Siemens SIMATIC S7 System Manual page 66

Installation
3.4 Wiring guidelines
A good suppressor solution is to use contactors and other inductive loads for which the
manufacturer provides suppressor circuits integrated in the load device, or as an optional
accessory. However, some manufacturer provided suppressor circuits may be inadequate
for your application. An additional suppressor circuit may be necessary for optimal noise
reduction and contact life.
For AC loads, a metal oxide varistor (MOV) or other voltage clamping device may be used
with a parallel RC circuit, but is not as effective when used alone. An MOV suppressor with
no parallel RC circuit often results in significant high frequency noise up to the clamp
voltage.
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 V DC 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
66
1N4001 diode or equivalent
8.2 V Zener (DC outputs),
36 V Zener (Relay outputs)
Output point
M, 24 V reference
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.
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART