Wiring I/O Units
Section 3-5
If the leakage current is less than 1.3 mA, there should be no problem. If the leak
age current is greater than 1.3 mA, determine the value and rating for the bleed
er resistor using the following formulas.
-
-
I = leakage current in mA
7.2
kW max.
R =
2.4 x I - 3
2.3
W =
W min.
R
where
I = leakage current in mA
R = Bleeder resistance (kW)
W = Bleeder resistor wattage (W)
Output Units
Output Short Protection
Output devices and Output Units can be damaged if the load connected to an
output terminal is shorted. Attach a fuse to the output circuit to protect your sys
-
tem. A fuse is recommended even if the Output Unit is provided with an internal
fuse (e.g., Transistor and Triac Output Units), to increase easy of maintenance
and provide extra protection.
Interlock Circuits
When the PC controls an operation such as the clockwise and counterclockwise
operation of a motor, provide an external interlock such as the one shown below
to prevent both the forward and reverse outputs from turning ON at the same
time.
Interlock circuit
MC2
00501
Motor clockwise
MC1
MC2
PC
MC1
00502
Motor counterclockwise
This circuit prevents outputs MC1 and MC2 from both being ON at the same
time. Even if the PC is programmed improperly or malfunctions, the motor is pro
-
tected.
Output Leakage Current
If a transistor or triac Output Unit is used to drive a low voltage load, the leakage
current may prevent the output device from turning OFF. To prevent this, con-
nect a bleeder resistor in parallel with the load as shown in the following table.
OUT
L
R
Load power supply
PC
Bleeder resistor
COM
Select the bleeder resistor using the following formula.
E
ON
R
I
where
I = leakage current in mA
R = Bleeder resistance (kW)
Eon = ON voltage of the load
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