I wonder if this works:

https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1

the idea being to just ask the DVM to measure the voltage or the
current. The DVM would be a benchtop, probably a Keithley 2100 or
something. The issue is whether the current path is always low
resistance.

Has anyone done this?

On Sat, 22 Apr 2023 11:21:18 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

>
>I wonder if this works:
>
>https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
>
>the idea being to just ask the DVM to measure the voltage or the
>current. The DVM would be a benchtop, probably a Keithley 2100 or
>something. The issue is whether the current path is always low
>resistance.
>
>Has anyone done this?

I have not, and have the strong impression that switching from amps to
volts under load will end badly. At the very least, the V and I
measurements will interfere with one another - this is not a use case
discussed in the manuals.

Joe Gwinn

On Sat, 22 Apr 2023 11:21:18 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

>
>I wonder if this works:
>
>https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
>
>the idea being to just ask the DVM to measure the voltage or the
>current. The DVM would be a benchtop, probably a Keithley 2100 or
>something. The issue is whether the current path is always low
>resistance.
>
>Has anyone done this?

We do this a lot. The shunt is always very low resistance.

On the order of 1 milliohm at least for measuring hundreds of amps.
Bidirectional current too.

A few millivolts of drop can offset your output load V measurement but
looks like you are measuring the input source V ?

boB

On Saturday, April 22, 2023 at 2:21:36 PM UTC-4, John Larkin wrote:
> I wonder if this works:
>
> https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
>
> the idea being to just ask the DVM to measure the voltage or the
> current. The DVM would be a benchtop, probably a Keithley 2100 or
> something. The issue is whether the current path is always low
> resistance.
>
> Has anyone done this?

The ammeter circuit is hard wired. Spec says 0.1 ohms for 1- and 3- Amp scale, and 5.1 ohm for the 10- and 100- mA. That means they have a 5.0R in series with the 0.1 ohm, the 5R gets shunted by a bidirectional FET switch on the high current scales. They have plenty of series R between that circuit and the input to A/D, so it's impossible to damage.

The current path is set and remains fixed once you select the measurement scale. Unless maybe if you select that auto-scale thing. Even then, autoscale may not work for currents. It wouldn't on the handhelds, but the 2100 has the one common jack so it probably should.

On Saturday, 22 April 2023 at 19:21:36 UTC+1, John Larkin wrote:
> I wonder if this works:
>
> https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
>
> the idea being to just ask the DVM to measure the voltage or the
> current. The DVM would be a benchtop, probably a Keithley 2100 or
> something. The issue is whether the current path is always low
> resistance.
>
> Has anyone done this?

Works fine.

On 2023-04-22, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
>
> I wonder if this works:
>
> https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
>
> the idea being to just ask the DVM to measure the voltage or the
> current. The DVM would be a benchtop, probably a Keithley 2100 or
> something. The issue is whether the current path is always low
> resistance.

It probably is, especially if the meter can measure power.

If it cannot measure power there is probably a single ADC and there may be
insufficient isolation between the current shunt and the voltage divider
such that signals on one could peteturb readings of the other.

> Has anyone done this?

I tried on a cheap hand-held and it didn't work well, current showed
on the voltage range.

Do you have the schematic for your instrument?

--
Jasen.
🇺🇦 Слава Україні

On Saturday, April 22, 2023 at 11:21:36 AM UTC-7, John Larkin wrote:
> I wonder if this works:
>
> https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
>
> the idea being to just ask the DVM to measure the voltage or the
> current. The DVM would be a benchtop, probably a Keithley 2100 or
> something. The issue is whether the current path is always low
> resistance.

In a typical VOM, the 'I' sense resistor is switched. So, doing an I
measurement ony happens when V measurement is antiselected.
A few high-current-range meters have an I terminal with a herky
resistor outside the switch connection, but if that's the circuit you
want, just use two V meters and a shunt, no need to trust the DVM
box's innards.

My Fluke 87 chirps a complaint if V and I probe plugs are both inserted...

On Sunday, April 23, 2023 at 2:14:58 AM UTC-4, Anthony William Sloman wrote:
> On Sunday, April 23, 2023 at 4:21:36 AM UTC+10, John Larkin wrote:
> > I wonder if this works:
> >
> > https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
> >
> > the idea being to just ask the DVM to measure the voltage or the
> > current. The DVM would be a benchtop, probably a Keithley 2100 or
> > something. The issue is whether the current path is always low
> > resistance.
> >
> > Has anyone done this?
> There's no end of optimistic idiots.
>
> https://en.wikipedia.org/wiki/Four-terminal_sensing
>
> does go into the right way of doing this. It's called a Kelvin connection..

