Model Railway Short-Circuit Beeper


Short circuits in the
tracks, points or wiring are almost inevitable when building or
operating a model railway. Although transformers for model systems must
be protected against short circuits by built-in bimetallic switches, the
response time of such switches is so long that is not possible to
immediately localise a short that occurs while the trains are running,
for example. Furthermore, bimetallic protection switches do not always
work properly when the voltage applied to the track circuit is
relatively low. The rapid-acting acoustic short-circuit detector
described here eliminates these problems. However, it requires its own
power source, which is implemented here in the form of a GoldCap storage
capacitor with a capacity of 0.1 to 1 F. A commonly available reed
switch (filled with an inert gas) is used for the current sensor, but in
this case it is actuated by a solenoid instead of a permanent magnet.

An adequate coil is provided by several turns of 0.8–1 mm enamelled
copper wire wound around a drill bit or yarn spool and then slipped over
the glass tube of the reed switch. This technique generates only a
negligible voltage drop. The actuation sensitivity of the switch
(expressed in ampère-turns or A-t)) determines the number of turns
required for the coil. For instance, if you select a type rated at 20–40
A-t and assume a maximum allowable operating current of 6 A, seven
turns (40 ÷ 6 = 6.67) will be sufficient. As a rule, the optimum number
of windings must be determined empirically, due to a lack of
specification data. As you can see from the circuit diagram, the
short-circuit detector is equally suitable for AC and DC railways. With
Märklin transformers (HO and I), the track and lighting circuits can be
sensed together, since both circuits are powered from a single secondary
winding.

Coil L1 is located in the common ground lead (‘O’ terminal), so the
piezoelectric buzzer will sound if a short circuit is present in either
of the two circuits. The (positive) trigger voltage is taken from the
lighting circuit (L) via D1 and series resistor R1. Even though the
current flowing through winding L1 is an AC or pulsating DC current,
which causes the contact reeds to vibrate in synchronisation with the
mains frequency, the buzzer will be activated because a brief positive
pulse is all that is required to trigger thyristor Th1. The thyristor
takes its anode voltage from the GoldCap storage capacitor (C2), which
is charged via C2 and R2.

The alarm can be manually switched off using switch S1, since
although the thyristor will return to the blocking state after C2 has
been discharged if a short circuit is present the lighting circuit, this
will not happen if there is a short circuit in the track circuit. C1
eliminates any noise pulses that may be generated. As a continuous tone
does not attract as much attention as an intermittent beep, an
intermittent piezoelectric generator is preferable. As almost no current
flows during the intervals between beeps and the hold current through
the thyristor must be kept above 3 mA, a resistor with a value of
1.5–1.8 kΩ is connected in parallel with the buzzer. This may also be
necessary with certain types of continuous-tone buzzers if the operating
current is less than 3 mA. The Zener diode must limit the operating
voltage to 5.1 V, since the rated voltage of the GoldCap capacitor is
5.5 V.
Author: R. Edlinger – Copyright: Elektor Electronics


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