User`s manual

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MELSEC-Q
11 I/O MODULE TROUBLESHOOTING
<Calculation example of Example 4>
Consider a switch with LED indicator connected to the QX40, giving a leakage current
of 4mA when a 24VDC power is turned on.
QX40
Input module
Leakage
current 4mA
24VDC
(1) The 1.7mA OFF current of the QX40 is not satisfied. Hence, connect a resistor as
shown below.
QX40
Iz=1.7mA
I
R=
2.3mA
24VDC
4mA
5.6K
Input impedance
R
(2) Calculate the resistor value R as indicated below.
To satisfy the 1.7mA OFF current of the QX40, the resistor R to be connected may
be the one where 2.3mA or more will flow.
I
R
: I
Z=
Z (Input impedance): R
R Z (Input impedance) = 5.6=4.13[k ]
2.3
1.7
I
R
I
Z
R<4.13k
.
Assuming that resistor R is 3.9k
, the power capacity W of resistor R is:
W = (input voltage)
2
÷ R = 28.8
2
÷ 3900 = 0.213(W)
(3) The power capacity of the resistor selected is 3 to 5 times greater than the actual
power consumption. A 3.9 (k
), 1 to 2 (W) resistor may therefore be connected to
the terminal in question.