Siemens S5-155U System Manual page 327

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
System Manual
C79000-B8576-C199-03
External quenching circuitry is needed when
• • 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);
• • disconnection of output lines can be expected;
• • 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:
• • The permissible overvoltage for the supply voltage (see
Section 8.3 "Common Technical Specifications")
• • 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.
Installation and Startup
8 - 29
8