When power to the layout is turned
off, the 3PDT relay reverts to its nor- mal position, but the stall motor switch machine stays where it is since it has no power.
When power is restored, the stall motor switch machine will return to its normal position. Diodes in the circuit isolate the ca- pacitor discharge and auxiliary power supplies from each other, and isolate the stall motor switch machine from the back current generated when the relay coils are deactivated.
Assembly I built my control modules on small
The author’s Harmon control panel (above) directly controls 57 sets of switch points, which are powered by all types of switch machines. In addition it can “straight line” an- other 24 sets of switch points, setting them all up for the mainline. This photo (below) shows one of the “ice cube” modules mounted on the benchwork beneath the layout.
don’t have a 3PDT or 4PDT, use a DPDT and a SPDT relay with the coils wired in parallel. Suitable relays are available from surplus electronics outlets for a few dollars each, and standard electronics supply houses have PC board mount relays for not much more. Use whatev- er you can get for a reasonable price. Avoid small reed relays for this use, however. They tend to be too fragile to withstand the fairly high voltage puls- es produced by the capacitor-discharge power supply.
The diodes in the circuit are general- purpose diodes (IN 4004), and cost a few pennies each. Some stall motor switch machines operate on less than 12 volts and will require a dropping re- sistor in series with the coil to operate from a 12-volt supply.
How it works
A control current pulse from a mo- mentary SPDT switch, push button, or diode matrix activates the 3PDT relay with current from the capacitor dis- charge unit. One pole of this relay is used to latch the relay on using the aux- iliary power supply (24- or 12-volts d.c.). The other two poles of the 3PDT re-
lay control polarity to the stall motor switch machine, which responds by op- erating in the desired direction. To restore the switch to normal, the control current pulse from the momen-
RAILROAD MODEL CRAFTSMAN 69
tary SPDT switch, push button, or diode matrix activates the SPNC (sin- gle-pole, normally closed) relay mo- mentarily with current from the capac- itor discharge unit, which interrupts current to the coil of the 3PDT relay and allows it to revert to normal posi- tion, thus reversing the stall motor switch machine.
blocks of plywood and mounted them under the layout near the stall motor switch machines they control. Where I used PC board relays, I used small pieces of PC board instead of plywood. Two wires are required to transmit control current from the control panels to the control modules, and auxiliary current is needed to power the control module relays and the stall motor switch machines. A connection to the common ground buss is of course also needed. Readers should note that a fil- tered power supply should be used to power the stall motor switch machines to avoid humming. Each of the control models takes an hour or so to build and test, and costs only a few dollars. The universal switch machine con- trol system has performed to my satis- faction. The Hudson Division of the New York Central Railroad can contin- ue to expand using the latest in switch machine technology.
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