rfeecs]

A better multimeter is a sourcemeter, like your Keithley 2400.
Set it to force zero volts and read back current.
No burden voltage.  The source cancels it out.

Of course this comes with its own set of limitations (compliance, stability, etc.).  Not to mention cost.

[Len]

Am I the only one that find the concept of burden voltage to be annoying? Why specify it in this way? Just tell me the shunt resistance, having it presented in mV/µA is much less intuitive to me and it does not make calculating the voltage drop or evaluating the effect on the circuit being measured any easier.

Yeah, specifying the burden resistance is more basic and sometimes more useful, but I also find it handy to know the full-scale burden voltage when I’m making measurements. I don’t often need to do a detailed calculation of the burden voltage, usually I need a general idea of “how bad is it?”

[ez24]

A better multimeter is a sourcemeter, like your Keithley 2400.
Set it to force zero volts and read back current.
No burden voltage.  The source cancels it out.

Of course this comes with its own set of limitations (compliance, stability, etc.).  Not to mention cost.

I think it is not clear that the "better" meter case has already been designed and probably molded, thus the physical size cannot change.  The only way to make better is with a few small components.  Not even a fuse can be changed.  I am surprised the proposed changes can be made at this stage.

Is there an estimate to market yet?  Sounds better every time.  I wonder if Fluke is getting worried, I think this will give them a run for their money.  Is there a model comparison to Fluke yet? 

Also I suggest Dave send out a dozen for beta testing so any "bugs" do not remain a problem forever like the LCD on the 235.

(FYI there are only 13 235's on Amazon)

[Stephan_T]

[...]

The higher resistance of the multimeter fuses is probably because they are super fast acting (FF), therefore they need to dissipate more power to melt in a shorter time.

Yes, power dissipation is a big part of the regular operation of the fuses. They need that power to burn through. FF plays a role in here, but also the HRC voltage. The HRC versions of fuses use sand to extinguish arcing. That also acts as a heat sink by design.
These two are reasons why they need larger voltage (power) than a cheap glass fuse

Another thing that can bite you is the thermo electric effect if you are not careful with the PCB layout and one end of the shunt resistor has a different temperature than the other end. When dealing with signals in the uV range this is a serious thing to consider.

Meters are essentially a sealed unit and the internals are pretty close to thermal equilibrium, so not an issue in practice.
But yes, the lower you go, the more this is an issue. A big issues in the likes of the 6.5/7.5 digit high end meters.

When you run the ampmeter at the top of the protected range, the fuse is at it's maximum power dissipation. Short before burnout. You don't want that anywhere near the sense wires of a microvolt measurement. . The fuse is also a major violation to the thermal symmetry (equilibrium) of your circuit.

In general, the current measurement with a multimeter (CAT-) rated for mains voltages or even higher seems like a (bad) compromise to me.

As a more pragmatic solution to the original problem (current measurement for a low voltage but high current device), I would rather like to have a power supply with external sense wires. This should be much cheaper to achieve than a SMU. 

Measuring milli- or microvolts across an external current shunt (right in the circuit under test) is probably a more precise and clear solution than gilding the Lilly inside the multimeter.

[brutester]

If having 0.1uV offset is a problem in amplifiers, what about MUX parasitic resistance and capacitance? I think they will cause much more issues than the amplifier offset voltage. I like Dr. Frank's approach above - switching on or off the x10 amp.
Some time ago I replaced one of the resistors of a typical amp with CD4052B (dual analog switch) which I was using to change the values of resitor+cap for custom filters. It was low-frequency business so it was actually doing quite good.

[f4eru]

Hello,

A couple of points :

1) don't forget you alsowant to measure AC (RMS ?), so a chopper amp can distort your AC measurement, and RMS calculation -> what is the frequency cutoff of your moldy meter, and your chopper amp ?

2) why not leave the 100Ohm resistor permanently connector to the uA jack ? there's no point in switching it. It also corrects the 1.01R to 0.9999R --> one switch needed only

3) no need of a 3-way mux for sensing. the 10A sense can go through the 1R resistor --> two input mux should be enough

4) diodes can be changed from 4 to 2 diodes if the burden voltage is reduced

[Craftplorer]

Is burden voltage also a problem in lab bench multimeter?
Like in an HP 3478A, Fluke 8840A or even one of the old HP once with lots of range switches?

Please excuse my bad english.

[EEVblog]

This is a very good explanatory video, about the difficulties of current measurements.
As you suggested, I'd like to add a better, more simple circuit, which may be found in mostly any higher grade DMM, in a similar way.
I only omitted the protection circuitry.
You don't need two MUX, and only a FET switch to change the gain.

Yes, I do this lower range sense resistor tap thing in my uCurrent.
I'd already come up with basically the same solution, yes it is better.
And of course this is where you would use the 99R and 0R99 if you wanted to keep nice round values.

[EEVblog]

Is burden voltage also a problem in lab bench multimeter?

Yes, and they usually aren't very good.

[EEVblog]

2) why not leave the 100Ohm resistor permanently connector to the uA jack ? there's no point in switching it. It also corrects the 1.01R to 0.9999R --> one switch needed only

Yes, I do this in the uCurrent.

[NiHaoMike]

Why not use active IV conversion for the uA range? For a mains powered meter, it would also be practical for the low mA ranges.

Use some high current diodes to protect the input and you'll be able to use a bigger fuse with a lower resistance.

The Mooshimeter uses a single 10m shunt and somehow manages a 1uA resolution. The current path is not switched at all.

Am I the only one that find the concept of burden voltage to be annoying? Why specify it in this way? Just tell me the shunt resistance, having it presented in mV/µA is much less intuitive to me and it does not make calculating the voltage drop or evaluating the effect on the circuit being measured any easier.

I prefer the term "insertion resistance".

1) don't forget you alsowant to measure AC (RMS ?), so a chopper amp can distort your AC measurement, and RMS calculation -> what is the frequency cutoff of your moldy meter, and your chopper amp ?

Switch to a higher bandwidth AC coupled preamp. Then usable bandwidth would likely be limited by stray inductance.

[EEVblog]

Probably worth doing a follow-up to this video on practical optimisation of the circuit.

[MobileWill]

I found this great app note with lots of different options.

https://cds.linear.com/docs/en/application-note/an105fa.pdf

I think I want to try MOSFET switching of 2 shunts.

I am looking at using the  LTC2945. http://www.linear.com/product/LTC2945

[julian1]

I haven't had a chance to look at the video yet so I might have missed some context. But I recently watched Dave's teardown of the Agilent SMU, and he showed it using simple mosfets to switch in shunts of different ranges, all in-line with the main linear regulator mosfets.

[EEVblog]

I haven't had a chance to look at the video yet so I might have missed some context. But I recently watched Dave's teardown of the Agilent SMU, and he showed it using simple mosfets to switch in shunts of different ranges, all in-line with the main linear regulator mosfets.

SMU's are somewhat different because they can and do source the current with a ton of available compliance voltage to make up for it.
DMM's have to drop as little as possible because it's affecting the DUT

[mrpackethead]

I struggled to find the enthuasim that you have for multimeters.  :-)

[obiwanjacobi]

What about using the fuse as the current detector?

I would SERIOUSLY doubt the accuracy of such an arrangement.  Precision resistance in a fuse is not required for it to provide it's prime function - especially when we are talking about a disposable, clip in field replaceable component.  Not to mention contact resistance between the fuse and holders at each end.

This was my initial thought too: use the fuse as shunt.
If the fuse has so much variation, why doesn't that affect the conventional design? At least for 50% ...
I understand half is better, but still...

[0xfede]

While I found the entire topic interesting from an engineering prospective I'm also not sure about the usefulness in the case study.
The possible solutions you will face in a labs are:
1: Use a SMU instead.
2: Use a lab PSU with sense wires and connect them near the DUT (after the multimeter).
3: if you are evaluating the performance of a battery operated device you must take in account that you have to simulate battery series resistance (e.g. it could even reach hundreds of ohms in Li-SOCl2) and so you have even to add series resistance to the DUT.

Otherwise IMHO you are trying to hammer a nail with a screwdriver and while it is still possible it is of course inconvenient.

Best,
0xfede

[f4eru]

This was my initial thought too: use the fuse as shunt.

The fuse wire is designed to have a big variation in resistance: a fuse is using the thermal runaway effect of a PTC for opening the circuit quickly

If the fuse has so much variation, why doesn't that affect the conventional design? At least for 50% ...

