Mitsubishi Electric MELCEC Q Series User Manual page 194

11. TROUBLESHOOTING
Table 11.3 Faults with the output circuit and the corrective actions (continued)
Situation
The load which
was turned off
is turned on for
Example
a moment at
6
power-off.
(Transistor
output)
Cause
The load [2] which was turned off may be turned on due to
back electromotive force at the time of power-off [1] if an
inductive load is used.
Output module,
combined module
Source output
ON
OFF
Output module,
combined module
Sink output
ON
OFF
[3]
Back electromotive
force
Load
TB1 ON
[2]
TB2 OFF
Load
[1]
Shut off
COM+
CTL-
[3]
Back electromotive
force
Load
TB1 ON
[2]
TB2 OFF
Load
[1]
Shut off
COM-
CTL+
11 - 27
Countermeasure
To prevent the generation of the back
electromotive force, connect diode in
parallel with load where the back
electromotive force has been generated.
Source output
[3]
Back electromotive
force
Load
Sink output
[3]
Back electromotive
force
Load