65
For High-density I/O Units:
29.3
5.9 x I – 5
––––––––
R =
kΩ max.
W =
2.3
R
–––
W min.
I = leakage current in mA
7-4 Output Circuits
Output Leakage Current If there is a possibility of leakage current causing a transistor or triac to malfunc-
tion, connect a bleeder resistor across the output as shown below.
PC
Load power supply
OUT
COM
Bleeder resistor
L
R
Determine the value and rating for the bleeder resistor using the following for-
mula.
E
on
I
–––
R
Where
E
on
= ON voltage of the load in V
I = leakage current in mA
R = bleeder resistance in kΩ
Output Surge Current When connecting a transistor or triac Output Unit to an output device having a
high surge current (such as an incandescent lamp), care must be taken to avoid
damage to the Output Unit. The transistor and triac Output Units are capable of
withstanding a surge current of ten times the rated current. If the surge current
for a particular device exceeds this amount, use the circuit shown below to pro-
tect the Output Unit.
R
L
+
OUT
COM
Another way of protecting the Output Unit lets the load draw a small current
(about one third the rated current) while the output is OFF, significantly reducing
the surge current. This circuit (shown below) not only reduces the surge current,
but also reduces the voltage across the load at the same time.
R
L
+
OUT
COM
Output Circuits Section 7-4
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