Should a shunt for a copper-coil moving coil meter be copper also?

[Peter]

I must be going stupid here but sure the answer is YES.

Take a 10A shunt for a 100uA moving coil meter whose resistance is say
1k and thus 100mV FSD.

The shunt (about 0.01 ohm) is going to be a very low impedance
*voltage* source to the meter.

The 100uA meter is 100mV FS but obviously what makes it move is
current, not voltage, but the current it draws is going to vary with
the tempco of the wire it is wound with.

So, surely, the shunt should be copper so that the output of the shunt
tracks the meter movement sensitivity.

Unless the meter movement is wound with nichrome... is it? I have
never heard of that.

All the shunts I can see are nichrome or similar which is surely wrong
for use with a copper-wound meter?

 

[[email protected]]

I must be going stupid here but sure the answer is YES.

Take a 10A shunt for a 100uA moving coil meter whose resistance is say
1k and thus 100mV FSD.

The shunt (about 0.01 ohm) is going to be a very low impedance
*voltage* source to the meter.

The 100uA meter is 100mV FS but obviously what makes it move is
current, not voltage, but the current it draws is going to vary with
the tempco of the wire it is wound with.

So, surely, the shunt should be copper so that the output of the shunt
tracks the meter movement sensitivity.

Unless the meter movement is wound with nichrome... is it? I have
never heard of that.

All the shunts I can see are nichrome or similar which is surely wrong
for use with a copper-wound meter?

My first thought was like you, that copper would have a horrid tempco,
but thinking about it a little more, it gets interesting. The
deflection of the meter is proportional to the current in the
windings, which will be proportional to the resistance of the shunt
and inversely proportional to the resistance of the coil. So, you
have a point, the tempco should drop out, neglecting any self-heating
differences.

A nichrome coil makes no sense. Resistance in the coil would make it
less efficient. These movements are all about efficiency (turning the
smallest possible current into the desired deflection). Resistance
just reduces accuracy (draws more power from the DUT).

 

[John S]

My first thought was like you, that copper would have a horrid tempco,
but thinking about it a little more, it gets interesting. The
deflection of the meter is proportional to the current in the
windings, which will be proportional to the resistance of the shunt
and inversely proportional to the resistance of the coil. So, you
have a point, the tempco should drop out, neglecting any self-heating
differences.

And therein lies the (minor) problem. Until the coil comes up to the
temperature of the shunt, there is an error.

 

[whit3rd]

I must be going stupid here but sure the answer is YES.

Take a 10A shunt for a 100uA moving coil meter whose resistance is say
1k and thus 100mV FSD.

All the shunts I can see are nichrome or similar which is surely wrong
for use with a copper-wound meter?

Copper and nichrome is (very similar to) a copper/chromel thermocouple.
So, you can expect a bit of a Seebeck coefficient, about 20 uV/K. It's not just
resistance that gives you a thermal effect, but thermal gradients (you wouldn't
want a nichrome shunt to be mounted vertically, self-heating would warm
the top more than the bottom).

The meter is a bit hard to judge; the magnetic field, the coil dimensions, the
hairspring return force, and the wiring resistance ALL have some thermal
response. It'd be easy to make the meter with a series thermistor if
copper-versus-nichrome-shunt TC of resistivity needed nulling.

 

[[email protected]]

And therein lies the (minor) problem. Until the coil comes up to the
temperature of the shunt, there is an error.

Of course but both should be sized such that there is minimal
self-heating. That error is impossible to correct so has to be
minimized in some other way.

 

[Grant]

I must be going stupid here but sure the answer is YES.

Take a 10A shunt for a 100uA moving coil meter whose resistance is say
1k and thus 100mV FSD.

The shunt (about 0.01 ohm) is going to be a very low impedance
*voltage* source to the meter.

The 100uA meter is 100mV FS but obviously what makes it move is
current, not voltage, but the current it draws is going to vary with
the tempco of the wire it is wound with.

So, surely, the shunt should be copper so that the output of the shunt
tracks the meter movement sensitivity.

Unless the meter movement is wound with nichrome... is it? I have
never heard of that.

All the shunts I can see are nichrome or similar which is surely wrong
for use with a copper-wound meter?

The meter drive of 100uA +/- copper variation is a minute fraction
of your 10A full scale measured value. Go figure the error due to
meter being in parallel to the shunt. Think of the shunt as a very
low source impedance voltage source, thus the meter resistance is
close to irrelevant.

Grant.

 

[Phil Allison]

"Peter"

I must be going stupid here but sure the answer is YES.

Take a 10A shunt for a 100uA moving coil meter whose resistance is say
1k and thus 100mV FSD.

