EEVblog #929 - Designing A Better Multimeter

[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.