Limit switches control dc-motor H bridge
Mechanical and electronic switches provide interlocks, preventing motors from overrunning their bounds.
Andreas Grün, Wedemark, Germany; Edited byMartin Rowe and Fran Granville -- EDN, March 5, 2009
You use limit switches to switch off a motor if it reaches one of its two end positions. Even if you build a microprocessor-based motor controller, you should switch off a motor with hardware by building a safety interlock. Such a circuit works by inhibiting movement in one direction but allowing movement in the other direction when the motor retracts from its end position. Figure 1 shows the circuit with mechanical switches. However, this ancient mechanical approach may be unsuitable in some cases because the motor current may be too high, or the switches may be closing switches or light barriers.
|
If you use an H bridge to drive the motor, you can achieve the same operation in a more versatile way. The circuit in Figure 2 shortens one input of the H bridge to ground so that movement is possible only in the other direction by turning on the other input. If the switches are opening at the end positions, the circuit in Figure 3 interrupts the connection to the driving circuit of one input and sets the input to low using a pulldown resistor. The same circuit works with phototransistors without modification (figure 4 and figure 5). The value of the resistors depends on the parts you use; a value of 10 kΩ should work in most designs.
-
Yes, one (and only one) input is shortened to ground to inhibit further movement in the direction controlled by this input
Andreas Gruen - 2009-19-3 12:59:00 PDT -
The circuit shortens one input to ground?
Nicholas Allen - 2009-6-3 13:37:00 PST
