270 HEIDENHAIN Technical Manual iTNC 530 HSCI
Output signals:
Calculation of the
voltage drop in "0"
state
Current (I
Off
= 500 µA) also flows in "0" state of the PLC output for component-
related reasons.
If resistive loads with high input impedance (> 15 kOhm) and a low-level lower
switching threshold are operated directly (without an interconnected relay) on
the PLC output, a shunt resistor may be necessary under certain
circumstances. Typical values of the shunt resistor are e.g. 5 kOhm or 10
kOhm. The voltage at the output in "0" state should be clearly (approx. 50 %)
below the lower switching threshold of the resistive load.
Calculation example—Resistive load does not require shunt resistor:
Reversing load relay directly on PLC output, data sheet specifications:
Rated control voltage U
c
:24 V DC
Rated operating current I
c
:3 mA
Switching threshold "0" state U
Low
: 9.6 V
Switching threshold "1" state U
High
: 19.2 V
In "0" state of the output, the voltage drop has to be less than 9.6 V. Calculation
of the input resistance R
c
of the reversing load relay:
Calculation of the voltage drop U
Low
in "0" state at the output:
With 4 V, the voltage drop is clearly below the switching threshold for the "0"
state (9.6 V). The reversing load relay can be operated directly at the PLC
output.
PLD-H
Min. output voltage for "1" signal 3 V below supply voltage
PLC outputs must neither be connected to a 24 V supply, nor to other PLC
outputs with a difference in potential. Otherwise, the voltage present at the
PLC outputs is transmitted to the power supply. As a result, the PLC
outputs that can be switched off may nevertheless be supplied with this
voltage.
R
c
Uc
Ic
---------
24
V
3 mA
------------------
8
kΩ== =
U
Low
R
c
I
Off
⋅ 8 kΩ 500 μA⋅ 4 V== =
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