Because the voltage drop is measured solely across the shunt resistor, not across the fuse. The acuracy is then not dependant on the fuse.

[EEVblog]

If the fuse has so much variation, why doesn't that affect the conventional design? At least for 50% ...

It does affect conventional designs in terms of burden voltage (i.e. it goes up as current goes up), and the current shunt doesn't change, so the measurement is always rocks solid regardless of what the fuse does.

[Dr. Frank]

Why is the FET switch better than using a MUX? I see more problems. For example the ON resistance of the FET affects the gain. Therefore you need to make the divider resistors rather high value to reduce the effect of the FET. Using a multiplexer, the ON resistance does not care, because it only switches the feedback voltage into the opamp either directly from the output (1x gain) or divided (10x gain).

Your concern is correct for high resolution DMMs, where a FET-MUX is used in the feedback path of the OpAmp, to give a very precise gain.

For lower resolution, this simple and cheaper solution would be sufficient.

The rds(on) will add up to the 1k feedback resistor, and their sum will be calibrated into the gain 10x constant.
As far as the FET changes its rds(on) to a few ten mOhm over temperature only, there will be no error visible, even on 50000 digits DMMs.

Frank

[tatus1969]

Dave, regarding your question around the fuse resistance. I'm not an expert in those, but here's my thought: these high breaking capacity fuses normally are filled with sand, which lowers their thermal impedance. So in order to maintain their capability to melt at their rated current, they need to dissipate more power, compared to an air-filled glass fuse. That would explain a higher resistance.

[Rolo]

Great video. I think having one meter that does all and meets all safety standards is always a compromise, specially in these low current ranges. For me it's always with battery feeded projects I need these low current measurments. I have build a shunt for this, works great. No fuses, no CAT rating, needs no battery and costs little money.
It's handy, you can leave the shunt in the power line of your project an use the multimeter for other measurements, it's also works on my scope, I can see short power peaks.
This "old" model is 1%, I have ordered a new resistor for it to improve accuraccy  (Precision resistor 0.6 W, 0.1% 10.0 Ohm).

[EEVblog]

Dave, regarding your question around the fuse resistance. I'm not an expert in those, but here's my thought: these high breaking capacity fuses normally are filled with sand, which lowers their thermal impedance. So in order to maintain their capability to melt at their rated current, they need to dissipate more power, compared to an air-filled glass fuse. That would explain a higher resistance.

Sounds quite plausible.

[EEVblog]

The rds(on) will add up to the 1k feedback resistor, and their sum will be calibrated into the gain 10x constant.
As far as the FET changes its rds(on) to a few ten mOhm over temperature only, there will be no error visible, even on 50000 digits DMMs.

If you want to eliminate Rds in gain circuit just put the switch in the zero current opamp input:


Rf could also be switched, but if it's orders bigger than R1 then it doesn't matter.

[Stephan_T]

Dave, regarding your question around the fuse resistance. I'm not an expert in those, but here's my thought: these high breaking capacity fuses normally are filled with sand, which lowers their thermal impedance. So in order to maintain their capability to melt at their rated current, they need to dissipate more power, compared to an air-filled glass fuse. That would explain a higher resistance.

Sounds quite plausible.

I already mentioned that in a previous post, but what about the role of the fuse as a heating element within a precision measurement device:

When you run the ampmeter at the top of the protected range, the fuse is at it's maximum power dissipation. Short before burnout. You don't want that anywhere near the sense wires of a microvolt measurement. . The fuse is also a major violation to the thermal symmetry (equilibrium) of your circuit.

You seem to ignore that aspect.

[nctnico]

And there is also the leakage current through the diode bridge. It seems a generic multimeter circuit isn't going to cut it when it comes to measuring small currents accurately.

[bittumbler]

Hi,

if you want to go for minimum voltage burden, why not use the attached circuit. It should have zero voltage burden, at least for the sense resistor.


Best Regards
Matthias

[G7PSK]

Dave, regarding your question around the fuse resistance. I'm not an expert in those, but here's my thought: these high breaking capacity fuses normally are filled with sand, which lowers their thermal impedance. So in order to maintain their capability to melt at their rated current, they need to dissipate more power, compared to an air-filled glass fuse. That would explain a higher resistance.

Dry sand is an insulator to heat compared to air, so it will lead to a more rapid heating of the fuse element when embedded in sand so it should increase the thermal impedance.

[thmjpr]

Perhaps we need to go back 70 years and have separate meters for current and voltage at those very low levels, that way you need not worry about some of the compromises that have to be made. A case of the right tool for the right job.

Yes I was thinking, why not make a low voltage electronics meter. Mains integration into projects isn't as big of a deal as it used to be.
Would let you use cheap glass fuses, and would make it easier to implement features like zener test, various current range tests for diodes, etc.

Of course you would risk/alienate some first time buyers in that they could not get a universal meter.

[Cerebus]

Dave, regarding your question around the fuse resistance. I'm not an expert in those, but here's my thought: these high breaking capacity fuses normally are filled with sand, which lowers their thermal impedance. So in order to maintain their capability to melt at their rated current, they need to dissipate more power, compared to an air-filled glass fuse. That would explain a higher resistance.

Dry sand is an insulator to heat compared to air, so it will lead to a more rapid heating of the fuse element when embedded in sand so it should increase the thermal impedance.

Eh? The thermal conductivity of dry sand is 0.15 to 0.25 W/m.k, air 0.024 W/m.K i.e. the thermal conductivity of sand is about ten times higher than air. Furthermore, as well as air conducting more heat than sand it will also absorb more heat than sand i.e. it has a specific heat capacity (taking account of density) of around 1000 times that of air.

[ez24]

Yes I was thinking, why not make a low voltage electronics meter. Mains integration into projects isn't as big of a deal as it used to be.
Would let you use cheap glass fuses, and would make it easier to implement features like zener test, various current range tests for diodes, etc.
Of course you would risk/alienate some first time buyers in that they could not get a universal meter.

Marketing

[David Hess]

Yes I was thinking, why not make a low voltage electronics meter. Mains integration into projects isn't as big of a deal as it used to be.
Would let you use cheap glass fuses, and would make it easier to implement features like zener test, various current range tests for diodes, etc.
Of course you would risk/alienate some first time buyers in that they could not get a universal meter.

Marketing

Lawsuits - the fuses are not there to primarily protect the meter.

And if you want a cheap multimeter which lacks safety features, I am sure someone in China already produces one.

[bittumbler]

One more try to make the voltage burden zero. This time including the fuse burden.
What do you think?

[tatus1969]

Dave, regarding your question around the fuse resistance. I'm not an expert in those, but here's my thought: these high breaking capacity fuses normally are filled with sand, which lowers their thermal impedance. So in order to maintain their capability to melt at their rated current, they need to dissipate more power, compared to an air-filled glass fuse. That would explain a higher resistance.

Dry sand is an insulator to heat compared to air, so it will lead to a more rapid heating of the fuse element when embedded in sand so it should increase the thermal impedance.

Huh? Did you maybe confuse thermal capacity and thermal impedance?

[phunkz]

What about two fuses in parallel with half the amps?

[EEVblog]

What about two fuses in parallel with half the amps?

1) Size
2) Cost
3) Likely non-consistent breakage characteristics.

[nctnico]

What about two fuses in parallel with half the amps?

It doesn't work! You can buy parallelled fuses for very high currents but these are stuck together in the factory and there are many things to consider to make sure the current is shared equally so it doesn't blow by accident.

[joeqsmith]

What about two fuses in parallel with half the amps?

It doesn't work! You can buy parallelled fuses for very high currents but these are stuck together in the factory and there are many things to consider to make sure the current is shared equally so it doesn't blow by accident.

I like Photonicinduction's channel.   Note the internal construction of this fuse.  It does not seem like this would ever work on a smaller scale with multiple fuses but I have never tired it. 

https://youtu.be/El7gp64PYnE?t=49

[Huluvu]

in case of low voltage and high current, when burden voltage really plays a role, I simply use an chunky 50A/50mV external 4 wire shunt and measure the voltage drop.

[rfeecs]

One more try to make the voltage burden zero. This time including the fuse burden.
What do you think?

You are getting closer and closer to an SMU.
Your circuit is a feedback ammeter, commonly used for low current measurements:
http://www.tek.com/sites/tek.com/files/media/document/resources/LowCurtMsmntsAppNote.pdf

Apparently the interest here is more to make a small tweek to the basic multimeter to improve burden voltage.