The shunt (about 0.01 ohm) is going to be a very low impedance
*voltage* source to the meter.

The 100uA meter is 100mV FS but obviously what makes it move is
current, not voltage, but the current it draws is going to vary with
the tempco of the wire it is wound with.

So, surely, the shunt should be copper so that the output of the shunt
tracks the meter movement sensitivity.
All the shunts I can see are nichrome or similar which is surely wrong
for use with a copper-wound meter?

** Using your own example, the shunt dissipates 1 watt at 10 amps load.

So it is gonna heat quite a bit while the coil of the amp meter does not.

Better use a low tempco material for the shunt.



..... Phil

 

[Peter]

Try it. There are two things to measure: the meter's input resistance
tempco, and the meters's current-deflection tempco. Or just its
deflection vs temp with a constant-voltage input.

The shunt may self-heat from the load current, which would complicate
matters if you used a copper shunt. Current shunts are usually
manganin. If you want a copper shunt, you'd probably have to make your
own.

How accurate do you need this to be? Copper is about 0.4% per degree
C.

OK - from the feedback so far is looks like I am right i.e. a shunt
with a "zero" TC with a copper-wound moving coil meter is going to
create a 0.4% degC error just due to the ambient temp variation.

Actually I think 4 % variation over a 10C range is a lot! One can read
even a cheap meter to 1% and this meter has a mirror scale.

It's a little project somebody is building. Funnily enough, in the
1970s when at college, I built a similar but much more sophisticated
version, which had the meter in the feedback loop of an op-amp based
constant current source so the meter's TC was irrelevant. Remember an
AD504 op-amp?

But the chap's point about shunt self heating is also valid and would
probably make a bigger error, because a 0.01 ohm shunt at 10A will
dissipate 1W which is a lot for a shunt of any realistic size. As a
rough order, 1 square foot of 16swg ally is 1degC/W.

So the best way to do this is to use a zero TC shunt (I have a 0.01
ohm precision resistor handy) and then separately temperature
compensate the meter itself with a thermistor os something...

 

[P E Schoen]

"Peter" wrote in message

I must be going stupid here but sure the answer is YES.

Take a 10A shunt for a 100uA moving coil meter whose resistance is
say 1k and thus 100mV FSD.

The shunt (about 0.01 ohm) is going to be a very low impedance
*voltage* source to the meter.

The 100uA meter is 100mV FS but obviously what makes it move
is current, not voltage, but the current it draws is going to vary
with the tempco of the wire it is wound with.

So, surely, the shunt should be copper so that the output of the
shunt tracks the meter movement sensitivity.

Unless the meter movement is wound with nichrome... is it? I have
never heard of that.

All the shunts I can see are nichrome or similar which is surely
wrong for use with a copper-wound meter?

I worked for many years at a company (Edgerly Instrument Labs, or EIL*),
where they modified and calibrated meters of all types, and I soon learned
that meters with a full scale current larger than the meter movement itself
contained a shunt made of manganin or constantan wire or bus bar, and the
meter movement had a "swamping" resistor in series that made it a voltmeter
with FS of 50 mV or 100 mV.

Analog meters designed for measuring DC current were usually about 4 mA and
10 mV (2.5 ohms), and a 10 ohm series resistor made it 50 mV. Thus the
tempco was "swamped" by a factor of 5 and good enough for standard 2% and 1%
meters. Also, the coils of these meters were often wound with a copper alloy
that had a lower tempco than pure copper. 100 uA meters usually are about
200-500 ohms and rarely as much as 1000, so they could also be used with a
swamping resistor for 100 mV shunts and sometimes also for 50 mV shunts. For
self-contained meters less than 1A FS, shunts could be more than 100 mV.

I also learned that blank scale meters from the factory had magnets that
were fully charged so that the sensitivity was about 10% higher than their
nominal rating. So you only needed to use a standard resistor value (based
on ohms per volt for voltmeters), and then the meters were demagnetized with
a pulsating magnetic field until they read correctly. A fully charged meter
movement would gradually lose some magnetism and sensitivity, and even after
demagnetizing they would drift as much as 1%, so for highest accuracy they
would be allowed to "age" for a few hours and then finally calibrated
exactly.

We made some analog meters that were specified as 1/4%, and they had to be
calibrated very carefully. They even needed hand drawn scales because the
movements were not exactly linear - probably slight variations in the
magnetic gap. I also found that the linearity could be tweaked slightly by
varying the angle of the movement in the demagnetizer coil.

IIRC, these meters also had a switch for several current ranges, and they
used a precision copper resistor as part of a temperature compensating
network.

