User`s manual
7-19
(3) Countermeasure leakage current on input device
In the following device, there is also leakage current at OFF. If the leakage current is higher than
the OFF level of the input module, the input module may not turn OFF, or noise margin at OFF
state may drop.
1) Limit switch with LED
2) Limit switch with neon lamp (the neon lamp is connected in parallel with the contact point.)
3) Proximity and photo switches of AC two lines system
In the AC two lines system, even at OFF there is leakage current from current consumption of
the internal circuit, and this might prevent the input module from falling in the OFF state. This is
nominated as “leakage current” in the specifications of photo switches etc. Make sure that this
value is less than the OFF level of the input module.
4) Built-in triac, thyristor and contact point output of surge killers
Some device has CR device as a surge killer for the purpose of avoiding the check mistakes of
triac and thyristor, and the leakage current of this CR may prevent the input module from falling
in the OFF state. In such a case, the best remedy is to remove the CR. If this is not possible, use
the C value of the CR of less than 0.033 mF for 100 VAC; and that of less than 0.015 mF, for 200
VAC.
Reference
Calculation of leakage current i
L
i
L
=
V: Power supply voltage
V
F
: Voltage drop in the forward direction of LED
R
S
: Current limit resistance
R
in
: Input impedance of input module
V – V
F
R
s
+ R
in
V
F
IN
COM
Rin
R
S
Leakage
current
Input module
iL
V
IN
COM
Rin
Leakage
current
Input module
iL
V
IN
COM
Rin
Leakage
current
Input module
Internal
circuit
Reference
Calculation of leakage current i
L
i
L
=
V: Power supply voltage
f: Power frequency (50/60Hz)
C: Capacitor
IN
COM
Rin
Leakage
current
Input module
R
C
V
Surge killer
V
2p fc