One issue with the feedback ammeter is your op-amp has to be capable of sinking/sourcing the current that you are trying to measure.  Also it has to be stable and not oscillate with whatever load you are presenting to it.

[Kleinstein]

The Feedback circuit (TIA) is only practical for low currents, that is about <1 mA for a battery powered meter and maybe 50 mA for a mains powered one. A few >(e.g. HP3457) high grade mains powered DMM actually use this for something like a 20 mA range. It might be attractive for the low current ranges (as this could allow even lower currents) and reduce leakage through the protection circuit.

The main problem here is the relatively high drop in the 500 mA range as here we have the fuse and quiet some drop on the shunt that add up.

The 1st. possible improvement is a really good (e.g. low offset and low noise) amplifier for the shunt voltage so one could use a lower drop on the shunt. Gain could be something like time 2 or times 20 - so always some amplification. Using just one switch as initially shown by Dr. Frank can he a slight advantage, as any extra switch / MUX can add thermal EMF. Whether switching the gain is using a FET or CMOS MUX is not that important, both ways are good enough and cheap. However the possible improvement is limited, maybe a factor of 2. Really low noise amps also need more current.

A 2nd possible improvement would be a slightly larger fuse. The problem could be getting such multimeter grade fuses at 1 or 2 A. The protection diodes (and maybe the shunts) also need to be a larger.

A 3rd way would be adding an extra shunt, so that one does not have to use them for a 1:100 range. The downside are extra costs for switching and the extra shunt(s) and maybe odd ranges. Even only an extra shunt for the 500 mA would already avoid the worst case.

[Chupacabras]

How to make that multiplexing mentioned in the end of the video?
Is it better to use some mosfets? Or is there any specialized IC?

[kvresto]

In the video Dave spoke of the sense wire across the 0R01 shunt resistor in the Amps range, and said he wont go into the details of how it would be layed out on the board. It seems to me that it would have to be close to the amp, but can someone elaborate and explain how to best layout this track?, pros, cons ect?

[Kleinstein]

The sense wire connection to the amplifier should be in such a way that there is no large area between this and the signal lines to the amplifier. It does not have to be extra short. The line resistance should not be that critical as the resistors to set the gain are relatively high in a battery operated circuit. The main concern would be inductive coupled external signals. One should also avoid vias as they could introduce minute thermal EMF. Ideally the layout around a 4 wire shunt is symmetric, so get equal temperature at the terminals. However space may not allow a fully symmetric layout and it is more like avoiding large temperature differences.

For the MUX one could use CMOS chips (e.g. DG4...), as the voltage is low and super low leakages is not an absolute priority. There are quite a lot of such chips to chose from. For just a single switch (especially one towards ground and only low voltage) one might use a separate MOSFET.

[tatus1969]

How to make that multiplexing mentioned in the end of the video?
Is it better to use some mosfets? Or is there any specialized IC?

that should all be done by the big rotary switch, no need for additional electronics.

[f4eru]

>>that should all be done by the big rotary switch, no need for additional electronics.
No. That's so '70s.
Today, every moldy meter has autoranging.

[tatus1969]

>>that should all be done by the big rotary switch, no need for additional electronics.
No. That's so '70s.
Today, every moldy meter has autoranging.

good point...

[Kleinstein]

Switching the shunts is usually still done mechanically - auto ranging with switching shunts can cause some upsets. Changing amplification is definitely better done under software control as auto-ranging.

Depending on the form of the 10 mOhms shunt, one may not even need to switch the sense voltage. Just have the shunts in series - especially also having the 100 Ohms one also in series is not problem. If at all having the 1 Ohms and 10 mOhms in series could be a slight problem as there will be some coper in the effective shunt part.  In this case only changing the amplification would be left to an electronic switch/FET.

[David Hess]

>>that should all be done by the big rotary switch, no need for additional electronics.
No. That's so '70s.
Today, every moldy meter has autoranging.

Automatic ranging is not a killer feature.  For those who prefer manual ranging, it is still available.  B&K for instance has 10 different models with manual ranging to complement their automatic ranging multimeters.

[EEVblog]

PART 2:

[riyadh144]

I have been thinking why don't we use a current mirror with an output ratio small enough? I am sure we can find Mosfets with low Rds, and so we can sense the current with that.

[Dr. Frank]

Using the (parasitic) reverse diodes of the MOSFETs for protection purposes will affect the precision of the sensitive ranges, due to their leakage currents. These lower ranges resolve nA, maybe pA currents.

Especially, if you apply a reverse current, in the "negative" direction of the DMMs input jacks, these 50mV burden voltage already create relevant forward currents in these MOSFET diodes, and if you combine two of such MOSFETs in parallel, this would be the case for either direction.

More common precision current protection schemes use either the low leakage of the B-E diode of a bipolar transistor, like in the HP3458A, which includes power dissipation protection by software, or they use at least two diodes of a rectifier bridge in series, combined with an OpAmp to reduce these diode leakage currents to near zero, like in most of HPs 6 1/2 digits DMMs.






Daves last circuit with one shunt for each decade can be used in DMMs with electronic gain calibration only, as the shunts for the different ranges have the sequence 0.01, 0.11, 1.11, 11.11 and 111.11, not a decade sequence.
Therefore this can't be used for an external front end, like the µcurrent.


Frank

[Psi]

Regarding the mosfet idea.

What happens when John uses his meter to measure 20A but at the full rated voltage of 600/1000V.

Unless i've missed something you're going to need some pretty HUGE mosfets to do 20A @ 1kv?
And high voltage mosfets usually have terrible on-resistance

Apologies if i missed something in the video, i did skip through a little. 


Looking at $8 for one that can do 50A at 600V with 70mR RDS
http://www.digikey.co.nz/product-detail/en/alpha-omega-semiconductor-inc/AOK53S60L/AOK53S60L-ND/3973573

Or $36 to get better RDS of 17mR  (150A/600V)
http://www.digikey.co.nz/product-detail/en/ixys/IXFB150N65X2/IXFB150N65X2-ND/5629500

[bktemp]

Regarding the mosfet idea.

What happens when John uses his meter for 20A but at the full rated voltage of 600/1000V.

Unless i've missed something you're going to need some pretty HUGE mosfets to do that?
And high voltage mosfets usually have terrible on-resistance

The voltage doesn't care, because ou are measuring current. The voltage drop across the current shunt is limited by the back to back diodes (mosfet's body diodes) to around +/-1V for a short time until autoranging kicks in and connects the 10mOhm shunt, limiting the voltage furthur until the fuse blows when the current is too high.
You can use low voltage (<10V) mosfets.

[Psi]

Ah, got ya

That's pretty neat!
Works by the fact that the two body diodes means the "off" fets don't conduct until ~1V and the current always takes the part of least resistance through the "on" fet.

[EEVblog]

Using the (parasitic) reverse diodes of the MOSFETs for protection purposes will affect the precision of the sensitive ranges, due to their leakage currents. These lower ranges resolve nA, maybe pA currents.

Gossen and Tek/Fluke get away with it just fine for 10nA resolution.

More common precision current protection schemes use either the low leakage of the B-E diode of a bipolar transistor, like in the HP3458A, which includes power dissipation protection by software, or they use at least two diodes of a rectifier bridge in series, combined with an OpAmp to reduce these diode leakage currents to near zero, like in most of HPs 6 1/2 digits DMMs.

Bench meters also usually have poor CAT rating.

Daves last circuit with one shunt for each decade can be used in DMMs with electronic gain calibration only, as the shunts for the different ranges have the sequence 0.01, 0.11, 1.11, 11.11 and 111.11, not a decade sequence.
Therefore this can't be used for an external front end, like the µcurrent.

Of course, just change the value if you wanted that.

[bktemp]

Using the (parasitic) reverse diodes of the MOSFETs for protection purposes will affect the precision of the sensitive ranges, due to their leakage currents. These lower ranges resolve nA, maybe pA currents.

Gossen and Tek/Fluke get away with it just fine for 10nA resolution.

I did some quick measurments: The leakage current of most low voltage power mosfets is <5nA (10pA-5nA depending on mosfet and polarity) at room temperature and +/-50mV. If you heat it up, the current goes much higher (>1uA).
Most datasheets specify <1uA at room temperature, so maybe you need to verify each mosfet or at least some batches at maybe 40°C or so before using it for such an application.