I'm sure JL knows what a Kelvin connection is.

Like the 6-digit resolution of the meter, it's not needed. But as long as the benchtop is available, it's much better than using a handheld with its bunchy leads and light weight begging to get knocked over or fall to the floor- and the no small matter of viewing its sh___y display, with microscopic measurement range/units, at just the right angle.

>
> https://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin
>
> He died in 1907, so the idea has been around for a while, though it may not have got to Tulane.
>
> --
> Bill Sloman, Sydney

On Sun, 23 Apr 2023 03:08:36 -0700 (PDT), Fred Bloggs
<bloggs.fredbloggs.fred@gmail.com> wrote:

>On Saturday, April 22, 2023 at 10:21:29?PM UTC-4, whit3rd wrote:
>> On Saturday, April 22, 2023 at 11:21:36?AM UTC-7, John Larkin wrote:
>> > I wonder if this works:
>> >
>> > https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
>> >
>> > the idea being to just ask the DVM to measure the voltage or the
>> > current. The DVM would be a benchtop, probably a Keithley 2100 or
>> > something. The issue is whether the current path is always low
>> > resistance.
>> In a typical VOM, the 'I' sense resistor is switched. So, doing an I
>> measurement ony happens when V measurement is antiselected.
>> A few high-current-range meters have an I terminal with a herky
>> resistor outside the switch connection, but if that's the circuit you
>> want, just use two V meters and a shunt, no need to trust the DVM
>> box's innards.
>>
>> My Fluke 87 chirps a complaint if V and I probe plugs are both inserted...
>
>Keithley 2100 has no such problems. The 4-wire is intended for precision resistance measurement, not current. It's main drawback is the front panel buttons need to be manipulated to display a reading. Meaning he doesn't get simultaneous V/I readings. He needs to use two meters for that.
>
>https://research.engineering.ucdavis.edu/woodall/wp-content/uploads/sites/84/2016/02/keithley_2100_manual.pdf

It will be in an automated test set, with a Python program running
everything. No need to push buttons.

The alternative would be a little box with a current shunt or two, and
some SSRs to switch between voltage and current measurements. That
would need logic signals to do the switching, but I can get that from
somewhere.

søndag den 23. april 2023 kl. 17.04.35 UTC+2 skrev Anthony William Sloman:
> On Monday, April 24, 2023 at 12:46:08 AM UTC+10, John Larkin wrote:
> > On Sun, 23 Apr 2023 03:14:11 -0700 (PDT), Fred Bloggs
> > <bloggs.fred...@gmail.com> wrote:
> >
> > >On Sunday, April 23, 2023 at 2:14:58?AM UTC-4, Anthony William Sloman wrote:
> > >> On Sunday, April 23, 2023 at 4:21:36?AM UTC+10, John Larkin wrote:
> > >> > I wonder if this works:
> > >> >
> > >> > https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
> > >> >
> > >> > the idea being to just ask the DVM to measure the voltage or the
> > >> > current. The DVM would be a benchtop, probably a Keithley 2100 or
> > >> > something. The issue is whether the current path is always low
> > >> > resistance.
> > >> >
> > >> > Has anyone done this?
> > >> There's no end of optimistic idiots.
> > >>
> > >> https://en.wikipedia.org/wiki/Four-terminal_sensing
> > >>
> > >> does go into the right way of doing this. It's called a Kelvin connection.
> > >
> > >I'm sure JL knows what a Kelvin connection is.
> >
> > I assume the folks at Fluke and Keithley do too.
> Some DVMs have extra terminals on their front panels to make it easy for people who know what they are doing to exploit the technique.

for current and voltage??

On Saturday, April 22, 2023 at 10:21:29 PM UTC-4, whit3rd wrote:
> On Saturday, April 22, 2023 at 11:21:36 AM UTC-7, John Larkin wrote:
> > I wonder if this works:
> >
> > https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
> >
> > the idea being to just ask the DVM to measure the voltage or the
> > current. The DVM would be a benchtop, probably a Keithley 2100 or
> > something. The issue is whether the current path is always low
> > resistance.
> In a typical VOM, the 'I' sense resistor is switched. So, doing an I
> measurement ony happens when V measurement is antiselected.
> A few high-current-range meters have an I terminal with a herky
> resistor outside the switch connection, but if that's the circuit you
> want, just use two V meters and a shunt, no need to trust the DVM
> box's innards.
>
> My Fluke 87 chirps a complaint if V and I probe plugs are both inserted....

