A normal switching output can turn on a motor. It cannot make it run in reverse. That's exactly what an H-bridge is for - and it's the reason why electric steps, valves, and actuators can be controlled without additional relay wiring.
What an H-Bridge Does
A DC motor runs in the direction dictated by the polarity. If positive and negative are reversed, it spins the other way. To switch this, conventionally two changeover relays are needed, which together reverse the polarity – and an interlock to prevent both from engaging simultaneously and causing a short circuit.
The H-bridge is the electronic form of this arrangement. The name comes from the circuit itself: four switching elements, between which the motor is connected - forming an H in the circuit diagram. Depending on which diagonal pair conducts, the current flows in one direction or the other.
The practical benefit lies less in the component itself than in the elimination of wiring: no two relays in the box, no interlock logic, no additional fuses. Two outputs and the motor lead are sufficient.
What It's Used For
Anything that has two end positions and needs to be moved in both directions:
- Electric Steps. Extending and retracting - the classic case in vehicle conversions.
- Motorized Valves. Switching water paths, isolating tanks, blocking inlets.
- Actuators and Flaps. Ventilation flaps, covers, hatches.
- Small Winches and Drives, as long as they stay within channel limits.
What's not included: anything that only runs in one direction. A pump, a fan, a compressor does not need an H-bridge, but a normal switching output. Using an H-bridge for a unidirectional consumer consumes channel capacity without benefit.
End Stops: The Actual Planning Point
A motor extending a step must stop at some point. There are three options for this, and the choice must be made before wiring.
Mechanical limit switches on the drive itself. The motor disconnects at the end position. This is the most robust solution because it works independently of the control system — even if the configuration is lost.
Time control. The output runs for a fixed time. Easy to implement, but inaccurate: if the drive runs slower in the cold, it won't reach the end position; if it runs lighter than planned, it will push against it.
Current monitoring. At the end position, the current increases because the motor is blocked. This is a useful shutdown criterion, but it requires that blocking current and starting current can be reliably distinguished - which is not always the case with every drive.
WarningA drive without a functioning limit switch will run against the stop in case of an error until something gives way. For movements where someone could be standing - such as a step - the mechanical limit switch is therefore not the most convenient, but the correct solution.
Design: The Starting Current Is Decisive
Motors draw several times their operating current during start-up, and more when starting against a load than when idling. Therefore, the nominal current of the drive is not decisive for channel design, but rather the peak at start-up under unfavorable conditions - cold, stiff, against resistance.
The output's current limiting must allow this peak to pass while remaining below the wiring's capacity. With drives, this is where the calculation often doesn't add up - and then the cross-section is chosen too small, not the threshold too low.
Commissioning
- Check rotation direction before the drive is mechanically coupled. A reversed pair will reliably move in the wrong direction.
- Test end-stop function, in both directions individually.
- Measure starting current, don't estimate — once cold and once warm.
- Check behavior in case of blockage. What happens if something is in the way?
- Make operation explainable. Whoever operates the drive must know how to stop it.
Typical Errors
- H-bridge used for unidirectional loads. Pumps and fans do not need a change of direction.
- Only time control as end stop. Works under constant conditions — and those don't exist on board.
- Designed according to nominal current. Starting against a load is the decisive factor.
- Direction of rotation checked only after installation. The test takes two minutes, the correction afterwards an hour.
- Blockage case not tested. The case will occur, and then it should be known what happens.
