Siemens S5-135U/155U - Manual (Page 338)

8-26
System Manual
C79000-G8576-C199-06
External quenching circuitry is needed when
S
the switching frequency of an output is greater than that specified in the
technical specifications (thermal stress of the integrated circuit for
quenching inductive loads);
S
disconnection of output lines can be expected;
S
disconnection 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:
S
The permissible overvoltage for the supply voltage (see Section 8.3
“Common Technical Specifications”)
S
Inductive breaking voltage of the output (see Section 8.4 “Description of
Individual Modules”)
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.
The following applies to digital outputs for DC voltage:
In general, load power supplies should be switched on the primary side to
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.
With switched-through digital outputs and disconnection of the supply line
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.
External
Quenching
Circuitry
Switching
Frequency of the
Output is Higher
than the
Permissible Value
Disconnecting the
Load Circuit
Disconnecting the
Supply Line L+
and L
Switched-Through
Outputs
Digital Input/Output Modules