[tszaboo]

What happens when John uses his meter to measure 20A but at the full rated voltage of 600/1000V.

How does that work? You have 1000V over the current terminals?
You got that Dave is talking about handheld DMM, that is why space, and price is important. Two cables connected only, and it is floating.

I'm not particularly fan of the multiplexer reduction. Probably it is better for the BOM, but MUXes are not expensive. A DG409, you would typically use for this, 2x4:1 is 80 cents. I like routing those analog signals close to each other, have symmetrical filtering, symmetrical tap from the shunt, and so on. Also, your last suggestion is with 5 shunts. A nice 0R1 or 1R shunt is expensive. Recently, I had to design a circuit to measure 500mA with 6.5 digit multimeter accuracy. The most expensive part was the shunt.
Also, probably the biggest issue I'm having with your setup: I can measure -100mA with my Fluke 87. Does this rely on the fact that the FET backwards will not conduct with a voltage of ~100mV? Surrely it will conduct some and it will be hugely temperature dependent. As will the FET conduct when it is open, up to few hundred microamps. These new, switching, low cost FETs will be especially bad for it, as all characteristic they care about is Rds and maybe gate charge. I worked with these FET switches for current measurement, they are all but ideal.
So, i encourage you to actually build the circuit. But then again, maybe it is fine for 3-4 digit.

[Brutte]

I have been thinking why don't we use a current mirror with an output ratio small enough? I am sure we can find Mosfets with low Rds, and so we can sense the current with that.

The current mirror is just primitive gain stage (A type amplifier) controlled in feedback to mirror the current of an input stage. So that is a simplified variation of an active circuit mentioned. The temperature dependence of both uni and bipolar mirrors can be matched maybe to 1% with the tongue at right angle. Dave is looking for something with 50000 counts so that is way too far from the goal.

For measuring 10A currents of a 3.3V power supply I think a simple and practical option is to use a remote shunt (R01) and a 50000 count voltmeter set to a 100mV full scale range. A kind of custom leads with shunt at the end maybe.
With standard leads, even 1mm diameter copper leads (2*0.5m) give over 0R02 resistance and 200mV drop at 10A so that won't work well with 3V3 supply.

[Dr. Frank]

I did some quick measurments: The leakage current of most low voltage power mosfets is <5nA (10pA-5nA depending on mosfet and polarity) at room temperature and +/-50mV. If you heat it up, the current goes much higher (>1uA).
Most datasheets specify <1uA at room temperature, so maybe you need to verify each mosfet or at least some batches at maybe 40°C or so before using it for such an application.

You need to measure the leakage of the protection/reverse diode, when the FET is explicitly switched off, and also the FORWARD current at 50mV bias, when the FET is explicitly switched off.

The FETs own leakage is also interesting, but has to be separated.

Either forward or leakage (reverse) current might be as high as many 100nA, up into the µA range, at higher temperatures.

Frank

[EEVblog]

Also, probably the biggest issue I'm having with your setup: I can measure -100mA with my Fluke 87. Does this rely on the fact that the FET backwards will not conduct with a voltage of ~100mV? Surrely it will conduct some and it will be hugely temperature dependent. As will the FET conduct when it is open, up to few hundred microamps. These new, switching, low cost FETs will be especially bad for it, as all characteristic they care about is Rds and maybe gate charge. I worked with these FET switches for current measurement, they are all but ideal.
So, i encourage you to actually build the circuit. But then again, maybe it is fine for 3-4 digit.

Both Gossen and Tek/Fluke got away with this MOSFET switching solution just fine in their high end meters with at east 10nA resolution.
Yes device selection will be important, but it works.

[bktemp]

You need to measure the leakage of the protection/reverse diode, when the FET is explicitly switched off, and also the FORWARD current at 50mV bias, when the FET is explicitly switched off.

I shorted source and gate together and measured the drain-source current at +50mV / -50mV and also at +100mV / -100mV. I didn't control the temperature, because I only wanted a rough estimate of the typical D-S leakage current for some mosfets at room temperature.
I also tried measuring gate-source current at +/-10V, but it is too low for the capabilities of my measuring equipment (1pA resolution).
At least at room temperature, both the forward and reverse leakage current is negligible for nA resolution.

The temperature is probably the biggest problem, maybe I do some measurements at different temperatures, because the leakage current probably increases exponentially:
Let's assume the mosfet used has 10mOhms RDson. Because there are two in parallel we have 5mOhms total. At 10A the mosfets will dissipate 0.5W total. The datasheet gives max. 50K/W for a mosfet in SO8 package. 0.25W per mosfet will rise the junction temperature to 12.5°C above pcb temperature (probably more, because they are close together). The 10mOhm shunt dissipates 1W and increases the temperature of the pcb near the mosfets probably to more than 10°C above ambient, giving a mosfet junction temperature of about 50°C. If you are trying to measure nAs immediately after measuring 10A the measurement will probably be not accurate.

[joeqsmith]

Both Gossen and Tek/Fluke got away with this MOSFET switching solution just fine in their high end meters with at east 10nA resolution.
Yes device selection will be important, but it works.

First, I enjoyed both videos.  I have a couple of questions/comments assuming this is what you plan to do with your meter. 

I agree that using a single high amp rated fuse may prevent some accidental fuses from being blown.  However, the flip side is when the user blows one, it is 100% assured it will take out a large very expensive HRC fuse.    I can't say that I have ever popped a 10A fuse in any of my meters but I have certainly taken out my fair share of low current ones.   Using the smaller package part you had originally planned on wouldn't be too expensive.   I think I paid less than $1.00/ea for the low current fuses last time.   After seeing the posts of the people soldering wire across a blown HRC fuse, I wonder how much of this would go on with a $1.00 fuse compared with a $10 fuse.   

When I benchmark meters like I did with your re-branded Brymen, it's very rare I do anything with the current inputs.  When designing the transient generator, obviously I was concerned with dv/dt ratings of the caps and surge current ratings for the parts.  The problem I ran into is many parts did not have data to the levels I needed.   This is rather a toy setup compared with an actual AC line.   I have not looked but imagine your requirements are pretty intense to meet the IEC standards.   How do you know if the solid state devices will handle all the conditions?    Does the meter actually need to survive the current tests beyond just replacing the fuse?   Does IEC require you to test the meter with a HV/HC supply?   

When they qualify this meter, it would be great to have to you go to the lab where it is certified and be involved.  I would really like to see a video like this done.

Looking forward to seeing what you come up with.

Couldn't understand what I wrote....

[f4eru]

Hello,

One problem with your design, Dave :

When you erroneously put your moldy meter Amp jacks directly onto mains voltage (Which is the use case for the HRC fuse),
then you briefly have 200V over 20+ mOhms (fuse + shunt + wires) ->> 10000A surge
During this short time there is 100V over the shunt
At this time the gates of the mosfets as well as the input of the ADC and the output of the drivers to the mosfet gates need protection

Add some 10k/1000V resistors in series with the gates, and a handfull of TVS, and it should be OK.
Proper layout is critical for everything to survive this surge ! (except the fuse, of course)

[unclebob]

Talking about Gossen meters: I want to get a second multimeter for my lab and I want it to be a really decent one. I can get the Metrahit Extra or the Metrahit Ultra for about the same price. The Ultra can go to lower ranges and has over 300k digits but the max. voltage is only half of the Extra. Do you agree that the Ultra will be more suitable for fiddeling around with low voltage uC electronics, maybe low power stuff running from batteries or have I missed a major drawback in the Ultra?

[Kleinstein]

For the HRC fuses, there is not that much difference in the price for a 11 A or 600 mA type. They both are at a similar price - maybe $1 less for the smaller one as this can be a slightly smaller form factor.

If leakage gets a problem for really small currents, one might have to use a separate input and fuse for these. So maybe the 400 mA fuse for the nA ranges. It could also help to have a TIA for the really low current ranges (e.g. < 50 µA) - so the voltage drop on the MOSFETs could go down from 50 mV to maybe 1 or 5 mV only. The low voltage is what helps with leakage - leakage specs are usually at a rather high voltage. Keep in mind that with AC current the peak voltage at the shunt might be higher than 50 mV. So peak currents higher than something like 3 times FS might cause extra leakage.
So it could be attractive to reduce the shunts even further with a good quality amplifier.

