Reverse Polarity: How It Controls a DC Linear Actuator

Why Reversing Polarity Does Not Stop the Motor

A DC linear actuator converts motor rotation into linear output through its internal drive mechanism. When the actuator reaches a mechanical limit or encounters an obstruction, the output can no longer move in the commanded direction.

If voltage remains applied, the motor enters a locked-rotor stall. When the actuator reaches a mechanical stop and the motor can no longer run, the current rises toward the motor’s stall current value.

If the actuator remains powered after it has stopped moving, heat can build up quickly inside the motor. Repeated or prolonged stalling can shorten the actuator’s life and may also damage the power supply, wiring, switch contacts, relay, or motor driver. The control circuit should therefore cut power when the actuator reaches its travel limit or becomes blocked.

How the motor is stopped depends on the actuator’s control option.

How S-Series Actuators Stop at the End of Stroke

Actuonix S-Series actuators are designed for two-wire extend-and-retract control. They contain built-in limit switches that stop motor power near each end of travel.

When the actuator mechanism reaches one limit, the corresponding switch opens the circuit in that direction. An internal diode provides a current path around the open switch when the supply polarity is reversed. The actuator can then move away from the limit, after which the switch returns to its normal state.

The L12-S datasheet , for example, states that its limit switches turn off motor power within 0.5 mm of the end of stroke and that internal diodes allow the actuator to reverse away from the activated switch. The exact switching position should be confirmed for the selected actuator model.

This arrangement provides automatic current interruption at the actuator’s factory-set stroke limits during normal operation. It does not protect against every possible stall. An external obstruction, excessive load, incorrect supply voltage, or mechanical interference can stop the actuator before it reaches an internal limit switch.

How P-Series Actuators Control Position and Stop

P-Series actuators also reverse direction when the polarity across their red and black motor leads is reversed. Unlike S-Series models, P-Series actuators provide potentiometer position feedback and do not rely on internal limit switches.

The potentiometer reports the actuator’s position, but it does not make the control decision by itself. An external circuit must read that feedback and determine when to energize, reverse, or stop the motor.

One option is the Actuonix Linear Actuator Control Board. The LAC compares the actuator’s feedback signal with the commanded position and controls the motor through an onboard H-bridge. It supports several command interfaces and provides adjustable speed, sensitivity, and stroke settings.

Another option is an external limit-switch circuit. The Actuonix limit switch kit contains two normally closed SPDT switches and two diodes. The mechanism being controlled activates each external switch at the selected travel point. The switch stops the flow of current in one direction, while the diode permits the actuator to move away when polarity is reversed.

External limit switches can also shorten the usable stroke of an S-Series actuator when an application must stop before the factory-set limit.

Choosing a Switch, Relay, or H-Bridge

Once the end-of-travel strategy has been established, the next decision is how the circuit will reverse current through the motor.

DPDT Switch

A double-pole, double-throw switch can reverse both motor connections simultaneously. It is appropriate when an operator will control the actuator directly.

A momentary center-off switch returns to its neutral position when released, giving the operator direct control over motion duration. A maintained or latching switch remains in its selected position, so the actuator or external control circuit must stop motor current safely.

DPDT switch kit for manual actuator direction control.

DPDT Relay

A DPDT relay performs the same polarity-reversing function through electrically operated contacts. It is useful when a sensor, timer, PLC output, or other low-current signal must control actuator direction.

The relay contacts must be rated for the actuator’s supply voltage and motor current. The design should account for stall current rather than using only the normal running current.

DPDT relay for electrically triggered actuator direction control.

H-Bridge Motor Driver

An H-bridge uses four electronic switching devices to reverse current through the motor without mechanical contacts. It is commonly used when a microcontroller or other digital controller determines actuator direction.

A microcontroller or single-board computer output pin should not power an actuator motor directly. The H-bridge must handle the actuator voltage, running current, stall current, and inductive switching produced by the motor.

How to Select the Correct Control Strategy

Before selecting the switching hardware, answer three questions.

What Control Option Does the Actuator Use?

Use polarity reversal across the motor leads of compatible S-Series and P-Series actuators. Keep the supply polarity fixed for R-Series and I-Series actuators, and command them through their supported signal inputs.

What Stops Motor Current?

For an S-Series actuator completing its full factory stroke, internal limit switches normally provide end-of-stroke interruption.

For a P-Series actuator, use a compatible position controller, a properly designed custom feedback controller, or external limit switches.

In either case, consider what happens if the load encounters an obstruction before the actuator reaches its intended stopping point.

What Device Will Reverse the Current?

Use a DPDT switch for direct manual control, a relay for simple electrically triggered control, or an H-bridge when an electronic controller must reverse the motor.

Whichever device is selected, verify its voltage rating, current rating, switching configuration, and compatibility with the actuator’s stall current.