A40B
Instruction Manual
18
Direct Read-out When Measuring Current
When using a Current Shunt with a meter that allows the displayed value to be
manipulated using math functions, the meter can be configured to give a direct reading of
current. For Example:
In the following scenario, a current source is measured with a 100 mA Current Shunt and
a Fluke 8508A.
Assumptions:
•
Calibrated DC resistance of the Current Shunt (
z
) = 7.999600
•
Calibrated AC-DC difference of the Current Shunt at this frequency = +7 ppm
•
Loading effect at this frequency = -9 ppm
1.
Combine the AC-DC difference and loading effect numbers:
Combined ppm errors = -(+7) -9 = -16 ppm
Note
AC-DC difference error is the amount the AC value would have to be
changed to make it the same as the DC value. To change this into a
frequency flatness error, reverse the sign.
2.
Convert the combined ppm errors into a multiplier (
m
)
000016
.
1
000
,
000
,
1
16
1
=
⎟
⎠
⎞
⎜
⎝
⎛
−
−
=
m
3.
Enter the
z
value (7.99960) into the
z store
of the 8508A
4.
Enter the
m
value (1.000016) into the
m
store
of the 8508
5.
Activate
divide by z
and
multiply by m
on the 8508A
The 8508A now gives a direct reading of the current being measured.
To measure the same current level at a different frequency; calculate a new value for
m
and enter the new value into the 8508A
m
store. The
z
value remains the same.
Theory of Operation
The Current Shunts are essentially a resistance through which a current is passed to
develop a voltage. They are intended for precision AC-DC current measurements in the
range 1 mA to 100 A at frequencies DC to 100 kHz. Inductance tends to make accurate
measurement difficult as current and frequency increases.
Coaxial Shunts
The coaxial shunts are especially designed for low-inductance. The design is similar to
that developed by Mendeleyev Institute, St Petersburg. The key components of the
coaxial shunts are the printed circuit board (PCB) construction and the precision resistive
elements. The design provides several parallel current paths (struts) each with its own
resistive element. Each current path is symmetrical to minimize mutual inductance.
Resistive elements comprise 1 to 8 discrete high precision bulk foil resistors per strut
depending on the current rating of the Current Shunt.
Individual PCB struts, collected in a cylindrical construction to reduce external magnetic
fields, form the path through the shunt. Figure 4 shows the current path through one of
the strut/resistor elements.
Summary of Contents for A40B
Page 6: ...A40B Instruction Manual iv...
Page 8: ...A40B Instruction Manual vi...
