Chapter 11 Troubleshooting 11-10
11.4.2 Output circuit troubles and corrective actions
The following describes possible troubles with input circuits, as well as their corrective actions.
Condition Cause Corrective Action
When the output is
off, excessive
voltage is applied to
the load.
yLoad is half-wave rectified inside (in some cases, it is true
of a solenoid)
yWhen the polarity of the power supply is as shown in ①,
C is charged. When the polarity is as shown in ②, the
voltage charged in C plus the line voltage are applied
across D. Max. voltage is approx. 2√2.
*) If a resistor is used in this way, it does not pose a
problem to the output element. But it may make the
performance of the diode (D), which is built in the load,
drop to cause problems.
y Connect registers of tens to hundreds KΩ across the
load in parallel.
The load doesn’t
turn off.
y Leakage current by surge absorbing circuit, which is
connected to output element in parallel.
y Connect C and R across the load, which are of registers
of tens KΩ. When the wiring distance from the output
module to the load is long, there may be a leakage current
due to the line capacity.
When the load is
C-R type timer, time
constant fluctuates.
y Leakage current by surge absorbing circuit, which is
connected to output element in parallel.
y Drive the relay using a contact and drive the C-R type
timer using the since contact.
y Use other timer than the C−R contact some timers have
half-ware rectified internal circuits therefore, be cautious.
The load does not
turn off.
y Sneak current due to the use of two different power
supplies.
E1<E2, sneaks. E1 is off (E2 is on), sneaks.
y Use only one power supply.
y Connect a sneak current prevention diode.
If the load is the relay, etc, connect a counter-electromotive
voltage absorbing code as shown by the dot line.
R
Load
R
Load
C
Output
Load
E
E
E1
Load
E2
Output
C
R
Load
Leakage current
Output
~
C
R
Load
Leakage current
Output
~
X
T Timer
Output~
C
R
Load
D
~
C
RLoad
R
D
~