The worst case power dissipation could also be quite high with the FETs: at 10 A and something like a 600 mV forward drop (with FETs off), this can be 6 W. So auto ranging is really needed to reduce the power. However this could be a possible problem with long time over-current when the meter is off  - might need to turn the FETs on even if the rest of the meter is off ! Otherwise think about a heat sink.

[bktemp]

The low voltage is what helps with leakage - leakage specs are usually at a rather high voltage.

That was what I thought initally. But the voltage does not help much: Yes, the current increases with voltage, but very slowely. Between 1V and 10V reverse bias the leakage current of most diodes I tested doubled. Increasing the voltage another order of magnitude doubles the current again. So the voltage dependency is almost negligible compared to the temperature dependency where the current doubles almost every 5°C.

Keep in mind that with AC current the peak voltage at the shunt might be higher than 50 mV. So peak currents higher than something like 3 times FS might cause extra leakage.
So it could be attractive to reduce the shunts even further with a good quality amplifier.

AC could be a problem if you need a uA AC range.
I did some measurements up to 50°C using a IRF8252. Why IRF8252? It was the mosfet with the lowest RDSon I could find in my part box. It's a 25V 2mOhm mosfet designed for synchronous buck converters or power switching, therefore its leakage is rather high compared to other mosfets. Because it takes some time to do the temperature sweep, I measured only 1 mosfet, but it should be enough to give a rough estimate of the typical leakage current over temperature.
The leakage current increases roughly 1.75x every 5°C. This mosfet should be useable for Dave's design with 50mV shunt voltage, even with multiple mosfets connected in parallel.
For shunt voltages of 100mA or higher the leakage current is too high if you need a uA range. For AC this mosfet is probably not the best choice if the mosfets get heated up by the shunt.
Maybe a series connecton + 4 external diodes is a better choice because it avoids any forward biased diodes. At 100mV the forward bias current is more than an order of magnitude higher than reverse bias current.

The worst case power dissipation could also be quite high with the FETs: at 10 A and something like a 600 mV forward drop (with FETs off), this can be 6 W. So auto ranging is really needed to reduce the power. However this could be a possible problem with long time over-current when the meter is off  - might need to turn the FETs on even if the rest of the meter is off ! Otherwise think about a heat sink.

That's a good idea. I have seen current sense circuits where a large relay shorted all shunts when the unit was powered off to protect all shunts from overcurrent. But this was mains powered, so it didn't care when the relay was powered permanently while the unit is switched on. For a battery powered DMM you could probably put some mosfets across the shunt resistors with a pullup connected to the battery. When the meter is switched on, it disables the protection mosfets.

[David Hess]

I agree that using a single high amp rated fuse may prevent some accidental fuses from being blown.  However, the flip side is when the user blows one, it is 100% assured it will take out a large very expensive HRC fuse.    I can't say that I have ever popped a 10A fuse in any of my meters but I have certainly taken out my fair share of low current ones.   Using the smaller package part you had originally planned on wouldn't be too expensive.   I think I paid less than $1.00/ea for the low current fuses last time.   After seeing the posts of the people soldering wire across a blown HRC fuse, I wonder how much of this would go on with a $1.00 fuse compared with a $10 fuse.

I like the solution I recently read about where an inexpensive fast blow fuse is placed in series with the HRC fuse so that in the event of a non-safety related failure, the cheap fuse will fail first protecting the expensive HRC fuse.

Another clever protection circuit found in old multimeters like the Tektronix DM502 and DM502A is to place a small high voltage incandescent lamp in series with the ohms converter current source instead of a fuse or other protection circuit.  These days I guess you could use a high voltage depletion mode MOSFET configured as a current limiter but incandescent lamps are less likely to fail with a short.

[EEVblog]

I have not looked but imagine your requirements are pretty intense to meet the IEC standards.   How do you know if the solid state devices will handle all the conditions?   

You take your best pick and test it I guess.

Does the meter actually need to survive the current tests beyond just replacing the fuse?   Does IEC require you to test the meter with a HV/HC supply?   

IIRC it does not have the survive it just has the "fail safe". At least one of the tests anyway.

[EEVblog]

Talking about Gossen meters: I want to get a second multimeter for my lab and I want it to be a really decent one. I can get the Metrahit Extra or the Metrahit Ultra for about the same price. The Ultra can go to lower ranges and has over 300k digits but the max. voltage is only half of the Extra. Do you agree that the Ultra will be more suitable for fiddeling around with low voltage uC electronics, maybe low power stuff running from batteries or have I missed a major drawback in the Ultra?

Nope, get the Ultra, it's a great meter.

[EEVblog]

For the HRC fuses, there is not that much difference in the price for a 11 A or 600 mA type. They both are at a similar price - maybe $1 less for the smaller one as this can be a slightly smaller form factor.
If leakage gets a problem for really small currents, one might have to use a separate input and fuse for these. So maybe the 400 mA fuse for the nA ranges. It could also help to have a TIA for the really low current ranges (e.g. < 50 µA) - so the voltage drop on the MOSFETs could go down from 50 mV to maybe 1 or 5 mV only. The low voltage is what helps with leakage - leakage specs are usually at a rather high voltage. Keep in mind that with AC current the peak voltage at the shunt might be higher than 50 mV. So peak currents higher than something like 3 times FS might cause extra leakage.
So it could be attractive to reduce the shunts even further with a good quality amplifier.

The Gossen Ultra gets away with MOSFETs (IRL3302 I think) with 1nA resolution.
I have not traced out the exact circuit though.

[David Hess]

How do you know if the solid state devices will handle all the conditions?    Does the meter actually need to survive the current tests beyond just replacing the fuse?   Does IEC require you to test the meter with a HV/HC supply?

Semiconductors are often the most fragile but they are not the only problem.  Properly derating passives for peak voltage, current, and power will exclude a lot of obvious in retrospect failures.  Semiconductors can be derated as well.  Watch out for creepage and clearance.  Certain parts tend to fail short or open.  Doubling up so that *two* failures are necessary to create a dangerous situation can help.  Some types of parts can be selected for specific failure modes which lead to greater safety; carbon composition resistors used to be used as slow blow fusible links and they make emitter ballast resistors which also act like fuses under overload conditions.

I worked on a military project where the power input had to operate properly over the nominal range of relatively low voltages and *not* destructively fail in the event of a roughly 120 volt surge.  When my boss asked how it was going, I demonstrated by plugging it into the 120VAC wall socket.  Of course knowing what I know now, it would have been even tougher at the same price.

[ez24]

Bench meters also usually have poor CAT rating.
 

Marketing problem - a good reading meter with poor CAT  or a poor reading meter with good CAT.    I would rather have a portable that is like a bench but I know that is not going to happen     So I know it will be poor reading (relative) with good CAT.  But for electronics seems that good reading and poor CAT is more desirable.  But CAT is a marketing gimmick, like HP in cars.  Too bad.   

[joeqsmith]

But CAT is a marketing gimmick, like HP in cars.  Too bad.

What's the gimmick for CAT and HP?

[ez24]

But CAT is a marketing gimmick, like HP in cars.  Too bad.

What's the gimmick for CAT and HP?

Ads would say:

CAT IV is better than CAT III which is better than CAT II which is better than CAT I   (or are they ?)

300 HP is better than 200 HP which is better than 100 HP or at least according to the ads  (or are they?)

I would rather have a better CAT I for electronic work than a worse CAT IV  (documented in the thread)

Dave stresses the CAT rating on his meter ( IV ?).  He does not say his meter is a really cool electronic CAT I, so his meter is good for high voltage work.  As an uneducated guess, I guess there are more low voltage people here than high voltage.  I just do not think there are many people measuring 1,000 volts.  It is all about marketing,  he can sell more 4's than 1's because it sounds good.

[David Hess]

Bench meters also usually have poor CAT rating.

Marketing problem - a good reading meter with poor CAT  or a poor reading meter with good CAT.    I would rather have a portable that is like a bench but I know that is not going to happen     So I know it will be poor reading (relative) with good CAT.  But for electronics seems that good reading and poor CAT is more desirable.  But CAT is a marketing gimmick, like HP in cars.  Too bad.

I suspect the difference in CAT rating represents a difference in application.  Bench meters are not likely to be used for unisolated off-line measurements outside of the controlled environment of relatively small off-line power supply design.  If I am going to be measuring the phases in a breaker panel or a big motor controller, I will be grabbing my double insulated battery powered handheld meter from my toolbox.

[joeqsmith]

I have not looked but imagine your requirements are pretty intense to meet the IEC standards.   How do you know if the solid state devices will handle all the conditions?   

