Mitsubishi QX10 User Manual page 141

11 I/O MODULE TROUBLESHOOTING
Table 11.2 Output Circuit Problems and Corrective Actions (Continued)
Condition
The load
operates only by
powering on the
external power
supply.
(transistor
output)
When an output
is turned on, a
load connected
to the other
output is also
turned on.
(transistor
output (source
type))
11 - 8
Cause
The polarity to connect the external power supply is reverse.
Transistor
output module
Y0
COM
Output element
protection diode
If the external power supply is connected with wrong polarity,
current may flow across an output element protection diode to
the load.
If the wire connecting 0V of an external power supply and a
common of a load is cut off or disconnected, a current flows to
the load that is off due to a parasitic circuit of the output
element that is off.
If a current keeps flowing under the above condition, a failure
may occur.
Load
Incorrect Correct
External
power
supply
MELSEC-Q
Corrective action
Connect the external power supply with
correct polarity.
Connect the external power supply and
loads correctly.
To prevent the condition described on the
left, connect a diode to each output terminal
as shown below.
11 - 8