January 2012 3.15 Digital PLC Inputs/Outputs 221
Calculation example—Resistive load requires shunt resistor:
Interlocking switch directly at PLC output, data sheet specifications:
Rated control voltage U
c
:28 V DC
Rated operating current I
c
: 2.2 mA
Switching threshold "0" state U
Low
: 3.6 V
Switching threshold "1" state U
High
: 17.4 V
In "0" state of the output, the voltage drop has to be less than 3.6 V. Calculation
of the input resistance R
c
of the interlocking switch:
Calculation of the voltage drop U
Low
in "0" state at the output:
The shunt resistor at the output should be such that the voltage drop at the
output in "0" state goes clearly below the lower switching threshold, e.g. R
p
=
5 kW:
With a voltage drop of 1.79 V in "0" state, the value falls below the lower
switching threshold of the interlocking switch. If the shunt resistor of 5 kW is
used, operation directly at the PLC output is possible in this case.
R
c
Uc
Ic
---------
28V
2.2 mA
-----------------------
12.73 kΩ== =
U
Low
R
c
I
Off
⋅ 12.73 kΩ 500μA⋅ 6.36 V== =
1
R
total
-------------------
1
R
c
--------
1
R
p
---------+=
1
R
total
-------------------
1
12.73 kΩ
------------------------------
1
5kΩ
-------------+=
R
total
3.59kΩ=
U
Low
3.59 kΩ 500μA⋅ 1.79 V==
To Next Page
Loading...
