lesson
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A microcontroller GPIO pin can safely output about 20 milliamperes of current at 3.3 volts β roughly 66 milliwatts of power. If you wire that pin directly to a motorized door lock or an electromagnetic relay, the pin will instantly burn out.
An actuator is an output transducer that converts electrical energy into physical motion, light, sound, or heat. Designing a circuit to power one safely requires calculating voltage, current draw, and power dissipation.
πInteractive diagram
How do we calculate the exact current and electrical power demanded by a continuous DC actuator like an electromagnetic solenoid?
DC Actuators and Solenoid Power
A solenoid is an electromechanical actuator consisting of a tightly wound coil of wire that pulls a magnetic armature when energized. In steady-state direct current operation, the coil behaves as a simple resistor with resistance R.
Electric power P represents the rate of energy transfer per unit time (1Β watt=1Β jouleΒ perΒ second). For a DC supply with voltage V and current I, electrical input power is given by:
Pinβ=VΓI=RV2β=I2R
πInteractive diagram
In the 1840s, James Prescott Joule showed that any electric current passing through a conductor generates thermal energy at a rate proportional to I2R, known as Joule heating. When a solenoid plunger finishes moving and holds its position, 100% of the input power converts directly into heat.
What happens when the actuator keeps moving continuously, like an electric motor spinning under load?
Motor Efficiency and Mechanical Power
A DC motor converts electrical power Pelecβ=VΓI into mechanical output power Pmechβ. The mechanical power delivered to a rotating shaft equals the product of torque Ο (in Nβ
m) and angular velocity Ο (in rad/s):