Rockwell Automation Publication 1606-RM055A-EN-P - December 2016
15
Basic Power Supply 5 A
Application Notes
Peak Current Capability
The unit can deliver peak currents (up to several milliseconds) which are
higher than the specified short-term currents.
This helps to start current demanding loads. Solenoids, contactors, and
pneumatic modules often have a steady state coil and a pick-up coil. The inrush
current demand of the pick-up coil is several times higher than the steady-state
current and usually exceeds the nominal output current. The same situation
applies when starting a capacitive load.
The peak current capability also verifies the safe operation of subsequent
circuit breakers of load circuits. The load branches are often individually
protected with circuit breakers or fuses. If there is a short or an overload in one
branch circuit, the fuse or circuit breaker need a certain amount of overcurrent
to open in a timely manner. This avoids voltage loss in adjacent circuits.
The extra current (peak current) is supplied by the power converter and the
built-in large sized output capacitors of the power supply. The capacitors get
discharged during such an event, which causes a voltage dip on the output. The
following two examples show typical voltage dips:
Back Feeding Loads
Loads such as decelerating motors and inductors can feed voltage back to the
power supply. This feature is also called return voltage immunity or resistance
against Back- E.M.F. (Electro Magnetic Force).
Figure 18 - Peak Load with 2x the Nominal Current for 50 ms, typ
Figure 19 - Peak Load with 5x the Nominal Current for 5 ms, typ
10ms/DIV
Output
Voltage
Output
Current
24V
0A
10A
17V
10A Peak load (resistive) for 50ms
Output voltage dips from 24V to 17V.
1ms/DIV
Output
Voltage
Output
Current
24V
0A
25A
9V
25A Peak load (resistive) for 5ms
Output voltage dips from 24V to 9V.
Peak Current Capability
Voltage Dip
Peak Load
Peak current voltage dips
typ
24…17V
at 10 A for 50 ms, resistive load
typ
24…13V
at 25 A for 2 ms, resistive load
typ
24…9V
at 25 A for 5 ms, resistive load