Siemens S5-135U/155U appendix Disconnecting, Following applies to digital outputs for DC voltage

Digital Input/Output Modules

External

Quenching

Circuitry

Switching Frequency of the Output is Higher than the Permissible Value

Disconnecting the Load Circuit

External quenching circuitry is needed when

Sthe switching frequency of an output is greater than that specified in the technical specifications (thermal stress of the integrated circuit for quenching inductive loads);

Sdisconnection of output lines can be expected;

Sdisconnection of the supply line can be expected.

When selecting or designing the quenching circuitry, the technical specifications for the digital output should be taken into account. These are:

SThe permissible overvoltage for the supply voltage (see Section 8.3 ªCommon Technical Specificationsº)

SInductive breaking voltage of the output (see Section 8.4 ªDescription of Individual Modulesº)

An external quenching circuit only provides thermal relief if its quenching voltage is lower than the breaking voltage specified for the module, allowing for unfavorable supply voltage conditions.

Example: 453 digital output

Inductive breaking voltage L+ - 47 V (tech. specifications) Max. supply voltage L+ = 30 V

Quenching voltage referred to L- (0 V ground) is - 17 V

The quenching device must be rated at approximately 15 V and the switched load current, e.g. 1 A.

A quenching device rated for the induced breaking current must be provided at the load (see Figure 8-14).

The quenching voltage at the load is independent of the module. The quenching devices must also ensure that the overvoltages at the switch and in the wiring do not exceed the disturbance voltage and voltage hazard limits permissible to VDE/IEC.

Disconnecting the	The following applies to digital outputs for DC voltage:
Supply Line L+	In general, load power supplies should be switched on the primary side to
and L	utilize the low-resistance secondary winding and smoothing capacitors of the
	power supply for energy compensation in the dropping of inductive loads.
	The load power supply switch should only be considered as a circuit breaker.
Switched-Through	With switched-through digital outputs and disconnection of the supply line
Outputs	L+, the output current is maintained during the quenching time by capacitors
	and the reverse voltage protection diode on the module. This severe electrical
	stress of the module should be avoided during operations, because it can
	result in a fault in the long term.

8-26	System Manual
	C79000-G8576-C199-06