H0 Sound Decoder MX640 Page 39
Low current servos (up to 200 mA) can be powered directly from the MX640! For all others,
the 5V
operating voltage must be supplied by an external voltage regulator such as the readily available
LM7805 as shown in the drawing.
Connection and control of an external energy source (capacitor) for
uninterrupted driving on dead track sections:
With the help of an electrolytic capacitor or a battery the
NOTE: ZIMO plans to introduce its own brand of a 5V regulator. Compared to the LM7805, it will be
easier to install and produce less heat!
- driving performance on dirty track sections (or wheels) can be improved
- flickering of lights is reduced
- and stalling of trains, especially when crawling, can be eliminated
In cases where power to the decoder is interrupted due to dirty rails, wheels or insulated frogs, the
decoder automatically keeps the engine going even though a currently active brake application
should bring the train to a stop. Only when power to the decoder is restored is the loco allowed to
stop, with subsequent testing to ensure power to the decoder is still available after the engine
stopped (if not, the engine is moved again a short distance).
The energy storage increases with the capacity of a condenser and from 100uF (Microfarad) on-
wards an effect will be noticed. 1000uF to 10’000uF are recommended if the necessary space is
available. The required voltage strength of the capacitor is given by the track voltage; 25V is suitable
for all cases. Smaller 16V capacitors should only be used if track voltage will never be higher than
that.
The capacitor is connected between ground (available on all ZIMO decoders as solder pad) and
power (blue wire) of the decoder. Note polarity!
When building your own power module, use the schematic above. The condenser is recharged
through the 100 ohm resistor. This is to prevent a shut down of the command station during start-up.
If a large number of loco’s so equipped are on the layout the command station could interpret the
current flow to these capacitors as a short circuit. The diode (e.g. 1N4007) is required to bypass the
resistor when power is needed by the decoder.
The outputs can be activated for servo control duty with CV’s #181 and CV #182 (the value in each
must be different than 0).
With the help of CV #181 and #182, the servo functions can be mapped to various function keys
(and direction) and selected for control with either one or two function keys.
NOTE: If signal stops by “asymmetrical DCC signal” (= Lenz ABC, implemented in ZIMO decoders
early 2005) is employed, the resistor-diode combination is necessary in any case (even when using
small capacitors) to ensure that the decoder can detect the asymmetry of the signal!
CV’s #161 to #169 define the servos end positions and rotating speed, see CV table.
CV #161 is also used to select the appropriate protocol. “Normal” for most servos are positive pulses
(which is also the default setting); furthermore a selection can be made whether the servo is pow-
ered only while it is being moved or remains powered at all times. The latter should only be used if
the servo position can be changed by mechanical influences. In any case, Bit 1 in CV #161 must be
set for
SmartServo’s
, which is CV #161 = 2!
The purpose of the resistor 3K3 shown in the drawing above (not required in all cases) is:
even though a large condenser supplies the motor and lights for just a few tenths of a second
(1000uF) or a few seconds (e.g. 4700uF) the remaining power, although at a voltage level below
what is required by the motor and lights, is sufficient power to keep the decoders memory alive for
quite some time (several minutes). This is sometimes a rather undesired effect. For example: If a
running loco is taken from the track and the speed then set to zero, the loco would briefly run at the
previous speed when it is set back on the track after about a minute. Using the above-mentioned re-
sistor would erase the memory after just a few seconds.