You take your best pick and test it I guess.

Guessing the design group has enough experience in this area to take some of the guess work out of it.   For me, I was guessing on some of the parts. 

Does the meter actually need to survive the current tests beyond just replacing the fuse?   Does IEC require you to test the meter with a HV/HC supply?   

IIRC it does not have the survive it just has the "fail safe". At least one of the tests anyway.

I'm sure your design house knows the ins and outs of the standards.  I did look up some of the areas concerning the current measurement.  Interesting stuff indeed.  Would really be great to see your meter run through the testing.   

Here's a few snippets from EVS-EN 61010-2-033:2012

Conformity is checked by inspection of the RATINGS of the overcurrent protection device and
by the following test.
If the protection device is a fuse, it is replaced with an open-circuited fuse. If the protection
device is a circuit breaker, it is set to its open position. A voltage of two times the highest
RATED voltage for any TERMINAL is applied to the TERMINALS of the overcurrent-protected
measuring circuit for 1 min. The source of the test voltage shall be capable of delivering
500 VA. During and after the test, no damage to the equipment shall occur.

A voltage equal to the highest RATED voltage for any TERMINAL is applied between the
TERMINALS of the measuring circuit for 1 min. The source of the test voltage shall be able to
deliver a current of at least the possible a.c. or d.c. short-circuit current as applicable. If the
function or range controls have any effect on the electrical characteristics of the input circuit,
the test is repeated with the function or range controls in every combination of positions.
During and after the test, no HAZARD shall arise, nor shall there be any evidence of fire,
arcing, explosion, or damage to impedance limitation devices or any component intended to
provide protection against electric shock, heat, arc or fire, including the ENCLOSURE and traces
on the printed wiring board. Any damage to a device used for current limitation shall be
ignored if other parts of the equipment were not affected during the test.
During the test, the voltage output of the source is measured. If the source voltage decreases
by more than 20 % for more than 10 ms, the test is considered inconclusive and is repeated
with a lower impedance source.

NOTE 2 This test can be extremely hazardous. Explosion shields and other provisions can be used to protect
personnel performing the test.

To me that seems like if the fuse blows, no big deal.  But nothing else better be damaged.   Wonder how the experts interpret it.

[joeqsmith]

But CAT is a marketing gimmick, like HP in cars.  Too bad.

What's the gimmick for CAT and HP?

Ads would say:

CAT IV is better than CAT III which is better than CAT II which is better than CAT I   (or are they ?)

300 HP is better than 200 HP which is better than 100 HP or at least according to the ads  (or are they?)

I would rather have a better CAT I for electronic work than a worse CAT IV  (documented in the thread)

Dave stresses the CAT rating on his meter ( IV ?).  He does not say his meter is a really cool electronic CAT I, so his meter is good for high voltage work.  As an uneducated guess, I guess there are more low voltage people here than high voltage.  I just do not think there are many people measuring 1,000 volts.  It is all about marketing,  he can sell more 4's than 1's because it sounds good.

Of course 300 HP is better than 100 HP!  If we put the 300HP into a motorcycle chassis, all the better!! 

Ok, I think I see where you are heading.  Agree, fear can be a powerful tool.  IMO, the CAT ratings make sense. 

Dave's re-branded meter is pretty tough.  My testing does not prove it meets it's standards in any way but it does show it is more robust than the vast majority of the other meters I have looked it.   The fact it has been certified to meet the EMC and safety standards is a good selling point and I see no reason why Brymen or Dave would not promote this.   

[EEVblog]

Bench meters also usually have poor CAT rating.

Marketing problem - a good reading meter with poor CAT  or a poor reading meter with good CAT.    I would rather have a portable that is like a bench but I know that is not going to happen     So I know it will be poor reading (relative) with good CAT.  But for electronics seems that good reading and poor CAT is more desirable.  But CAT is a marketing gimmick, like HP in cars.  Too bad.

I suspect the difference in CAT rating represents a difference in application.  Bench meters are not likely to be used for unisolated off-line measurements outside of the controlled environment of relatively small off-line power supply design.  If I am going to be measuring the phases in a breaker panel or a big motor controller, I will be grabbing my double insulated battery powered handheld meter from my toolbox.

Correct, and this is why bench meters don't have high CAT rating, they are designed for different applications.

[BobC]

For a dual-display multimeter, I would very much like to see a readout of the actual burden voltage while measuring amps, measured at the amps jack.

Any problem adding such a feature?  Do any existing hand-held multimeters do so?

[EEVblog]

For a dual-display multimeter, I would very much like to see a readout of the actual burden voltage while measuring amps, measured at the amps jack.
Any problem adding such a feature?  Do any existing hand-held multimeters do so?

None that I know of.
This might be possible!

[David Hess]

For a dual-display multimeter, I would very much like to see a readout of the actual burden voltage while measuring amps, measured at the amps jack.

Any problem adding such a feature?  Do any existing hand-held multimeters do so?

Obviously the meter knows the portion of the burden voltage across the current shunt and at low currents that is most of it.  At high currents the rest could be estimated but I wonder how high it is outside of the shunt.

If I need to know the burden voltage, I just measure it with another meter or measure the output side of the current shunt.

[nixfu]

This was one of my favorite EEVBlog videos in a long time. 

[bittumbler]

Why were the Mosfets put on the "high" side?

Wouldn't it be easier if they are on the low side?

[Kleinstein]

The circuit has to work for both polarities of the current - so there is no such thing as a well defined high side and low side. The voltage over the shunts is rather small (e.g. < 50-100 mV) compared to the gate drive (e.g. 5 V) - so it does not really matter. The important factor to know is that the mosfets work for current in both directions.

For low AC currents the leakage problem might occur with high peak currents. Usually this should not such a big problem, as accuracy in the AC ranges is often not that good anyway. If there are really peaks above about 100 mV one should give a warning if (DC coupled) peak measurement is active. So one might want to avoid using a possible 50µA AC range all the way to 50 µA, as the might be slight errors at more than 30 µA. So auto ranging should be a little more intelligent than just checking the AC value.

One may also like more hysteresis in automatic switching ranges in current ranges, as switching the shunt might change the actual current in low voltage circuits. So even a kind of "scale hold" might be a good idea, to prevent of automatic changes to a larger shunt if needed.

[bittumbler]

The circuit has to work for both polarities of the current - so there is no such thing as a well defined high side and low side. The voltage over the shunts is rather small (e.g. < 50-100 mV) compared to the gate drive (e.g. 5 V) - so it does not really matter. The important factor to know is that the mosfets work for current in both directions.

I agree for normal usage. But at least to me it would "look nicer", if the Mosfets were at "Common" or "GND".
And in case of a big overload the shunts can briefly have a much higher voltage drop (many Volts), until the fuse acts. Thus pushing the "high side" Mosfets in linear mode or even off mode.

Use depletion mode Mosfets to have them "on" while the multimeter is off? Or do they have other drawbacks?

[SeanB]

I agree for normal usage. But at least to me it would "look nicer", if the Mosfets were at "Common" or "GND".
And in case of a big overload the shunts can briefly have a much higher voltage drop (many Volts), until the fuse acts. Thus pushing the "high side" Mosfets in linear mode or even off mode.

Use depletion mode Mosfets to have them "on" while the multimeter is off? Or do they have other drawbacks?

Problem is the on resistance of the MOSFET is not a thing that is stable with time, current or temperature, so you now have to make a differential measurement of the current, and the differential amplifier has to cope with high voltages along with the small measured current. Much simpler to have a single ended measurement and the switching MOSFETs have a small change in gate voltage instead. If all switches are off then you have a zero voltage measurement, and if you accidentally connect the current range to a voltage source, the only voltage applied to the amplifier or ADC is that developed across the lowest value shunt, not the full input voltage across the shunt diodes dynamic resistance.

[Kleinstein]

Usual depletion mode FETs are high R_on and only for low current.  There are special ones (e.g. GaN based) for high currents but these are really expensive and still not that low in R_on. So depletion mode fets are not an option.

One might be able to use a depletion mode FET to control the gate, so that in off mode the fets will be on - as long as the battery is still alive. So not using the meter to conduct a high current with the batteries empty would be the users responsibility.

Depending on cooling the meter might still survive quite some current even without power - as the fet gets hot, the forward voltage will drop and thus reduce the power a little. I have not seen thermally operated switches that turn on at high temperature - this would be a option for this rare case.

