H-Bridge Symbol
Definition: The H-Bridge symbol represents a four-switch power topology — named for its H-shaped circuit arrangement of four transistors (or MOSFETs) bridging a DC motor or load between the supply voltage and ground — that enables bidirectional current flow through the load, allowing a motor to be driven forward, in reverse, or braked, as used in motor driver schematics and described in IEC 60617 block-diagram conventions.
Also known as: H bridge, full bridge driver, motor bridge, four-quadrant driver, bidirectional motor driver, dual half-bridge.
What the H-Bridge symbol means
The H-Bridge symbol denotes a power electronics circuit block consisting of four switching elements (typically power MOSFETs, IGBTs, or BJTs) arranged in a bridge configuration. The load (motor or other inductive load) is connected between the mid-points of two half-bridge legs. By closing diagonal pairs of switches (high-side left + low-side right, or high-side right + low-side left) the motor current can flow in either direction, enabling forward and reverse rotation. With both high-side or both low-side switches closed, the motor terminals are shorted together for active braking.
In schematic block diagrams the H-Bridge symbol is a rectangular block with five pins: In A (left control input, for the left half-bridge), In B (right control input, for the right half-bridge), Motor (the load/motor output connection), VCC (the motor supply voltage), and GND (the power return). The inputs accept logic-level PWM signals; the internal gate drivers amplify these to switch the power transistors at the motor supply voltage.
How to identify the H-Bridge symbol
The H-Bridge symbol is drawn as a rectangular block labelled 'H-BRIDGE' or 'MOTOR DRIVER'. On the left side are two control signal pins: In A (upper) and In B (lower). On the right side is the Motor output pin. VCC appears at the top centre and GND at the bottom centre. Inside some representations the four switch positions are shown schematically as four transistor symbols in the 'H' arrangement — two high-side switches connected to VCC and two low-side switches connected to GND, with the motor connected across the bridge mid-points.
Function in a circuit
The H-Bridge operates by selectively closing pairs of switches to route current through the motor in the desired direction. For forward rotation: In A high activates the upper-left and lower-right transistors, allowing current to flow from VCC through the motor to GND from left to right. For reverse rotation: In B high activates the upper-right and lower-left transistors, reversing the current direction. PWM (pulse-width modulation) on the input pins controls average motor voltage and therefore speed. Logic-level protection circuits prevent both high-side and low-side switches on the same leg from being on simultaneously (shoot-through protection), which would short VCC to GND.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a unique symbol for an H-Bridge specifically. It is represented as a general functional block per IEC 60617 standard block conventions, with annotations for motor supply (VCC), control inputs (In A, In B), and motor output. Power electronics textbooks (IEC TC22 series) describe the H-Bridge as a 'single-phase full bridge converter'. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 (ANSI Y32.2) does not define an H-Bridge symbol. It is drawn as a labelled rectangular IC/module block with input and output pin annotations per general IEEE 315 IC block conventions. In power electronics schematics it may be shown with the four individual MOSFET symbols in the bridge arrangement. |
| Key difference | Both IEC and IEEE/ANSI represent the H-Bridge as a functional rectangular block when used as an abstracted module symbol. When drawn at the transistor level, IEC and ANSI use slightly different MOSFET and BJT symbols (IEC uses a different body-diode representation than ANSI), but the bridge topology and labelling are identical. |
Terminals / pins
| Pin | Name |
|---|---|
| in_a | In A |
| in_b | In B |
| motor | Motor |
| vcc | VCC |
| gnd | GND |
Typical values
Supply voltage VCC: 2 V to 48 V typical (load-dependent). Continuous output current: 0.6 A (DRV8833) to 30 A (L6208) depending on IC. PWM frequency: up to 200 kHz for MOSFET-based drivers. Typical R_DS(on) per switch: 0.3–3 Ω for integrated drivers. Shoot-through dead-time: 200 ns to 1 µs.
Where the H-Bridge symbol is used
- DC motor control for robotics — H-bridges on drive wheels allow forward, reverse, and pivoting (differential drive) from MCU PWM outputs
- Stepper motor bipolar drive — each winding of a bipolar stepper is driven by one H-bridge to allow full-step and microstepping current control
- BLDC motor drives — three H-bridges (one per phase) constitute the three-phase inverter for brushless DC motor control
- Automotive actuator drivers — power window motors, seat adjusters, and mirror controllers use H-bridges for bidirectional control
- Linear vibration actuators and haptic feedback devices — H-bridges drive voice-coil actuators with alternating current waveforms
- Solenoid valve drivers — H-bridges apply forward voltage to energise and reverse voltage to quickly de-energise inductive solenoids
- Battery management systems — bidirectional DC-DC converters in BMS cells use H-bridge topologies for charge and discharge path control
Example
In a two-wheel robot schematic, two H-Bridge blocks are shown — one per motor. Each H-Bridge block has its In A and In B pins connected to two Arduino PWM output pins (e.g., D5 and D6), its Motor pin connected to one DC motor terminal (the other motor terminal connects to GND through the bridge), VCC connected to the 7.4 V LiPo battery pack, and GND shared with the Arduino ground. By varying the PWM duty cycle on In A, the robot controls motor speed; toggling In B HIGH for the right motor while In A drives the left motor enables pivot turns.
