Power Flip-Flop Using A Triac

Modern electronics is
indispensable for every large model railroad system, and it provides a
solution to almost every problem. Although ready-made products are
exorbitantly expensive, clever electronics hobbyists try to use a
minimum number of components to achieve optimum results together with
low costs. This approach can be demonstrated using the rather unusual
semiconductor power flip-flop described here. A flip-flop is a toggling
circuit with two stable switching states (bistable multivibrator). It
maintains its output state even in the absence of an input pulse.

Flip-flops can easily be implemented using triacs if no DC voltage
is available. Triacs are also so inexpensive that they are often used by
model railway builders as semiconductor power switches. The decisive
advantage of triacs is that they are bi-directional, which means they
can be triggered during both the positive and the negative half-cycle by
applying an AC voltage to the gate electrode (G). The polarity of the
trigger voltage is thus irrelevant. Triggering with a DC current is also
possible. Figure 1 shows the circuit diagram of such a power flop-flop.
A permanent magnet is fitted to the model train, and when it travels
from left to right, the magnet switches the flip-flop on and off via
reed switches S1 and S2.

Circuit diagram:

In order for this to work in both directions of travel, another pair
of reed switches (S3 and S4) is connected in parallel with S1 and S2.
Briefly closing S1 or S3 triggers the triac. The RC network C1/R2, which
acts as a phase shifter, maintains the trigger current. The current
through R2, C1 and the gate electrode (G) reaches its maximum value when
the voltage across the load passes through zero. This causes the triac
to be triggered anew for each half-cycle, even though no pulse is
present at the gate. It remains triggered until S2 or S4 is closed,
which causes it to return to the blocking state.The load can be
incandescent lamps in the station area (platform lighting) or a

solenoid-operated device, such as a crossing gate. The LED
connected across the output (with a rectifier diode) indicates the
state of the flip-flop. The circuit shown here is designed for use in a
model railway system, but there is no reason why it could not be used
for other applications. The reed switches can also be replaced by normal
pushbutton switches. For the commonly used TIC206D triac, which has a
maximum current rating of 4 A, no heat sink is necessary in this
application unless a load current exceeding 1 A must be supplied
continuously or for an extended period of time. If the switch-on or
switch-off pulse proves to be inadequate, the value of electrolytic
capacitor C1 must be increased slightly.

Author: R. Edlinger – Copyright: Elektor July-August 2004

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