Siemens S5-135U/155U appendix DC Voltage AC Voltage

Digital Input/Output Modules

Two-Wire Switches With two-wire switches and AC voltage outputs:

Disconnection of the supply voltage during operation is not permissible without additional protective circuitry. Quenching of the inductive load is achieved here via the load power supply unit or system. Since, as two-wire switches, the outputs have no L-/N (0 V ground) terminal, the inductive load cannot be quenched on the module when the supply line is disconnected. With the 453 and 457 modules using two-wire switches, the overvoltages can result in destruction of the module.

If the load voltage must be switched for safety reasons, you must provide external quenching circuitry at each inductive load (see Figure 8-15). Instead, with the 453 and 457 modules using two-wire switches, you can insert a quenching device (diode) with adequate ratings after contact K from the L+ line to L-.

DC Voltage	AC Voltage
L+	L
Common Diode	Contact, Switch
only on 453/457
Digital Output	Digital Output
	ELR	ELR	ELR
Contactor,
Solenoid Valve, etc.
L±			N
Quenching Device
(Diode, Zener Diode, R/C Network or Varistor)

Figure 8-15 Quenching of Inductive Loads when Switching the Supply Line L+ and L

System Manual	8-27
C79000-G8576-C199-06