[David Hess]

IXYS has high current depletion mode MOSFETs available but they are not cheap.  The ones made by Supertex (bought by Microchip) are lower current.

[splin]

Using the (parasitic) reverse diodes of the MOSFETs for protection purposes will affect the precision of the sensitive ranges, due to their leakage currents. These lower ranges resolve nA, maybe pA currents.

Especially, if you apply a reverse current, in the "negative" direction of the DMMs input jacks, these 50mV burden voltage already create relevant forward currents in these MOSFET diodes, and if you combine two of such MOSFETs in parallel, this would be the case for either direction.

More common precision current protection schemes use either the low leakage of the B-E diode of a bipolar transistor, like in the HP3458A, which includes power dissipation protection by software, or they use at least two diodes of a rectifier bridge in series, combined with an OpAmp to reduce these diode leakage currents to near zero, like in most of HPs 6 1/2 digits DMMs.



Frank

Does anyone have any information on those transistors? They are listed in the BOM as SQW5011A (or similar) 'TRANSISTOR-PNP SI TO-220AB PD=1.67W'. I can't find anything having tried various combos such as 5QWSOIIA, etc.

Presumably they were selected for very low leakage which I'm guessing is unusual in a power transistor.

Also, what might be the purpose of the purpose of the FET switch Q206? My guess is that it's used to reduce the leakage of those transistors when performing an auto-calibration, with K201 disconnecting the input. The leakage could originate from the PCB round the unguarded traces to the diodes and relay, which they perhaps wouldn't want to guard because, being normally grounded, could couple ground noise into the signal lines or perhaps excessive leakage into the guard trace.

Not very convinced though, and if so, why not simply have the protection transistors on the other side of the relay, or use the other relay contact to apply the protection when needed? The current input is earthed by K201 when it is disconnected so the transistors wouldn't dissipate any power when an overload is applied when K201 is off.

Presumably the protection transistors and high current shunts are protected from excessive dissipation by disconnecting K201 in the event of an overload, but how is it done quickly if the meter is in a low current range? I guess that when it sees an over-range current it would set the DC amplifier to x1 and make a quick measurement to detect an excessive voltage which could damage one or more current shunts or see the 6 to 8V [EDIT: wrong, 2V max - see follow on post] or so that would indicate a protection transistor + diode conducting. This wouldn't be particularly quick though if a 10NPLC, 200ms (the longest integration time) measurement had just started.

In the meantime the protection transistor could be carrying 3A (1A fast acting fuse) [EDIT: 3A for the 200ms before the fuse blows] dissipating 15 to 21W. [EDIT: 3 to 4W] - I don't suppose that's likely to be a problem thermally for a TO220 device but what about the leakage through that device subsequently, or if it took a hit with a much larger current, say 50A for 1ms? Reverse base-emitter breakdown can damage a transistor so perhaps these were selected because of their ability to handle this scenario. I suppose the meter is designed to remain operational, within spec, after any type of current overload provided the correct fuse is fitted - after all they didn't seem to see the need to add a H/W overload cct to reset the relay ASAP as they did for over voltage protection.

[Cerebus]

Also, what might be the purpose of the purpose of the FET switch Q206? My guess is that it's used to reduce the leakage of those transistors when performing an auto-calibration, with K201 disconnecting the input. The leakage could originate from the PCB round the unguarded traces to the diodes and relay, which they perhaps wouldn't want to guard because, being normally grounded, could couple ground noise into the signal lines or perhaps excessive leakage into the guard trace.

The signal 'boot' at the source of Q206 is a low impedance copy of whatever the input signal is (in this case the 'HI' end of the shunt voltage) and is used to bootstrap the protection 'diodes' Q207 and Q208 to reduce the leakage across them by making sure they have the same voltage at both sides. Q206 can be used to apply or remove the bootstrap voltage under control of the 'HBST' signal (you can guess that the BST in that signal name stands for 'bootstrap').

K201 can isolate the input and also lifts the ground from the protection 'bridge' at the same time. This looks like the way you'd want things during autocal and with the 'bridge' floating it would make sense to turn off the bootstrap drive to it to reduce leakage of the bootstrap current.

My guess is that K201 is quite delicate and K200 comparatively meaty and that a little dance is done between them so that K201 never has to make or break the input without K200 being closed while this goes on. I suspect that neither K200 or K201 are used as protection, there's a fuse upstream of them and that, the spark gap and the 'diode' 'bridge' are enough to do the protection task way more effectively than any relay ever will. I think the relays are purely for isolation to keep any outside influences (such as a lead left plugged in) from affecting calibration cycles.

[splin]

Also, what might be the purpose of the purpose of the FET switch Q206? My guess is that it's used to reduce the leakage of those transistors when performing an auto-calibration, with K201 disconnecting the input. The leakage could originate from the PCB round the unguarded traces to the diodes and relay, which they perhaps wouldn't want to guard because, being normally grounded, could couple ground noise into the signal lines or perhaps excessive leakage into the guard trace.

The signal 'boot' at the source of Q206 is a low impedance copy of whatever the input signal is (in this case the 'HI' end of the shunt voltage) and is used to bootstrap the protection 'diodes' Q207 and Q208 to reduce the leakage across them by making sure they have the same voltage at both sides. Q206 can be used to apply or remove the bootstrap voltage under control of the 'HBST' signal (you can guess that the BST in that signal name stands for 'bootstrap').

I made a mistake when I was trying to work out this circuit thinking that diodes CR201/would clamp the collector of Q207 to +/-.6V preventing them being bootstrapped when measuring current - overlooking that the maximum signal and bootstrap are +/-100mV in this mode. But the question remains as to why would you ever want to remove the bootstrap using q206? [Edit: could it be to allow the protection transistors to be checked as part of the self-test? Except that would require K201 to be operated and thus the input jack would have to be open circuit to allow a voltage to be applied via the acal signal]

K201 can isolate the input and also lifts the ground from the protection 'bridge' at the same time. This looks like the way you'd want things during autocal and with the 'bridge' floating it would make sense to turn off the bootstrap drive to it to reduce leakage of the bootstrap current.

My guess is that K201 is quite delicate and K200 comparatively meaty and that a little dance is done between them so that K201 never has to make or break the input without K200 being closed while this goes on. I suspect that neither K200 or K201 are used as protection, there's a fuse upstream of them and that, the spark gap and the 'diode' 'bridge' are enough to do the protection task way more effectively than any relay ever will. I think the relays are purely for isolation to keep any outside influences (such as a lead left plugged in) from affecting calibration cycles.

Good points. K201 is a Panasonic DS2E-SL2 with 2A/30V contact rating and K200 is an even smaller custom Panasonic reed relay, probably with even lower contact ratings. So those relays probably aren't used for protection.

Doh! More importantly I've just realized why most of my question was rubbish - for some odd reason I had it in mind that Q207/208 were NPN and was using the reverse base-emitter breakdown as an ultra-low leakage protection mechanism - except that they are quite obviously PNPs (as I actually pasted into my previous post)! 

Thus the protection will clamp the input to 2 diode drops - say 2V maximum rather than the 6V to 7V if they had been NPNs.

Anyway the type of transistor is still interesting; the bootstrap/signal offset is calibrated out with a DAC so I'm guessing the maximum base-emitter voltage on Q207/208 would be a few uVs - perhaps a little more given how far the guard travels on the board, especially if the board isn't particularly clean and the total leakage into the guard increases significantly. So how much 'leakage' current might a standard PNP power transistor have with a few microvolts of base voltage and 100mV collector-emitter, at 55C ambient (say 60C internally)? The transistors are Nat-semi parts. Could they be specially designed for this type of application?

Even with a 2V clamp, the 9ohm, 10mA shunt R211 would be dissipating nearly three times its 125mW rating so the software will have to detect an overload and disconnect K202. As before, I believe this could take slightly more than 200ms worst case, but that's not likely to be long enough to cause damage, and any shift in value due to the shunt's overload would be calibrated out at the next autocal. Similarly the 1ohm 100mA shunt, rated at 3W could be dissipating 4W with a 2V clamp but the fuse would blow within 150ms at 4A - so all is well after all. 

[Kleinstein]

The clamp voltage should be less than 2 V, more like 1.5 V. So the power ratings for the shunts should be OK, though just barely.

