In article ,
alan wrote:
On 03/01/2013 11:05, Dave Plowman (News) wrote:
In article ,
alan wrote:
As long as the connecting leads can handle the current and are "low"
resistance, the resistance of the leads makes no difference to the
measurement.

Swapping leads on my various DVMs - using what appears to be decent
quality leads - will give different resistance readings for very low
values. Try it.


It doesn't matter if the measuring leads are 0.01ohms or 1 ohm when
measuring the _Voltage_ across a shunt resistor with a DVM (or
voltmeter) with a high impedance input. There may be 100s of Amps
flowing through the shunt resistor but microAmps in the measuring leads.

But those connecting leads I was referring to will make a difference to
the results. In the same way as test leads do when measuring very low
resistance. As when doing that you're actually measuring voltage drop
across a resistance.

--
*Isn't it a bit unnerving that doctors call what they do "practice?"

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
alan wrote:
On 03/01/2013 11:05, Dave Plowman (News) wrote:
In article ,
alan wrote:
As long as the connecting leads can handle the current and are "low"
resistance, the resistance of the leads makes no difference to the
measurement.
Swapping leads on my various DVMs - using what appears to be decent
quality leads - will give different resistance readings for very low
values. Try it.


It doesn't matter if the measuring leads are 0.01ohms or 1 ohm when
measuring the _Voltage_ across a shunt resistor with a DVM (or
voltmeter) with a high impedance input. There may be 100s of Amps
flowing through the shunt resistor but microAmps in the measuring leads.

But those connecting leads I was referring to will make a difference to
the results. In the same way as test leads do when measuring very low
resistance. As when doing that you're actually measuring voltage drop
across a resistance.

If you're measuring the voltage drop due to 100 amps across a few
milliohms as in the case proposed, then the error due to probe
resistance should be negligible, and well within the tolerances of
anything outside a standards lab. Assuming 2kpv for the meter, (500
microamp/ volt FSD), the meter impedance will be on the order of dozens
of ohms, with a current of less than 100 microamps. If the meter is the
now standard? (Electronic multimeter) 10 Megohm input impedance, then
the current in the leads will be hundredths of a microamp. The
resistance of the leads and their connection to the shunt should be
hundredths of an ohm at most, given clean contacts and a steady hand.
The major errors are going to be bad contact between the probes and the
shunt, thermal noise in the shunt, thermal noise in the leads ands
meter, and induced currents from nearby AC circuitry, especially stuff
like switch mode supplies, which can generate some very strange induced
waveforms.

The biggest error in this measurement will probably come from the change
in resistance of the circuit being measured by adding and removing the
shunt, unless the shunt is permanently in circuit, with a proper meter
connection for it. In this case only, the effect of the meter being
connected or not on the circuit being measured will be minimal. Now, how
accurate is the meter, and will these effects be swamped by meter error?

One way round this problem is to use the supply lead as the shunt
resistor, and calibrate the voltmeter accordingly.

--
Tciao for Now!

John.

In article ,
John Williamson wrote:
But those connecting leads I was referring to will make a difference to
the results. In the same way as test leads do when measuring very low
resistance. As when doing that you're actually measuring voltage drop
across a resistance.

If you're measuring the voltage drop due to 100 amps across a few
milliohms as in the case proposed, then the error due to probe
resistance should be negligible, and well within the tolerances of
anything outside a standards lab

Quite. But that wasn't my point. It was made in relation to the comment
that the connecting leads to the high current shunt wouldn't make any
difference. I'm saying they will and must be taken into account. Unless
they are *vastly* oversized.

--
*Never miss a good chance to shut up *

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
John Williamson wrote:
But those connecting leads I was referring to will make a difference to
the results. In the same way as test leads do when measuring very low
resistance. As when doing that you're actually measuring voltage drop
across a resistance.

If you're measuring the voltage drop due to 100 amps across a few
milliohms as in the case proposed, then the error due to probe
resistance should be negligible, and well within the tolerances of
anything outside a standards lab

Quite. But that wasn't my point. It was made in relation to the comment
that the connecting leads to the high current shunt wouldn't make any
difference. I'm saying they will and must be taken into account. Unless
they are *vastly* oversized.

Ah, right. yes, unless the shunt and connections are vastly oversized,
they will affect the readings by changing the circuit conditions. One
reason for using the existing leads as the shunt or a clamp meter if
possible.

--
Tciao for Now!

John.

On Tue, 1 Jan 2013 12:19:55 -0800 (PST), wrote:

As part of another project, would like to take some High Current DC measurements.

I have a couple of multimeters ... but obviously not going to be any good for around 80-100A

At the price these are, with shunt, I don't see the point of ****ing
about, although making your own is good for a laugh just out of
principle.
http://stores.ebay.co.uk/indu-equip/... 4634.c0.m322

On 03/01/2013 13:48, Dave Plowman (News) wrote:
In article
,
Man at B&Q wrote:
On Jan 3, 11:05 am, "Dave Plowman (News)"
wrote:
In article ,
alan wrote:

As long as the connecting leads can handle the current and are "low"
resistance, the resistance of the leads makes no difference to the
measurement.

