15
Current Sensing Resistors
The current sensing resistor should have low resistance (to
minimize power dissipation), low inductance (to minimize
di/dt induced voltage spikes which could adversely aect
operation), and reasonable tolerance (to maintain overall
circuit accuracy). Choosing a particular value for the resis-
tor is usually a compro-mise between minimizing power
dissipation and maximizing accu-racy. Smaller sense re-
sistance decreases power dissipation, while larger sense
resistance can improve circuit accuracy by utilizing the full
input range of the HCPL -7800(A).
The rst step in selecting a sense resistor is determining
how much current the resistor will be sensing. The graph
in Figure 20 shows the RMS current in each phase of a
three-phase induction motor as a function of average
motor output power (in horsepower, hp) and motor drive
supply voltage. The maximum value of the sense re-sis-
tor is determined by the current being measured and the
maxi-mum recommended input voltage of the isolation
amplier. The maximum sense resistance can be calculated
by taking the maxi-mum recommended input voltage and
dividing by the peak current that the sense resistor should
see during normal operation. For example, if a motor will
have a maximum RMS current of 10 A and can experience
up to 50% overloads during normal op-eration, then the
peak current is 21.1 A (=10 x 1.414 x 1.5). Assuming a
maximum input voltage of 200 mV, the maximum value of
sense resistance in this case would be about 10 mΩ.
The maximum average power dissipation in the sense
resistor can also be easily calculated by multiplying the
sense resistance times the square of the maximum RMS
current, which is about 1 W in the previous example. If
the power dissipation in the sense resistor is too high, the
resistance can be decreased below the maximum value
to decrease power dissipation. The minimum value of the
sense resistor is limited by precision and accuracy require-
ments of the design. As the resistance value is reduced, the
output voltage across the resistor is also reduced, which
MOTOR PHASE CURRENT - A (rms)
15
5
40
10 25 30
0
35
0 35
25
10
20
440 V
380 V
220 V
120 V
30
20
5
15
MOTOR OUTPUT POWER - HORSEPOWER
means that the oset and noise, which are xed, become
a larger percentage of the signal amp-litude. The selected
value of the sense resistor will fall somewhere between
the minimum and maximum values, depending on the
particular requirements of a specic design.
When sensing currents large enough to cause signicant
heating of the sense resistor, the temperature coecient
(tempco) of the resistor can introduce nonlinearity due
to the signal dependent temperature rise of the resistor.
The eect increases as the resistor-to-ambient thermal
resistance increases. This eect can be minimized by reduc-
ing the thermal resistance of the current sensing resistor
or by using a resistor with a lower tempco. Lowering the
thermal resistance can be accomplished by repositioning
the current sensing resistor on the PC board, by using
larger PC board traces to carry away more heat, or by
using a heat sink.
For a two-terminal current sensing resistor, as the value
of resistance decreases, the re-sistance of the leads become
a signicant percentage of the total resistance. This has
two primary eects on resistor accuracy. First, the eective
resistance of the sense resistor can become dependent
on factors such as how long the leads are, how they are
bent, how far they are inserted into the board, and how
far solder wicks up the leads during assembly (these is-
sues will be discussed in more detail shortly). Second, the
leads are typically made from a material, such as copper,
which has a much higher tempco than the material from
which the resistive element itself is made, resulting in a
higher tempco overall.
Both of these eects are eliminated when a four-terminal
current sensing resistor is used. A four- terminal resistor
has two additional terminals that are Kelvin-connected
directly across the resistive element itself; these two ter-
minals are used to monitor the voltage across the resistive
element while the other two terminals are used to carry the
load current. Because of the Kelvin connection, any voltage
drops across the leads carrying the load current should have
no impact on the measured voltage.
When laying out a PC board for the current sensing resis-
tors, a couple of points should be kept in mind. The Kelvin
connections to the resistor should be brought together
under the body of the resistor and then run very close to
each other to the input of the HCPL-7800(A); this mini-
mizes the loop area of the connection and reduces the
possibility of stray magnetic elds from interfering with
the measured signal. If the sense resistor is not located
on the same PC board as the HCPL-7800(A) circuit, a
tightly twisted pair of wires can accomplish the same
thing.
Also, multiple layers of the PC board can be used to
increase current carrying capacity. Numerous plated-
through vias should surround each non-Kelvin terminal
of the sense resistor to help distribute the current be-
tween the layers of the PC board. The PC board should
Figure 20. Motor Output Horsepower vs. Motor Phase Current
and Supply Voltage.