Keithley 2100 has no such problems. The 4-wire is intended for precision resistance measurement, not current. It's main drawback is the front panel buttons need to be manipulated to display a reading. Meaning he doesn't get simultaneous V/I readings. He needs to use two meters for that.

https://research.engineering.ucdavis.edu/woodall/wp-content/uploads/sites/84/2016/02/keithley_2100_manual.pdf

On Sun, 23 Apr 2023 03:14:11 -0700 (PDT), Fred Bloggs
<bloggs.fredbloggs.fred@gmail.com> wrote:

>On Sunday, April 23, 2023 at 2:14:58?AM UTC-4, Anthony William Sloman wrote:
>> On Sunday, April 23, 2023 at 4:21:36?AM UTC+10, John Larkin wrote:
>> > I wonder if this works:
>> >
>> > https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
>> >
>> > the idea being to just ask the DVM to measure the voltage or the
>> > current. The DVM would be a benchtop, probably a Keithley 2100 or
>> > something. The issue is whether the current path is always low
>> > resistance.
>> >
>> > Has anyone done this?
>> There's no end of optimistic idiots.
>>
>> https://en.wikipedia.org/wiki/Four-terminal_sensing
>>
>> does go into the right way of doing this. It's called a Kelvin connection.
>
>I'm sure JL knows what a Kelvin connection is.

I assume the folks at Fluke and Keithley do too.

>
>Like the 6-digit resolution of the meter, it's not needed. But as long as the benchtop is available, it's much better than using a handheld with its bunchy leads and light weight begging to get knocked over or fall to the floor- and the no small matter of viewing its sh___y display, with microscopic measurement range/units, at just the right angle.

The DVM will be rackmount and interfaced to a computer running
automated tests.

>
>>
>> https://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin
>>
>> He died in 1907, so the idea has been around for a while, though it may not have got to Tulane.
>>
>> --
>> Bill Sloman, Sydney

søndag den 23. april 2023 kl. 18.08.01 UTC+2 skrev Anthony William Sloman:
> On Monday, April 24, 2023 at 1:19:18 AM UTC+10, Lasse Langwadt Christensen wrote:
> > søndag den 23. april 2023 kl. 17.04.35 UTC+2 skrev Anthony William Sloman:
> > > On Monday, April 24, 2023 at 12:46:08 AM UTC+10, John Larkin wrote:
> > > > On Sun, 23 Apr 2023 03:14:11 -0700 (PDT), Fred Bloggs
> > > > <bloggs.fred...@gmail.com> wrote:
> > > >
> > > > >On Sunday, April 23, 2023 at 2:14:58?AM UTC-4, Anthony William Sloman wrote:
> > > > >> On Sunday, April 23, 2023 at 4:21:36?AM UTC+10, John Larkin wrote:
> > > > >> > I wonder if this works:
> > > > >> >
> > > > >> > https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
> > > > >> >
> > > > >> > the idea being to just ask the DVM to measure the voltage or the
> > > > >> > current. The DVM would be a benchtop, probably a Keithley 2100 or
> > > > >> > something. The issue is whether the current path is always low
> > > > >> > resistance.
> > > > >> >
> > > > >> > Has anyone done this?
> > > > >> There's no end of optimistic idiots.
> > > > >>
> > > > >> https://en.wikipedia.org/wiki/Four-terminal_sensing
> > > > >>
> > > > >> does go into the right way of doing this. It's called a Kelvin connection.
> > > > >
> > > > >I'm sure JL knows what a Kelvin connection is.
> > > >
> > > > I assume the folks at Fluke and Keithley do too.
> > >
> > > Some DVMs have extra terminals on their front panels to make it easy for people who know what they are doing to exploit the technique.
> >
> > for current and voltage??
> Can't remember the details, but if memory serves it was it would have been fours sets of banana plug sockets to which you could also clamp spade terminals. You'd clamp the shunt resistor between the current terminals, clamp a pair of flat metal links up to the voltage terminals, and feed the current to be measured into the lower (current) pair of banana plug sockets, using cables terminated with banana plugs . Nothing all that exciting, but handy.
>

