Another scratchy drawing. This is a quick and dirty way of wiring two sticks to control a single axis trim system. Hold on to your hat for the explanation!
There are two spdt relays. Notice that both relays hold the common contact (c) to ground, through the normally closed contact (n/c). The upper relay is connected to the motor through the red wire and the lower relay is connected to the motor through the black wire. If either trim switch is pushed to the CW position, power is applied to the upper relay, moving the (c) contact from the (n/c) to the (n/o) contact. When this happens, trim motor power, from a breaker, is applied to the motor through the red wire. And since the lower relay still has the black motor wire tied to ground, the trim motor will turn in a clockwise direction.
If either switch is moved to the CCW position, the same, but opposite, function will take place. The upper relay will keep the red motor wire tied to ground but the lower relay will apply power to the black lead. This will cause the motor to move in a CCW direction.
Each relay coil is tied to ground through a 'bump switch'. This is the travel limit switch. They need to be mounted where the surface being moved (like the horizontal stab) will hit the switch when it is at maximum travel, one in each direction. When the switch is contacted it will open the ground path for the relay coil and the relay contacts will return to the (c) to ground through the (n/c) contact. This prevents continued travel in that direction but still allows for travel in the opposite direction.
A separate breaker needs to feed power to each trim switch. If the pilot needs to prevent the opposite stick from operating the trim, simply pop the breaker for that switch.
If there is a runaway, either, through malfunction or someone frozen on the other trim switch, the pilot can simply push his trim switch in the opposite direction. This will energize the relay previously providing ground, thereby removing the ground path for the trim motor. This action prevents the motor from moving in either direction, and allows the pilot time to pop circuit breakers without the danger of electrical or mechanical damage.
Fire at will!
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