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Home > Archive > Electrical Engineering > July 2006 > Current shunt application, use with an ammeter or a voltmeter?
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Current shunt application, use with an ammeter or a voltmeter?
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| itsme.ultimate@gmail.com 2006-07-22, 1:25 pm |
| X-No-Archive: Yes
I have a onemili ohm current shunt rated for 50A and I have two ways of
measuring its output.
I could use a 100 ohm resistance =B5A meter to yield 10=B5A/A reading or
a 10Mohm input voltmeter to yield 1mV/1A reading.
In theory, either works well, but which is the preferred method for a
real life application if it matters at all?
ammeter specifications:
500=B5A F.S. 0.01=B5A resolution (1mA resolution)
0=2E25% + 20 dgt
Voltmeter specifications:
50mV F.S. 1=B5V (1mA resolution, but LSD is too jumpy to be of real
use)
0=2E1% + 20 dgt
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| Phat Bytestard 2006-07-22, 5:25 pm |
| On 22 Jul 2006 09:27:09 -0700, itsme.ultimate@gmail.com Gave us:
>X-No-Archive: Yes
>
>I have a onemili ohm current shunt rated for 50A and I have two ways of
>measuring its output.
>
>I could use a 100 ohm resistance ?A meter to yield 10?A/A reading or
>a 10Mohm input voltmeter to yield 1mV/1A reading.
Wrong. One measures voltage across a current shunt to get a table of
reading that correspond to amperage in the resistor/circuit.
>
>In theory, either works well, but which is the preferred method for a
>real life application if it matters at all?
See above. BTW, your method changes the circuit.
>ammeter specifications:
>500?A F.S. 0.01?A resolution (1mA resolution)
>0.25% + 20 dgt
>
>Voltmeter specifications:
>50mV F.S. 1?V (1mA resolution, but LSD is too jumpy to be of real
>use)
>0.1% + 20 dgt
One reads volts across a current shunt and extrapolates the
corresponding current value for the voltage presented across the
resistor. Basic Ohm's Law.
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| BFoelsch 2006-07-22, 5:25 pm |
| On 22 Jul 2006 09:27:09 -0700, itsme.ultimate@gmail.com wrote:
>X-No-Archive: Yes
>
>I have a onemili ohm current shunt rated for 50A and I have two ways of
>measuring its output.
>
>I could use a 100 ohm resistance µA meter to yield 10µA/A reading or
>a 10Mohm input voltmeter to yield 1mV/1A reading.
>
>In theory, either works well, but which is the preferred method for a
>real life application if it matters at all?
>
>
>ammeter specifications:
>500µA F.S. 0.01µA resolution (1mA resolution)
>0.25% + 20 dgt
>
>Voltmeter specifications:
>50mV F.S. 1µV (1mA resolution, but LSD is too jumpy to be of real
>use)
>0.1% + 20 dgt
Practically, use the voltmeter method. The accuracy of the ammeter
method depends directly on the specified 100 ohm resistance, but you
have not provided an accuracy for that parameter. As a corollary, the
voltmeter method will be acceptably accurate with any commercially
available voltmeter, the ammeter method will work only with a 100 ohm
ammeter, which may or may not always be available.
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| Don Kelly 2006-07-23, 3:25 am |
| ----------------------------
<itsme.ultimate@gmail.com> wrote in message
news:1153585629.518260.5710@p79g2000cwp.googlegroups.com...
X-No-Archive: Yes
I have a onemili ohm current shunt rated for 50A and I have two ways of
measuring its output.
I could use a 100 ohm resistance µA meter to yield 10µA/A reading or
a 10Mohm input voltmeter to yield 1mV/1A reading.
In theory, either works well, but which is the preferred method for a
real life application if it matters at all?
ammeter specifications:
500µA F.S. 0.01µA resolution (1mA resolution)
0.25% + 20 dgt
Voltmeter specifications:
50mV F.S. 1µV (1mA resolution, but LSD is too jumpy to be of real
use)
0.1% + 20 dgt
----------
Either will work. B Foelsch suggests the voltmeter.
