Siemens S7-1200 System Manual page 74

Installation
3.4 Wiring guidelines
A well controlled turn-off transient will have a ring frequency of no more than 10 kHz, with
less than 1 kHz preferred. Peak voltage for AC lines should be within +/- 1200 V of ground.
Negative peak voltage for DC loads using the PLC internal suppression will be ~40 V below
the 24 VDC supply voltage. External suppression should limit the transient to within 36 V of
the supply to unload the internal suppression.
Note
The effectiveness of a suppressor circuit depends on the application and must be verified for
your particular usage. Ensure that all components are correctly rated and use an
oscilloscope to observe the turn-off transient.
Typical suppressor circuit for DC or relay outputs that switch DC inductive loads
①
②
③
④
Typical suppressor circuit for relay outputs that switch AC inductive loads
①
②
③
If you design your own suppressor circuit, the following table suggests resistor and capacitor
values for a range of AC loads. These values are based on calculations with ideal
component parameters. I rms in the table refers to the steady-state current of the load when
fully ON.
74
1N4001 diode or equivalent
8.2 V Zener (DC outputs),
36 V Zener (Relay outputs)
Output point
M, 24 V reference
See table for C value
See table for R value
Output point
In most applications, the addition of a diode (A)
across a DC inductive load is suitable, but if your
application requires faster turn-off times, then the
addition of a zener diode (B) is recommended. Be
sure to size your zener diode properly for the amount
of current in your output circuit.
Ensure that the working voltage of the metal oxide
varistor (MOV) is at least 20% greater than the
nominal line voltage.
Choose pulse-rated, non-inductive resistors, and
capacitors recommended for pulse applications
(typically metal film). Verify the components meet
average power, peak power, and peak voltage
requirements.
System Manual, 03/2014, A5E02486680-AG
S7-1200 Programmable controller