220 HEIDENHAIN Technical Manual MANUALplus 620
If the holding brakes of motors are not driven exclusively over the inverter
outputs for brakes, a protective circuit in the form of a varistor must be used.
Due to the inductance of the holding brakes and any relays used, a voltage
peak that may exceed 1000 V occurs when the exciting current is switched
off. This may destroy other electronics, such as connected PLC inputs/
outputs.
A protective circuit is not necessary only if the holding brakes are driven
exclusively over the inverter outputs for brakes, because the voltage is
limited internally by electronic switches in the inverters.
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 impedances (> 15 kW) 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 circum-
stances. Typical values of the shunt resistor are 5 kW or 10 kW, for example.
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 out-
put.
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 out-
puts that can be switched off may nevertheless be supplied with this volt-
age.
R
c
Uc
Ic
---------
24V
3mA
----------------
8kΩ
== =
U
Low
R
c
I
Off
⋅ 8kΩ 500μA⋅ 4V== =
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