makes no sense

mandag den 24. april 2023 kl. 06.55.37 UTC+2 skrev Anthony William Sloman:
> On Monday, April 24, 2023 at 2:22:39 AM UTC+10, Lasse Langwadt Christensen wrote:
> > søndag den 23. april 2023 kl. 18.08.01 UTC+2 skrev Anthony William Sloman:
> > > On Monday, April 24, 2023 at 1:19:18 AM UTC+10, Lasse Langwadt Christensen wrote:
> > > > søndag den 23. april 2023 kl. 17.04.35 UTC+2 skrev Anthony William Sloman:
> > > > > On Monday, April 24, 2023 at 12:46:08 AM UTC+10, John Larkin wrote:
> > > > > > On Sun, 23 Apr 2023 03:14:11 -0700 (PDT), Fred Bloggs
> > > > > > <bloggs.fred...@gmail.com> wrote:
> > > > > >
> > > > > > >On Sunday, April 23, 2023 at 2:14:58?AM UTC-4, Anthony William Sloman wrote:
> > > > > > >> On Sunday, April 23, 2023 at 4:21:36?AM UTC+10, John Larkin wrote:
> > > > > > >> > I wonder if this works:
> > > > > > >> >
> > > > > > >> > https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
> > > > > > >> >
> > > > > > >> > the idea being to just ask the DVM to measure the voltage or the
> > > > > > >> > current. The DVM would be a benchtop, probably a Keithley 2100 or
> > > > > > >> > something. The issue is whether the current path is always low
> > > > > > >> > resistance.
> > > > > > >> >
> > > > > > >> > Has anyone done this?
> > > > > > >> There's no end of optimistic idiots.
> > > > > > >>
> > > > > > >> https://en.wikipedia.org/wiki/Four-terminal_sensing
> > > > > > >>
> > > > > > >> does go into the right way of doing this. It's called a Kelvin connection.
> > > > > > >
> > > > > > >I'm sure JL knows what a Kelvin connection is.
> > > > > >
> > > > > > I assume the folks at Fluke and Keithley do too.
> > > > >
> > > > > Some DVMs have extra terminals on their front panels to make it easy for people who know what they are doing to exploit the technique.
> > > >
> > > > for current and voltage??
> > > Can't remember the details, but if memory serves it was it would have been fours sets of banana plug sockets to which you could also clamp spade terminals. You'd clamp the shunt resistor between the current terminals, clamp a pair of flat metal links up to the voltage terminals, and feed the current to be measured into the lower (current) pair of banana plug sockets, using cables terminated with banana plugs . Nothing all that exciting, but handy.
> > >
> > makes no sense
> Didn't stop the meter being produced in large enough volume for one of my employers to buy one. It did come off a production line.

but what would be the point?

On Sunday, 23 April 2023 at 15:46:08 UTC+1, John Larkin wrote:
> On Sun, 23 Apr 2023 03:14:11 -0700 (PDT), Fred Bloggs
> <bloggs.fred...@gmail.com> wrote:
>
> >On Sunday, April 23, 2023 at 2:14:58?AM UTC-4, Anthony William Sloman wrote:
> >> On Sunday, April 23, 2023 at 4:21:36?AM UTC+10, John Larkin wrote:
> >> > I wonder if this works:
> >> >
> >> > https://www.dropbox.com/s/dz1k4d46fwxj8b5/DVM_E_I.jpg?raw=1
> >> >
> >> > the idea being to just ask the DVM to measure the voltage or the
> >> > current. The DVM would be a benchtop, probably a Keithley 2100 or
> >> > something. The issue is whether the current path is always low
> >> > resistance.
> >> >
> >> > Has anyone done this?
> >> There's no end of optimistic idiots.
> >>
> >> https://en.wikipedia.org/wiki/Four-terminal_sensing
> >>
> >> does go into the right way of doing this. It's called a Kelvin connection.
> >
> >I'm sure JL knows what a Kelvin connection is.
> I assume the folks at Fluke and Keithley do too.
> >
> >Like the 6-digit resolution of the meter, it's not needed. But as long as the benchtop is available, it's much better than using a handheld with its bunchy leads and light weight begging to get knocked over or fall to the floor- and the no small matter of viewing its sh___y display, with microscopic measurement range/units, at just the right angle.
> The DVM will be rackmount and interfaced to a computer running
> automated tests.
> >
> >>
> >> https://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin
> >>
> >> He died in 1907, so the idea has been around for a while, though it may not have got to Tulane.

Reality is it depends on the meter. So you can design a universally compatible setup or restrict it to ones that work ok with your setup.