At a voltage in the low mV range, the forward current of a transistor or diode can be really low. Further up in reply #79 bktemp did a measurement for a big MOSFET (which also has a diode inside). There the current was below about 200 pA even at 50 mV.  So the leakage in the transistors should be of similar size, possibly even smaller. I don't think the transistors need to be that special.

For dave's proposed circuit, bootstrapping the protection diodes is not an option as, as there is just one diode drop. Even this can cause a power problem with the 11 A fuse and the small case. For possibly critical low current ranges (e.g. less than 50 µA) there would be the option to use an transimpedance amplifier and this way keep the voltage even lower than the currently planed 50 mV, and this way reduce leakage a little.

[Cerebus]

Anyway the type of transistor is still interesting; the bootstrap/signal offset is calibrated out with a DAC so I'm guessing the maximum base-emitter voltage on Q207/208 would be a few uVs - perhaps a little more given how far the guard travels on the board, especially if the board isn't particularly clean and the total leakage into the guard increases significantly. So how much 'leakage' current might a standard PNP power transistor have with a few microvolts of base voltage and 100mV collector-emitter, at 55C ambient (say 60C internally)? The transistors are Nat-semi parts. Could they be specially designed for this type of application?

You seem to be missing that Q207/Q208 have the base-collector leads shorted, i.e. they are 'diode wired' with the base-emitter junctions being used purely as low reverse leakage diodes with lousy reverse breakdown voltages. My go-to transistor for that usage is a selected 2N3904/MMBF3904 but under these circumstances it'd be too small for the currents under consideration. Note that the BOM calls out a Pd of under 2W yet a TO-220 case - that suggests a low-power power transistor. A non obsolete part that might fit the bill (depending on leakage) might be a BD139/140 or a ZTX851/951 (the latter 5A continuous/20A peak rated but in an e-line case or there's a SOT-223 part available).

The offset between the main signal and bootstrap signal is going to be on the close order of 100uV. The offset correction DAC is 8 bit and (from memory) has an effective adjustment range of +/-30mV giving one LSB = 234 uV.

[phunkz]

Hi
i'm again the guy who asked the stupid question why not to use 2 fuses in parallel.

Here I do have a  late 70's Hartmann & Braun Multavi3, 30 Amps, analogue Multimeter with an automatic fuse.
(I don't want to send it in for a teardown, i like it very much, therefore some additional information:
http://www.radiomuseum.org/r/hartmann_multavi_3.html :-)

But what about an automatic fuse? Regardless of price and size, because you are planning an absolut high-end-multimeter...
As far as i know they do have 2 paths using slow bimetal and fast magnetic switching, so they should met any specs in terms of speed or current...

Sorry, just another stupid question...

[Kleinstein]

The difficult part is getting the high DC voltage rating - normal circuit breakers are not good at this. So it would be a really special automatic fuse.

The circuit is still for a relatively small battery powered meter - so some compromises need to be made to make is small and power power.

For the proposed circuit the burden voltage for the 10 A range would be more than 100 mV, as this range would also have to got through the MOSFETs - so maybe another 20-50 mV. So the combined power dissipation might become an issue. Using a separate plug like before would be unsafe with just one fuse. One less plug would also free up some space.

[mqsaharan]

Hi everybody.
First of all, a big thanks to Dave for his efforts on this video blog. I have always found his blog very informative.
Secondly, after watching this video, I started searching for commercially available DMMs with low burden voltages and found that Gossen is using such technique to lower the burden voltages but they are prohibitively expensive for hobby work. So, I kept searching and found another company that is using the same idea, Rishabh. I do not know how good their DMMs are. I have attached the specifications for their cheaper meters. On paper at least, they have low burden voltages but cannot say how do they behave in practice.

Qasim.

[Kleinstein]

My guess is the Rishabh meter is ignoring the voltage drop at the fuse. The values look to low just for an HRC fuse alone.  The 66 mV burden value is otherwise suspicious for a 6600 count meter. So this is just the voltage drop on the shunt.
The low resolution also helps in having a low burden, as the needed voltage resolution is not that high. There is also a very limited number of ranges and at low current the mechanical switch can be use.

[Chupacabras]

It might be a stupid question, but:
Is it possible to omit fuse in design? Just use mosfets (possibly several of them), and switch them off immediately when higher current is detected?
Would it work or be dangerous?

[bktemp]

It might be a stupid question, but:
Is it possible to omit fuse in design? Just use mosfets (possibly several of them), and switch them off immediately when higher current is detected?
Would it work or be dangerous?

Try finding a 1kV mosfet (or even more, look up the voltage levels for CAT ratings) with less resistance than a fuse.
So no, it won't work, and yes it would be dangerous, because mosfets typically fail short circuit if you overload them (too high current or too high voltage).

[analogNewbie]

Since the body diode is always there, there is forward current through the protection MOSFET body diode, why does not it matter in the uA range? Because the burden voltage is only 50mV?

[David Hess]

Since the body diode is always there, there is forward current through the protection MOSFET body diode, why does not it matter in the uA range? Because the burden voltage is only 50mV?

If you are talking about in the HP3458A schematic, those are JFETs.

Some designs use lateral MOSFETs with a separate substrate connection so the body diode does not conduct.

[Cerebus]

Since the body diode is always there, there is forward current through the protection MOSFET body diode, why does not it matter in the uA range? Because the burden voltage is only 50mV?

If you are talking about in the HP3458A schematic, those are JFETs.

Some designs use lateral MOSFETs with a separate substrate connection so the body diode does not conduct.

No, I think the question is about Dave's proposal, where 2 back to back MOSFETs body diodes are used as replacements for the more common bridge rectifier as a protection element in parallel with the current shunt.

[bktemp]

Since the body diode is always there, there is forward current through the protection MOSFET body diode, why does not it matter in the uA range? Because the burden voltage is only 50mV?

I did some measurements here:
https://www.eevblog.com/forum/blog/eevblog-929-designing-a-better-multimeter/msg1042459/#msg1042459
At 50mV there is a small current flowing through the diode, but it is low enough even for uA range. At 100mV or 150mV the current can be a problem for uA range at elevated temperatures.

[kvresto]

Hi Everyone.
In the pic I've uploaded Dave has drawn up a circuit to illustrate his idea. I’m not too clear on how this circuit deals with common high mode voltages? I hope someone can clear it up for me.

I assume the DMM will be battery powered, so the mux and opamps will run off this voltage, but if I insert this in a system that runs say 20V, would I have to select components that have a high common mode voltage?

I see GND is marked, but this is not really GND, after all its in series with the system being measured.

EDIT: what if this cct was powered by the system under test?

[Kleinstein]

The DMM has no other external connection. So the circuit ground will just float with a external voltage. So the circuit does not have to deal a lot on that, except for shielding so that higher frequency external field will not disturb the readings like it does with some of the newer Aligent meters.

[drussell]

Secondly, after watching this video, I started searching for commercially available DMMs with low burden voltages and found that Gossen is using such technique to lower the burden voltages but they are prohibitively expensive for hobby work. So, I kept searching and found another company that is using the same idea, Rishabh. I do not know how good their DMMs are. I have attached the specifications for their cheaper meters. On paper at least, they have low burden voltages but cannot say how do they behave in practice.

You could always just buy one of Dave's uCurrent units...  They are specifically designed to be a better current measuring device than most multimeters.

P.S.: Welcome to the forum! 

[kvresto]

Absolutely sensational Dave!!    I don't actually work in the industry and rely on Blogs such as this one for information, and I always find it here. A couple of well placed dumb questions always seems to clear things up. Cant wait for what's to come. Thanking all those answering my questions.

[Yansi]

How does the Gossen meter (and possibly others) get over the fact, that when the meter is powered off and current applied accross the current terminals, the mosfet won't overheat, due to excessive current being pushed through their body diodes?

10A through the body diode of the protection mosfet, is something like 10W of heat. How do they get away with that? Haven't seen any heatsinking in there.

[f4eru]

3 possible solutions to that:

The easy solution : With a multimeter in off state you could still put voltage to the gate to hold the mosfet shorted.

The complex solution : a LV oscillator-step up ("joule thief" like) to get charge into the gate, from the diode voltage.

The expensive solution : use a depletion mode mosfet, which is saturated @ Vgs=0V

[Yansi]

I think the second two won't happen, as we know the Gossen uses IRL3302, not depletion mode mosfets. And joulethief-like converter is most likely just an imagination. Probably only the battery voltage on the gate in off state.

But does the user manual state then the meter should never get amps going through when the battery goes flat, otherwise it's toast?