Key facts
- The H-Bridge is a four-switch power circuit named for its H-shaped topology that enables bidirectional current flow through a motor or load, allowing forward drive, reverse drive, and active braking from a single DC supply.
- The H-Bridge symbol block has five pins: In A (left half-bridge control input), In B (right half-bridge control input), Motor (load/motor output), VCC (motor supply voltage), and GND (power return).
- Forward rotation is achieved by closing the upper-left and lower-right switches (activating In A); reverse rotation by closing the upper-right and lower-left switches (activating In B); braking by closing both low-side switches simultaneously.
- Shoot-through protection (dead-time control) is essential in H-bridge design: closing both the high-side and low-side switch of the same leg simultaneously would short VCC to GND, causing destructive overcurrent.
- PWM modulation on the H-bridge input signals controls average motor voltage and speed: at 50% duty cycle the average voltage across the motor is VCC/2; at 100% the motor runs at full supply voltage.
- Common integrated H-bridge ICs include the L298N (dual H-bridge, up to 2 A per channel, 46 V), TB6612FNG (dual H-bridge, 1.2 A, 15 V), and DRV8833 (dual H-bridge, 1.5 A, 10 V), each equivalent to the generic H-Bridge block symbol.
- A three-phase inverter for BLDC motors is formed by three H-bridge legs (six switches total), where each leg is controlled independently to produce three-phase AC from a DC bus.
Frequently asked questions
What does the H-bridge symbol mean in a circuit diagram?
The H-Bridge symbol represents a four-switch motor driver circuit that enables bidirectional current flow through a DC motor or load. The 'H' shape describes the four transistors arranged around the load: two high-side switches connected to VCC and two low-side switches connected to GND, with the motor connected across the bridge mid-points. The symbol has In A and In B control inputs, a Motor output, VCC supply, and GND.
What does an H-bridge do?
An H-bridge drives a DC motor in forward or reverse directions from a single DC supply. By closing diagonal pairs of switches, it reverses the direction of current through the motor, reversing rotation. PWM control of the input signals adjusts motor speed. It can also provide active braking by short-circuiting the motor terminals through the low-side switches.
What are the pins on the H-bridge symbol?
The H-Bridge symbol block has five pins: In A (the control input for the left half-bridge leg), In B (the control input for the right half-bridge leg), Motor (the output connection to the motor load), VCC (the motor supply voltage), and GND (the power ground return).
What is shoot-through in an H-bridge?
Shoot-through (also called cross-conduction) occurs when both the high-side and low-side switches of the same H-bridge leg conduct simultaneously, creating a direct short from VCC to GND. This causes destructive overcurrent. Integrated H-bridge ICs prevent shoot-through by inserting a dead-time (200 ns to 1 µs) during which both switches are off between transitions, ensuring one switch is fully off before the other turns on.
What standard defines the H-bridge symbol?
IEC 60617 and IEEE 315-1975 (ANSI Y32.2) do not define a specific symbol for the H-Bridge. It is represented as a labelled rectangular functional block in both standards. When drawn at the transistor level in power electronics schematics, the individual MOSFET or BJT symbols are used per IEC or IEEE conventions, showing the four switches in the bridge arrangement.
What is the difference between an H-bridge and a half-bridge?
A half-bridge uses two switches (one high-side, one low-side) connected in series between VCC and GND, with the load connected to the mid-point and one end (VCC or GND). A half-bridge can only switch current in one direction. An H-bridge uses two half-bridge legs with the load connected between the two mid-points, enabling bidirectional current and therefore bidirectional motor control.
What common ICs implement the H-bridge?
Common H-bridge ICs include the L298N (dual H-bridge, 2 A per channel, up to 46 V, DIP package), TB6612FNG (dual H-bridge, 1.2 A, 13.5 V, SMD), DRV8833 (dual H-bridge, 1.5 A, 10 V, integrated current limiting), and L9110S (dual H-bridge, 0.8 A, 12 V). Each integrates all four switches, gate drivers, and shoot-through protection in a single IC, corresponding to the H-Bridge block symbol.
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