H-Bridge Current Calculator

Estimated Motor Current:

0.00 A

What is an H-Bridge?

An H-bridge is an electronic circuit that allows a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards. It consists of four switches (usually MOSFETs or BJTs) arranged in an "H" configuration. By closing specific pairs of switches, the polarity of the power applied to the motor changes, reversing its rotation.

How the Calculation Works

The steady-state current flowing through a motor in an H-bridge configuration is determined by Ohm's Law, but it must account for several voltage drops and the control signal. The effective voltage is modified by the Pulse Width Modulation (PWM) duty cycle. Additionally, the internal resistance of the switches (RDS(on)) and the Back Electromotive Force (Back EMF) generated by the spinning motor significantly impact the total current draw.

The formula used in this calculator is: I = ((Vcc × Duty) - Vbemf) / (Rm + Rds_total). Note that in a standard H-bridge, current passes through two switches at any given time, so the total RDS(on) should represent the sum of those two active components.

Factors Affecting Motor Current

Several variables influence how much current your motor will draw during operation:

  • Duty Cycle: Reducing the PWM percentage reduces the average voltage, thus lowering the current.
  • Back EMF: As the motor spins faster, it generates a counter-voltage (Back EMF) that opposes the supply voltage, reducing the current. When the motor is stalled (0 RPM), Back EMF is zero, leading to maximum "Stall Current."
  • Switch Resistance: High-resistance MOSFETs generate heat and limit the power delivered to the motor. Using low RDS(on) components improves efficiency.

Frequently Asked Questions

Q: Why is my calculated current higher than the measured current?
A: Real-world factors like wire resistance, battery sag under load, and motor inductance (which limits current rise time in high-frequency PWM) can cause actual current to be lower than the theoretical steady-state calculation.

Q: How do I find my motor resistance?
A: You can measure it using a multimeter across the motor terminals when it is disconnected, or check the manufacturer's datasheet for the "Terminal Resistance" or "Stall Current" (where R = V/I_stall).