3-3
Section
Sharing the Communications Power Supply
32
•
If the nodes are already located at both sides of the power supply, move the
power supply in the direction that requires the larger current capacity.
•
If thin cable is being used, replace it with thick cable.
If, after following the above procedure, B is still less than A, go to Step 2 and de-
termine the actual position of the nodes by the formula calculation method.
Example: Uneven Current Consumption on Left and Right
The following example shows a Network that requires power to be supplied for
240 m on thick cable. The power supply must be located off the center of the
Network. Because the current consumption is not the same on the left and right
sides of the Network, the current flow to one side will be insufficient if the power
supply is connected in the very center of the Network. Placing it off center just a
little allows power to be supplied to all nodes.
Communications
power supply
120 m
120 m
3 m
max.
1.25 A
0.5 A
0.25 A
0.25 A
0.85 A
Node
Node
Node
Node
Node
1.1 A
Node
Trunk line
(5-wire cable)
Terminating resistor
Trunk line
Power supply cable
Terminating resistor
Trunk line
(5-wire cable)
Total power supply length on left = Total power supply length on right = 120 m
Total current consumption on left: 1.1 + 1.25 + 0.5 = 2.85 A
Total current consumption on right: 0.25 + 0.25 + 0.85 = 1.35 A
Maximum current for thick cable for 120 m (see previous table) = approx. 2.5 A
(using straight line approximation between 100 to 150 m)
Because the total current flow on the left side (2.85 A) is greater than the maxi-
mum current on the left side (2.56 A), the power supply cannot be placed at the
center of the network and supply current to all nodes.
This problem can be corrected by moving the communications power supply as
shown in the following diagram.
Communications
power supply
100 m
140 m
3 m
max.
1.25 A
0.5 A
0.25 A
0.25 A
0.85 A
Node
Node
Node
Node
Node
1.1 A
Node
Trunk line
(5-wire cable)
Terminating resistor
Trunk line
Power supply cable
Terminating resistor
Trunk line
(5-wire cable)
Total power supply length on left = 100 m
Total power supply length on right = 140 m
Total current consumption on left: 1.1 + 1.25 = 2.35 A
Total current consumption on right: 0.5 + 0.25 + 0.25 + 0.85 = 1.85 A
Maximum current for thick cable for 100 m (see previous table) = 2.93 A
Maximum current for thick cable for 140 m (see previous table) = 2.1 A
(using straight line approximation between 100 to 150 m)
Because the total current flow on both the left and right sides is now less than the
maximum current, the power supply can be placed as shown in the diagram and
supply current to all nodes.
