Siemens SIMATIC S7-200 System Manual page 36

S7-200 Programmable Controller System Manual
Guidelines for Inductive Loads
You should equip inductive loads with suppression circuits to limit voltage rise when the control
output turns off. Suppression circuits protect your outputs from premature failure due to high
inductive switching currents. In addition, suppression circuits limit the electrical noise generated
when switching inductive loads.
Tip
The effectiveness of a given suppression circuit depends on the application, and you must verify
it for your particular use. Always ensure that all components used in your suppression circuit are
rated for use in the application.
DC Outputs and Relays That Control DC Loads
The DC outputs have internal protection that is adequate for most applications. Since the relays
can be used for either a DC or an AC load, internal protection is not provided.
Figure 3-3 shows a sample suppression
circuit for a DC load. In most
applications, the addition of a diode (A)
across the 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 your Zener diode properly for the amount
of current in your output circuit.
AC Outputs and Relays That Control AC Loads
The AC outputs have internal protection that is adequate for most applications. Since the relays
can be used for either a DC or an AC load, internal protection is not provided.
Figure 3-4 shows a sample suppression
circuit for an AC load. When you use a
relay or AC output to switch 115 V/230
VAC loads, place resistor/capacitor
networks across the AC load as shown
in this figure. You can also use a metal
oxide varistor (MOV) to limit peak
voltage. Ensure that the working voltage
of the MOV is at least 20% greater than of the MOV is at least 20% greater than
the nominal line voltage.
Warning
When relay expansion modules are used to switch AC inductive loads, the external
resistor/capacitor noise suppression circuit must be placed across the AC load to prevent
unexpected machine or process operation. See Figure 3-4.
Guidelines for Lamp Loads
Lamp loads are damaging to relay contacts because of the high turn-on surge current. This surge
current will nominally be 10 to 15 times the steady state current for a Tungsten lamp. A
replaceable interposing relay or surge limiter is recommended for lamp loads that will be switched
a large number of times during the lifetime of the application.
22
A B (optional)
Output
Point
DC Inductive Load
Figure 3-3 Suppression Circuit for a DC Load
.1 µ F
MOV
Output
Point
AC Inductive Load
Figure 3-4 Suppression Circuit for an AC Load
A - - I1N4001 diode or equivalent
B - - 8.2 V Zener for DC Outputs
36 V Zener for Relay Outputs
100 to 120 Ω