Irrelevant. The meter is measuring the *VOLTAGE* dropped across the
reference resistor. No current at all flows in the meter leads.

OK a few fA or less must flow but with modern FET front ends even that
is probably an exaggeration. The voltage dropped in the meter leads
would not be measurable even with state of the art kit.

Swapping leads on my various DVMs - using what appears to be decent
quality leads - will give different resistance readings for very low
values. Try it.

But there the meter leads are both supplying the current and trying to
measure the voltage drop in the load. Four terminal measurement does not
have that problem - that is exactly why it is done that way!

The connecting leads referred to are clearly those to the shunt, hence
the reference to the current rating. They make no difference to the
reading across the shunt for a given set of test leads and DVM.

Well, yes. But my point is in practice they will make a difference.

No it shouldn't if the DVM is measuring voltage. Modern meters have such
a high input impedance that they do not affect the measurement.

Exactly where you measure the voltage between could do, but provided
that the system is a four terminal shunt then the measured voltage
should be stable and reproducible no matter what test leads are used on
the meter.

--
Regards,
Martin Brown

On 02/01/2013 21:52, Rick Hughes wrote:

I don't have such a clamp meter, and impractical to buy one for just one
set of measurement.

A Hall-effect sensor such as
http://www.panucatt.com/Current_Sens..._p/cs-100a.htm
is quite a neat solution for high-current measurement - but it does need
a supply voltage and some interface circuitry.

--
Andy

On Jan 3, 4:56*pm, "Dave Plowman (News)" wrote:
In article ,
* *alan wrote:

On 03/01/2013 11:05, Dave Plowman (News) wrote:
In article ,
* * alan wrote:
As long as the connecting leads can handle the current and are "low"
resistance, the resistance of the leads makes no difference to the
measurement.

Swapping leads on my various DVMs - using what appears to be decent
quality leads - will give different resistance readings for very low
values. Try it.

It doesn't matter if the measuring leads are 0.01ohms or 1 ohm when
measuring the _Voltage_ across a shunt resistor with a DVM (or
voltmeter) with a high impedance input. *There may be 100s of Amps
flowing through the shunt resistor but microAmps in the *measuring leads.

But those connecting leads I was referring to will make a difference to
the results. In the same way as test leads do when measuring very low
resistance. As when doing that you're actually measuring voltage drop
across a resistance.


All measuring instruments change whatever they are trying to measure.
Can't be helped.
In the case of voltmeters, the higher ohms per volt the better as it
draws less current.

On Jan 3, 9:18*pm, Martin Brown
wrote:
On 03/01/2013 13:48, Dave Plowman (News) wrote:

In article
,
* * Man at B&Q wrote:
On Jan 3, 11:05 am, "Dave Plowman (News)"
wrote:
In article ,
* * alan wrote:

As long as the connecting leads can handle the current and are "low"
resistance, the resistance of the leads makes no difference to the
measurement.

Irrelevant. The meter is measuring the *VOLTAGE* dropped across the
reference resistor. No current at all flows in the meter leads.

OK a few fA or less must flow but with modern FET front ends even that
is probably an exaggeration. The voltage dropped in the meter leads
would not be measurable even with state of the art kit.



Swapping leads on my various DVMs - using what appears to be decent
quality leads - will give different resistance readings for very low
values. Try it.

But there the meter leads are both supplying the current and trying to
measure the voltage drop in the load. Four terminal measurement does not
have that problem - that is exactly why it is done that way!

The connecting leads referred to are clearly those to the shunt, hence
the reference to the current rating. They make no difference to the
reading across the shunt for a given set of test leads and DVM.

Well, yes. But my point is in practice they will make a difference.

No it shouldn't if the DVM is measuring voltage. Modern meters have such
a high input impedance that they do not affect the measurement.

Exactly where you measure the voltage between could do, but provided
that the system is a four terminal shunt then the measured voltage
should be stable and reproducible no matter what test leads are used on
the meter.

--
Regards,
Martin Brown

Of course current flows in the voltmeter, how else could it work?
The better the quality of the instrument, the lower current it needs
for full scale deflection, hence the less effect it has on the circuit
measured.

On 03/01/2013 23:24, Andy Wade wrote:
On 02/01/2013 21:52, Rick Hughes wrote:

I don't have such a clamp meter, and impractical to buy one for just one
set of measurement.

A Hall-effect sensor such as
http://www.panucatt.com/Current_Sens..._p/cs-100a.htm
is quite a neat solution for high-current measurement - but it does need
a supply voltage and some interface circuitry.



