Mitsubishi Electric MELSEC-L Series User Manual page 79

(3) A load momentarily turns on when powering on the external power supply.
An incorrect output occurs due to floating capacitance(C) between collector and emitter of photocoupler.
When a high sensitivity load (such as solid state relay) is used, this incorrect output may occur.
Photocoupler
Cause
When the rise time of voltage of the external power supply is 10ms or less, current (Ic) flows to gate of transistor (Tr1) of next stage due to
floating capacitance (C) between collector and emitter of photocoupler. Then, output Y0 turns on for approx. 100µs.
Action 1: Check that the rise time of the external power supply is 10ms or more. And then, install a switch (SW1) for turning on or off
external power supply to the primary side of it.
Action 2: When installing the SW1 to the secondary side of it is required, make the rise time to 10ms or more and connect a capacitor and
resistor as shown below.
For the following source output modules, take Action 1 on the above due to no effect of Action 2 by the characteristics of the external power
supply circuit.
• LY40PT5P
• LY41PT1P
• LY42PT1P
• LH42C4PT1P
Action
Sink type output
12/24VDC
Output module
C
Tr1
Ic Y0
12/24VDC
Constant-voltage
COM
circuit
24VDC
Primary side
External power supply
SW1
Y0
R1: Several tens of ohms
Power capacity ≥ (external power supply current*
Load
C1: Several hundreds of microfarads 50V
Example R1 = 40Ω, C1 = 300μF
R1
SW
C1
*1 Check the consumption current of the external power supply for modules used.
*2 Select the power capacity of resistance to be 3 to 5 times lager than the actual power
COM
consumption.
24VDC
Case
Output Y0
Load
SW
SW: External
power supply
(24VDC) at ON
Secondary side
Output module
External power supply terminal
Programmable
controller
Time constant = C1 × R1 = 300 × 10
CHAPTER 8 TROUBLESHOOTING
Approx.100 s
10ms or less
2
1
) × resistance value × (3 to 5)
-6
× 40
-3
= 12 × 10 [s]
= 12 [ms]
8
* 2
77