EEVblog #929 - Designing A Better Multimeter

[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.