Mitsubishi Electric A173UHCPU User Manual page 129

5. INSPECTION AND MAINTENANCE
Symptom
Input signal does not
turn OFF
Table 5.7 Troubleshooting Input Circuits (cont.)
Cause
• Sneak path due to use of two power supplies.
1 2
E E
1 2
E >E
<Resistor Resistance Calculation for Example 4>
For the case with a limit switch with LED indicator connected to A1SX40,
causing 4 mA leak current.
3.6K
1) This circuit does not turn OFF because the A1SX40 OFF current of 1 mA is not
reached. Therefore, the connection of a resistor is required, as shown below.
3.6K
2) Resistance calculation
To achieve the A1SX40 OFF current of 1 mA, a resistor should be connected
such that a current of 3 mA min. flows through the resistor.
A resistance value of R < 1.1 k
If a 1 k resistor is used, the resistor R power capacity (W) is given by:
W = (Current value)
3) In practice, a 1 [k ] 0.5 [W] resistor, which has a power capacity 3 to 5
times the actual power consumption, is connected across the terminals where
the problem exists.
• Use a single power supply.
• Connect a diode to prevent sneak paths, as
shown in the diagram.
Input module
A1SX40
4mA leak current
Input module
24VDC
A1SX40
4mA
I =1mA
Z
Input impedance
I =3mA
R
3.3K
24VDC
2 2
R = 0.003 (A) 1000 ( ) = 0.009 (W)
5 − 18
Corrective Action
L
1 2
E E
Input module