Assuming resistances are exact (not true)
Ammeter
At 50A, current in shunt =49.9995 A
current in meter =499.995 microamp Error is negligable
Voltmeter
At 50A current in shunt =50.0 as near as damn
current in meter 0.005 microamps. near as damn
B Foelsch is right but on the other hand, if you want a continuously
available reading, you could use the ammeter and save the voltmeter for
moving around taking other measurements. The error is negligable.
Take your pick
Phats is wrong. Chances are that your ammeter is made as a 50mvFS voltmeter
(say several megohms) with an internal shunt of about 100 ohms.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
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| itsme.ultimate@gmail.com 2006-07-23, 9:25 am |
| X-No-Archive: Yes
One advantage I thought might be present with the current method is
that 100 ohm shunt is inside the meter. I measured the resistance of
the meter using a different meter and its within 1% of 100 ohm.
Since the voltage is in the order of 10=B5V, I thought maybe the
exposed, unshielded high impedance loop to the meter might be
suspecible to magnetic interference and things like thermoelectric
voltage at connections.
Basically, the question is whether use the =B5A range or the mV range on
the same multimeter. Using one of the other doesn't free the meter for
other purpose.
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| no_one 2006-07-23, 1:25 pm |
| mV is the preferred method. Using the ammeter assumes that you are able to
reliably share the current between the shunt and the meter (forming a
current divider). We use this sort of calibrated shunt in many applications
where I work.
<itsme.ultimate@gmail.com> wrote in message
news:1153651021.240008.3280@b28g2000cwb.googlegroups.com...
X-No-Archive: Yes
One advantage I thought might be present with the current method is
that 100 ohm shunt is inside the meter. I measured the resistance of
the meter using a different meter and its within 1% of 100 ohm.
Since the voltage is in the order of 10µV, I thought maybe the
exposed, unshielded high impedance loop to the meter might be
suspecible to magnetic interference and things like thermoelectric
voltage at connections.
Basically, the question is whether use the µA range or the mV range on
the same multimeter. Using one of the other doesn't free the meter for
other purpose.
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| Palindr☻me 2006-07-23, 1:25 pm |
| itsme.ultimate@gmail.com wrote:
> X-No-Archive: Yes
>
> I have a onemili ohm current shunt rated for 50A and I have two ways of
> measuring its output.
>
> I could use a 100 ohm resistance µA meter to yield 10µA/A reading or
> a 10Mohm input voltmeter to yield 1mV/1A reading.
>
> In theory, either works well, but which is the preferred method for a
> real life application if it matters at all?
>
>
> ammeter specifications:
> 500µA F.S. 0.01µA resolution (1mA resolution)
> 0.25% + 20 dgt
>
> Voltmeter specifications:
> 50mV F.S. 1µV (1mA resolution, but LSD is too jumpy to be of real
> use)
> 0.1% + 20 dgt
>
Just one small point which doesn't seem to have been raised. The
possible effects of component or circuit failure.
For example, if the shunt could become open circuit with the meter in
place, then the outcome could be rather different if a low impedance
ammeter is used instead of a high impedance voltmeter.
It is a rather essential part of any engineering project to look at
abnormal, as well as normal, conditions.
--
Sue
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| Don Kelly 2006-07-24, 3:25 am |
| ----------------------------
<itsme.ultimate@gmail.com> wrote in message
news:1153651021.240008.3280@b28g2000cwb.googlegroups.com...
X-No-Archive: Yes
One advantage I thought might be present with the current method is
that 100 ohm shunt is inside the meter. I measured the resistance of
the meter using a different meter and its within 1% of 100 ohm.
Since the voltage is in the order of 10µV, I thought maybe the
exposed, unshielded high impedance loop to the meter might be
suspecible to magnetic interference and things like thermoelectric
voltage at connections.
Basically, the question is whether use the µA range or the mV range on
the same multimeter. Using one of the other doesn't free the meter for
other purpose.
Since it is the same meter-used the voltage range. On the ammeter range,
with the external shunt, a failure of the internal shunt would not be a
problem except for wrong data being displayed, but as the meter* is
basically a voltmeter with "add on" current measurements-why bother.
* it is a digital meter which, as opposed to an analogue meter, is basically
a voltmeter with high impedance bridged by an internal shunt of 100 ohms.
Apology to phat- he was not wrong-but half right.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
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