It is possible to buy a DC clamp meter.

e.g. http://www.maplin.co.uk/dc-ac-curren...timeter-629713 ~£40

On 04/01/2013 08:43, harry wrote:

Of course current flows in the voltmeter, how else could it work?
The better the quality of the instrument, the lower current it needs
for full scale deflection, hence the less effect it has on the circuit
measured.




Even a 4 quid multimeter will have an input impedance in the mega Ohm
range. The inaccuracies due the the meter impedance and measurement
leads is vanishingly small and completely swamped by the specified
accuracy of the shunt resistor and voltmeter.

--
mailto:news{at}admac(dot}myzen{dot}co{dot}uk

On Jan 4, 9:53*pm, alan wrote:
On 04/01/2013 08:43, harry wrote:

Of course current flows in the voltmeter, how else could it work?
The better the quality of the instrument, the lower current it needs
for full scale deflection, hence the less effect it has on the circuit
measured.

Even a 4 quid multimeter will have an input impedance in the mega Ohm
range. The inaccuracies due the the meter impedance and measurement
leads is vanishingly small and completely swamped by the specified
accuracy of the shunt resistor and voltmeter.

--
mailto:news{at}admac(dot}myzen{dot}co{dot}uk

Depends on what you are trying to measure.

If it is the voltage in an electronic device, it may draw more current
than the device.
If it was a washing machine, then neither here nor there.

You have to understand the limitations of your equipment.

In article
,
harry wrote:
Even a 4 quid multimeter will have an input impedance in the mega Ohm
range. The inaccuracies due the the meter impedance and measurement
leads is vanishingly small and completely swamped by the specified
accuracy of the shunt resistor and voltmeter.

--
mailto:news{at}admac(dot}myzen{dot}co{dot}uk

Depends on what you are trying to measure.

If it is the voltage in an electronic device, it may draw more current
than the device.
If it was a washing machine, then neither here nor there.

You have to understand the limitations of your equipment.

In the days when moving coil meters were the norm, allowance had to be
made for the loading of the meter on the measurement. If this was a
problem, you'd use a valve voltmeter with a high input impedance - an
expensive device. Modern DVMs have a similar high input impedance for
pennies. And in general don't need an any allowance made when measuring.

What is more of a problem in practice is a high impedance meter will read
mains voltages where there is no actual connection - caused by
capacitive/inductive coupling. A moving coil meter which draws more
current shows near zero.

--
*Why is a boxing ring square?

Dave Plowman London SW
To e-mail, change noise into sound.

On 01/01/2013 20:19, wrote:
As part of another project, would like to take some High Current DC measurements.



In the end I followed a different route - and on advice here got myself
a nice used Megger AC/DC Clamp Meter DCM2039 off eBay for under £30.

I can always sell it after the job ... but I'm sure I'll find and excuse
to keep it.

On Friday, January 4, 2013 9:53:47 PM UTC, alan wrote:
On 04/01/2013 08:43, harry wrote:

Of course current flows in the voltmeter, how else could it work?
The better the quality of the instrument, the lower current it needs
for full scale deflection, hence the less effect it has on the circuit
measured.

Even a 4 quid multimeter will have an input impedance in the mega Ohm
range. The inaccuracies due the the meter impedance and measurement
leads is vanishingly small and completely swamped by the specified
accuracy of the shunt resistor and voltmeter.

Analogue meters are much lower R than that. Good ones are usually 50k/V, cheapies more like 1/10th that.


NT

On 12/01/13 23:40, wrote:
On Friday, January 4, 2013 9:53:47 PM UTC, alan wrote:
On 04/01/2013 08:43, harry wrote:

Of course current flows in the voltmeter, how else could it work?
The better the quality of the instrument, the lower current it needs
for full scale deflection, hence the less effect it has on the circuit
measured.

Even a 4 quid multimeter will have an input impedance in the mega Ohm
range. The inaccuracies due the the meter impedance and measurement
leads is vanishingly small and completely swamped by the specified
accuracy of the shunt resistor and voltmeter.

Analogue meters are much lower R than that. Good ones are usually 50k/V, cheapies more like 1/10th that.


NT

Ive yet to see one much less than 20kohm/V

--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to
lead are elected by the least capable of producing, and where the
members of society least likely to sustain themselves or succeed, are
rewarded with goods and services paid for by the confiscated wealth of a
diminishing number of producers.

On 12/01/2013 23:40, wrote:

Analogue meters are much lower R than that. Good ones are usually 50k/V, cheapies more like 1/10th th

The OP was attempting to measure 100Amps.

By putting a 5K Ohm resistor in parallel with a 0.01 Ohm shunt resistor
the effective shunt resistance changes by 0.0002%. This is completely
swamped by the accuracy of the shunt resistor and voltmeter.

--
mailto:news{at}admac(dot}myzen{dot}co{dot}uk