An email was recently received by
TDK-Lambda’s Technical Support from a technician who was fault-finding a video
system. His questions about how an EMC
filter should be connected were relevant and warranted a blog article on the
subject!
As indicated in Figure 1, the EMC filter is
situated between the AC input to the system and the power supply providing DC
voltage(s) to the system load. The
filter’s function is to reduce incoming noise from the AC input and/or outgoing
noise to the AC input from the power supply.
Figure
1: System block diagram
Regarding connection to the filter, let us now look at a
typical EMC filter - the 250Vac 10A rated RSEN-2010 for example (see Figure 2
and 3).
Figure
2: RSEN-2010 filter
Figure
3: RSEN schematic
Reviewing Figure 2, the nomenclature
“LINE/LOAD” on the left hand and right hand side of the label indicates that
either set of the terminals can be used for the input or load connection. It can be also noted that there is no
indication of where the Line or Neutral should be wired to, just numbers 1, 2,
3 & 4. Which connections are made to
these terminals is very important.
On the right hand side of the label and
schematic (Figure 3), two additional capacitors, known as “Y” capacitors can be
seen. These provide a low impedance path
to earth ground to reduce high frequency common-mode noise. When the filter is used to reduce system
noise from the power supply reaching the AC source, terminals 3 and 4 should be
connected to the power supply. If
the filter is being used to reduce externally generated high frequency noise
from entering the system then terminals 3 and 4 should be connected to the AC
source input.
It is essential that if the AC line is
connected to terminal “1”, then terminal “4” should be connected to the power
supply Line terminal. Likewise with
terminals “2” and “3” for the neutral wiring as shown in Figure 4.
Figure
4: Connection of the filter to the AC and the power supply
If the wiring to “3” and “4” is crossed,
there could be safety issues for a power supply with a single input fuse situated
internally in series with the AC Line terminal.
If the AC is correctly wired (Figure 5a),
in the event the internal power supply fuse was to open due to a fault, the
internal circuitry is isolated. It is important
to note that at the breaker panel, where the AC first comes into the building,
the neutral input is grounded to earth.
Figure
5a: Correct AC connection to a single fuse power supply
If the Line and Neutral is reversed to the
power supply due to incorrect wiring (Figure 5b), in the event of the fuse
opening the internal circuit is still live.
Two issues can arise with this scenario.
A service technician fault-finding the system could receive an
electrical shock, particularly with an open frame power supply without a cover. Secondly, if an internal short was to occur
in the power supply between Line and the earthed chassis, the fuse would be out
of circuit and not open. Even the simple
use of a mounting screw that is too long could cause this!
Figure
5b: Incorrect AC connection to a single fuse power supply
Always ensure that adequate inspection and
testing techniques are in place with AC wiring.
Power Guy
