Mitsubishi Electric MELCEC Q Series User Manual page 190

11. TROUBLESHOOTING
Table 11.2 Faults with the input circuit and the corrective actions (Continued)
Situation
Example Input signal does not
5 turn off.
<Sample calculation for Example 4>
When a switch with LED indicator, giving leaking current of 3mA at maximum when
24VDC power is supplied to the A1SX40.
(1)
(2)
(3)
(4)
Cause
• Sneak path due to the use of two power
supplies.
E1
E2
E1 E2
Leakage current 3mA
1mA or less OFF current of the A1SX40 is not satisfied. Hence, connect a resistor as
shown below.
I =2mA
R
24VDC
Calculate the resistance value R as shown below.
To satisfy 1mA or less OFF current of the A1SX40, connect a resistor which flows
2mA or more.
I :I =Z(Input impedance):R
R Z
I
Z
R Z(Input impedance)
I
R
Supposing that the resistance R is 1.5k , the power capacity W of resistor R is:
2
W = (Input voltage) /R = 26.4
Connect a resistor of 1.5(k ) and 2 to 3(W) to a terminal which may cause an error,
since the power capacity of a resistor is selected so that it will be 3 to 5 times greater
than the actual power consumption.
Also, OFF voltage when resistor R is connected will be as follows.
1
1 1
+
1.5[k ] 3.3[k ]
• This satisfies 4V or less OFF voltage of A1SX40.
• Use only one power supply.
• Connect a diode to prevent the sneak path
DC input
Input
module
A1SX40
Input module
24VDC
A1SX40
3mA
I =1mA
Z
Input impedance
R
3.3k
1.0
3.3 1.65[k ]
2.0
2
/1500 = 0.465 [W]
3[mA] = 3.09[V]
11 - 23
Countermeasure
(figure below).
E1
E2
DC input
Input
module