46 Collectable Model Trains: Maintenance Tips: I'm all confused by whistle trigger polarity.
Description
This article is from the Model Trains FAQ, by Christopher D Coleman with numerous contributions by others.
46 Collectable Model Trains: Maintenance Tips: I'm all confused by whistle trigger polarity.
There are three kinds of whistle triggers available. The first,
original, and oldest is the electromechanical whistle/horn relay
discussed earlier. It does not care which way the superimposed DC
current goes, just that there is one. The transformers designed to
activate these use a diode to rectify part of the AC signal. A
resistor was in parallel with a diode to allow most of the AC through
to power the train, and rectify only enough DC to trigger the
whistle/horn relay. The result was a sine wave modified to a sine wave
with attenuated (smaller) peaks on one polarity. This is not a true DC
offset, but does change the RMS (average) voltage to a non-zero value.
That is sufficient for whistle relays of that era. The higher priced
models used a two position trigger in which the first intermediate
postion, all current passes through the diode, and in the second, the
bypass resistor is introduced. This provides an initial strong DC
'current' to activate the relay and then a smaller one to sustain it
in its position. These models also usually cut in a 5 volt 'booster'
coil that compensates for the additional current drain placed on the
transformer by the whistle motor.
The second trigger emerged in the early Fundimensions era of Lionel
and is electronic. It uses a circuit board to sense the presence of a
net DC current. Because these units are electronic, they are more
sensitive and discerning of input signals. They were designed for use
with solid state circuits. The older diodes used for mechanical relay
triggering were ineffecient and noisy. This tends to confuse the
electronic detector. Often it is necessary to hold older controllers
in the intermediate position (all current through the diode) in order
to get enough DC to trigger newer triggers. Additionally the
electronic triggers are polarity sensitive. One polarity activates the
primary function (whistle for steamers or horn for diesels or
electrics) and the opposite polarity for an optional second feature,
usually a clanging bell sound. The correspondence of available power
to this trigger arrangement will be discussed in a moment.
The third type is part of the TrainMaster Control system. This system
uses encoded digital signal transmitted on the track to trigger
decoders mounted in the locomotives. This uses wholey unrelated
methodology. TM is, however, equipped with a retroactive horn/whistle
and bell activators for track blocks. This presumably works the same
as the previous electronic activators.
Now back to the original and its diodes.
- Cathode-----|<------Anode +
The anode is positive and the cathode negative, and since current runs
from positive to negative, it runs only in the direction the arrow
points. This is an electronics convention. For our discussion we will
use the common Lionel ZW and 1033 as examples. The ZW is usually wired
with the common U terminal wired to the common outer rail, and the
1033 with terminal A wired to the common outer rail. According to
Lionel's schematics, when the whistle is triggered, the 5V booster
coil (which compensates for the whistle motor and diode loss drain) is
put in series with the variable coil and a diode-resistor combination.
According to the Lionel diagram the anode is positioned toward the
center rail terminal.
Lionel's questionable diagram:
common common
1033 +--------- A --+ ZW +----------- U --+
| - + | load | - + | load
+--|<----- U <-+ +--|<------- A or D <-+
diode power diode power
This would imply a negative charge on the power rail relative to the
common rail for a current to flow in the direction the diode indicates
(Current always flows positive to negative). Another way to look at it
is that since the diode is conductive in the direction of the current
shown, the diode 'pulls' the power rail negative. Concluding from the
diagram, the horn trigger uses a center rail negative DC current, but
this is not the case. There are three test that confirm this.
o Testing continuity with a silicon diode in series with the copper
Lionel diode. The combination will only conduct when the two are
anode to anode or cathode to cathode (according to the Lionel
diagram). This can be done with a plain diode and a continuity
testor or with two AAs and a LED. Same results.
o The horn of a current direction dependent locomotive can be
activated just as the built-in whistle controller does by adding
a silicon diode in the circuit to the track. The successful
direction of the diode is the opposite of that shown in the
Lionel diagram.
o Inserting a AA into the circuit to the track activates the horn
of a current direction dependent locomotive when the positive end
of the battery is contacting the center rail. The battery would,
in effect, add a positive offset to the AC from the transformer.
This is a positive offset on center relative to outer. The
reverse configuration does not activate the horn.
A modern MRC 027 transformer is reported to actually use negative
center rail DC for horn activation. This may have resulted from 1) MRC
taking Lionels diagrams verbatim or 2) not caring which way the DC
current went, as where it is irrelevant for electromechanical whistle
relays.
It has been reported that MTH (QSI) do use positive center rail DC for
whistle/horn activation and negative for bell control. MRC and the
Lionel schematic drawers made mistakes.
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