Pag. 9
Technical Book
MDS
MDS
Another important point to take into account (regarding the decoders) is that the voltage along the bus-line
may drop, due to the wire resistance. To avoid this, it is necessary to install additional 12 Vdc power
supplies to regenerate the 12 Vcc in long installations.
You can use for this the REF.8840 Power Supply or the REF.8837 (12 Vdc 0,75 A), which can be enough, in
somes cases, to keep the voltage level in range. See table and explanation below.
On the other hand, in installations where the decoder bus wires are very long we recommend using a
bigger wire section.
The next table shows how many decoders can be installed in terms of the distance and section of wire
utilised.
It shows the number of decoders that can be connected in relation to the power supply point (before having
to instal an additional one), the wire section used and the type of power supply, ref.8837 or ref.8840 (in blue,
between brackets).
* In the left-hand column is the distance that a power supply can cover.
* In the top row the section of the wires to be used.
* In the intersection is the maximum number of decoders that may be connected with these specifica-
tions. (In blue, in brackets, if using a Ref. 8840 supply, whereas the others refer to Ref. 8837.)
Example:
We have to stretch 20 decoders along 500 metres (1 decoder each 25 mts aproximatelly) in a very
long installation.
* We are using 10 decoders every 250m, so the best option is to use 3 mm
2
wires. Then we only need
one support power supply at 250m from the central unit. The first 250m are powered by the power
supply in the central unit.
* If using 1 mm
2
wires, we would need to add one power supply every 150m (we are using
approximately 7 decoders every 150m). We would need 3 additional Ref. 8837 power supplies.
'LVW6HFFLyQ PP
PP
PP
PP
P
10 10 (15) 10 (22) 10 (30)
P
7 9 10 (15) 10 (22)
P
57910 (17)
P
46810 (13)
P
4 5 7 10 (11)
P
3469
P
2357
P
2346
P
1234
P
1224
P
1123
.P
1123
We can "play" with these parameters in order to find
the most suitable relation between the wire section
and the number of power supplies required.
Note:
* The wire section of the Ref. 5918 cable recommended by FERMAX for bus installation is 1 mm
2
.
* If we can plan the structure of the installation, it is best to use "star" topology as much as possible . This
would reduce the total distance between the central unit and the last decoder in each branch, and
consequently the number of additional power supplies.
* In any case, using thick wires in very long installations is the best, because this is the only way to avoid the
audio level decreasing.
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