We wanted to control a dc permanent-magnet motor in a system containing limit switches to prevent overtravel. The control had to be accomplished using only one "hot" wire and the common ground lead which were available in the existing cable design.
The figure shows our solution to the problem. Two sources of power were provided, one of each polarity. When the control switch is held in the right position the motor runs until the right limit switch opens. Since diode CR1 will not conduct with a negative potential across it, the motor will stop.
With the switch in the left position a positive potential is applied and CR1 conducts. Since the left limit switch is still closed, the motor will run in the opposite direction until the left limit switch opens.
For the small 28-v, 100-ma motor used in this application there are many satisfactory types of diodes. 1N91 was used in our case. For larger motors, diodes must be selected to handle the starting current of the motor. Their inverse voltage rating must be greater than the sum of the supply potential and the voltage transient which occurs when a limit switch opens. (Electronic Design, May 25, 1960, p. 121)
These types of circuits from the "Ideas for Design" section are almost timeless. The Idea came from Charles C. Defir, senior engineer, Hallamore Electronics Co., Anaheim, Calif.