These were special meters used for a military application, and sold for
several hundred dollars (1974 dollars). They probably only needed to be 1%,
but each middleman in the chain of command probably demanded twice the
accuracy and so we had to take extra time and effort to get them to an
accuracy much better than they were originally designed for. But we still
made a nice profit, since the blank meters were only maybe $10 or so.

Paul

*http://search.juneauempire.com/fast...tring="EDGERLY INSTRUMENTS LABORATORIES INC."
*http://www.railroadsignals.us/amis/eil/index.htm

 

[Peter]

Analog meters designed for measuring DC current were usually about 4 mA and
10 mV (2.5 ohms), and a 10 ohm series resistor made it 50 mV. Thus the
tempco was "swamped" by a factor of 5 and good enough for standard 2% and 1%
meters. Also, the coils of these meters were often wound with a copper alloy
that had a lower tempco than pure copper.

That makes complete sense.

On this project, I can make the shunt voltage 300mV rather than 100mV
and that that will improve the ambient tempco by 3x.

We also have a linear resistance range (done with a floating current
source done with an op-amp, with several resistors switched to give
different currents i.e. resistance ranges) and that feeds into the
rest of the multimeter on a 1V range, so the tempco of the movement
will be swamped by 10x.

100 uA meters usually are about

I think times have moved on in that nobody makes the 10uA movements
which were used in the best analog multimeters in the 1970s (100k ohms
per volt) and the smallest meters one can easily get are 100uA (maybe
50uA if you are not fussy about the other stuff) and the ones I have
seen are 1k.

 

[Phil Hobbs]

"Peter"

** Using your own example, the shunt dissipates 1 watt at 10 amps
load. So it is gonna heat quite a bit while the coil of the amp meter
does not. Better use a low tempco material for the shunt.

Good point. There are materials that have low tempcos and a low
thermocouple coefficient against copper. I think the best is manganin,
which is about -600 nV/K against copper. Temperature-compensating the
meter itself is a better idea.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Hobbs]

"Peter" wrote in message







I worked for many years at a company (Edgerly Instrument Labs, or EIL*),
where they modified and calibrated meters of all types, and I soon
learned that meters with a full scale current larger than the meter
movement itself contained a shunt made of manganin or constantan wire or
bus bar, and the meter movement had a "swamping" resistor in series that
made it a voltmeter with FS of 50 mV or 100 mV.

Analog meters designed for measuring DC current were usually about 4 mA
and 10 mV (2.5 ohms), and a 10 ohm series resistor made it 50 mV. Thus
the tempco was "swamped" by a factor of 5 and good enough for standard
2% and 1% meters. Also, the coils of these meters were often wound with
a copper alloy that had a lower tempco than pure copper. 100 uA meters
usually are about 200-500 ohms and rarely as much as 1000, so they could
also be used with a swamping resistor for 100 mV shunts and sometimes
also for 50 mV shunts. For self-contained meters less than 1A FS, shunts
could be more than 100 mV.

I also learned that blank scale meters from the factory had magnets that
were fully charged so that the sensitivity was about 10% higher than
their nominal rating. So you only needed to use a standard resistor
value (based on ohms per volt for voltmeters), and then the meters were
demagnetized with a pulsating magnetic field until they read correctly.
A fully charged meter movement would gradually lose some magnetism and
sensitivity, and even after demagnetizing they would drift as much as
1%, so for highest accuracy they would be allowed to "age" for a few
hours and then finally calibrated exactly.

We made some analog meters that were specified as 1/4%, and they had to
be calibrated very carefully. They even needed hand drawn scales because
the movements were not exactly linear - probably slight variations in
the magnetic gap. I also found that the linearity could be tweaked
slightly by varying the angle of the movement in the demagnetizer coil.

IIRC, these meters also had a switch for several current ranges, and
they used a precision copper resistor as part of a temperature
compensating network.

These were special meters used for a military application, and sold for
several hundred dollars (1974 dollars). They probably only needed to be
1%, but each middleman in the chain of command probably demanded twice
the accuracy and so we had to take extra time and effort to get them to
an accuracy much better than they were originally designed for. But we
still made a nice profit, since the blank meters were only maybe $10 or so.

Paul

*http://search.juneauempire.com/fast...tring="EDGERLY INSTRUMENTS LABORATORIES INC."

*http://www.railroadsignals.us/amis/eil/index.htm

Interesting stuff, thanks. I'm a huge fan of old Avometers, especially
the Model 8 Mk IV. (The Mk V was the beginning of the downward slope.)
I have a Mk IV from the collection of a guy who also collected Rolls
Royces--not a trace of stickiness, and accuracy well within 0.5% on
volts and amps.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Hobbs]

Zeranin is even better than manganin; lower tempco and lower emf against copper.
But it's harder to get than manganin.

And of course True Audiophiles turn up their noses at anything less.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[boB]

Hadn't heard of Zeranin before, I don't think.

We use Cupron wire for shunts.

boB

 

[josephkk]

I must be going stupid here but sure the answer is YES.

Take a 10A shunt for a 100uA moving coil meter whose resistance is say
1k and thus 100mV FSD.

The shunt (about 0.01 ohm) is going to be a very low impedance
*voltage* source to the meter.

The 100uA meter is 100mV FS but obviously what makes it move is
current, not voltage, but the current it draws is going to vary with
the tempco of the wire it is wound with.

So, surely, the shunt should be copper so that the output of the shunt
tracks the meter movement sensitivity.

Unless the meter movement is wound with nichrome... is it? I have
never heard of that.

All the shunts I can see are nichrome or similar which is surely wrong
for use with a copper-wound meter?

Well, let us consider what the delta temperature of the meter coil is for
full scale deflection and the change in resistance in delta percent. For
valid readings we would want both the short term and the long term delta
percent resistance to be the same, ignoring the differences in delta
temperature itself. Does that lead to using the same materials for both
devices?

?-)

 

[Phil Allison]

"joseph kook"

All the shunts I can see are nichrome or similar which is surely wrong
for use with a copper-wound meter?

Well, let us consider what the delta temperature of the meter coil is for
full scale deflection and the change in resistance in delta percent.

** Nonsense.

The dissipation in a 1000 ohm coil passing 100uA is only **10uW** at FSD.

Ambient temp error is the only issue.


For
valid readings we would want both the short term and the long term delta
percent resistance to be the same, ignoring the differences in delta
temperature itself. Does that lead to using the same materials for both
devices?

** Cooper is impractical as a shunt cos the resistance is too low.

Eg: for 10mohms, 44cm of of 1mm dia wire is needed.

If made from Nichrome, the wire is then only 0.67cm long.


..... Phil

 

[Peter]

"joseph kook"

Well, let us consider what the delta temperature of the meter coil is for
full scale deflection and the change in resistance in delta percent.

** Nonsense.

The dissipation in a 1000 ohm coil passing 100uA is only **10uW** at FSD.

Ambient temp error is the only issue.

Yes - and quite big an issue.

 

[Phil Allison]

"Peter"

"Phil Allison"

Yes - and quite big an issue.

** Only if you imagine it is.

Even in the *worst case* with no compensation or swamping resistor in
series, the error in reading is - 0.4% per degree C rise in temp. For normal
lab/workshop conditions, this is perfectly acceptable as the ambient temp
range is small.

BTW:

I just ran a test with a old, Japanese 20kohms/V multimeter of mine.

It has a 50mV, 50uA movement with 1050 ohms internal resistance plus a
1200ohm resistor permanently in series.

The movement *alone* shows the expected 0.4% per degree C change in reading.

But in the meter, the change becomes less that half of that, which is
insignificant in normal use.

If ever I need to know a DC current accurately, I get out my DMM.



..... Phil

 

[P E Schoen]

"Phil Allison" wrote in message

"Peter"

** Only if you imagine it is.

Even in the *worst case* with no compensation or swamping resistor in
series, the error in reading is - 0.4% per degree C rise in temp. For
normal lab/workshop conditions, this is perfectly acceptable as the
ambient temp range is small.

I just ran a test with a old, Japanese 20kohms/V multimeter of mine.

It has a 50mV, 50uA movement with 1050 ohms internal resistance plus a
1200ohm resistor permanently in series.

The movement *alone* shows the expected 0.4% per degree C change in
reading.

But in the meter, the change becomes less that half of that, which is
insignificant in normal use.

If ever I need to know a DC current accurately, I get out my DMM.

For higher current readings, such as 100 amps, a 100 mV shunt dissipates 10
watts, which can cause significant heating above ambient. So unless the
meter movement is in close thermal proximity, the temperature of the shunt
will be much higher. This is why shunts are made from low tempco materials.

The 1000 ohms for the 50 uA movement is about what I would have expected and
close to my recollection that a 100 uA meter would be about 500 ohms. In
fact, it may even be less, since it requires half the number of turns for
twice the current, and those turns could be a thicker wire.

That being said, I have a 100-0-100 uA Model 1120 blank scale edgewise
meter, essentially 200 uA FS, and the movement measures 536 ohms at 62F.

Paul

 

[George Herold]

On Sun, 20 Oct 2013 14:02:51 -0400, Phil Hobbs

Hadn't heard of Zeranin before, I don't think.

We use Cupron wire for shunts.

Hmmm, I hadn't heard of either of them. Cupron seems to be a trade name for constantan. (AFAICT)

George H.