Specifications

Installing an S7-200 PLC
2-17
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
Protecting Relays That Control DC Power
Resistor/capacitor networks, as shown in Figure 2-14, can be used for low voltage
(30 V) DC relay applications. Connect the network across the load.
+VDC
where minimum R = 12 Ω
RC
I
L
Inductor
R +
V
DC
I
L
where K is 0.5 µF/A to 1 µF/A
C + I
L
K
Figure 2-14 Resistor/Capacitor Network on Relay-Driven DC Load
You can also use diode suppression, as shown in Figure 2-12 and Figure 2-13, for
DC relay applications. A threshold voltage of up to 36 V is allowed if you use a
reverse zener diode.
Protecting Relays That Control AC Power
When you use a relay to switch 115 VAC/230 VAC inductive loads, you should
place resistor/capacitor networks across the relay contacts as shown in
Figure 2-15. 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 the
nominal line voltage.
R > 0.5 x Vrms for relay
C = 0.002 µF to 0.005 µF for each
10 VA of steady-state load
R
C
MOV
Inductor
Figure 2-15 AC Load with Network across Relay
The capacitor allows leakage current to flow around the open switch. Be sure that
the leakage current, I (leakage) = 2 x 3.14 x f x C x Vrms, is acceptable for the
application.
For example: A NEMA size 2 contactor lists 183 VA coil inrush and 17 VA sealed
coil load. At 115 VAC, the inrush current is 183 VA/115 V = 1.59 A, which is within
the 2-A switching capability of the relay contacts.
The resistor = 0.5 x 115 = 57.5 W; choose 68 W as a standard value.
The capacitor = (17 VA/10) x 0.005 = 0.0085 µF; choose 0.01 µF as the value.
The leakage current = 2 x 3.14 x 60 x 0.01 x 10
-6
x 115 = 0.43 mA rms.