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Constant Current Source Using Transistor (MEC131E)

This article was written based on my book Robotics, Mechatronics and Artificial Intelligence (sold out) published in the United States.The described circuit can be used to control speed and torque of a DC motor or to drive other loads such as SMAs or heaters.

An interesting circuit for robotics and mechatronics designers is the one shown in Figure 1. This circuit maintains a constant current flow across a load. This can be important to keep the motor speed constant even when its load changes, or to keep the current across a heater constant when its temperature changes, thus altering its resistance.

* Remember that constant voltage sources here are the batteries normally used to power the robots and other appliances. These are different from the current sources. Some constant voltage sources are described in other parts of this book.


Figure 1     Constant current source using transistor.
Figure 1 Constant current source using transistor.


The zener diode is a 400 mW or 1 W type, and P1 is used to adjust the current across the load. This circuit can be used to maintain constant currents up to 500 mA. If larger currents are involved, a Darlington transistor can be used.

P1 must be a wire-wound potentiometer, and its dissipation rate is important if larger loads are controlled. The current adjustment can be made using a current meter in series with the load.

If higher voltages are used at the input, the 1 kΩ resistor must be increased in value, and the zener must have larger dissipation power. If the load is a motor, it is convenient to add a decoupling capacitor in parallel. Electrolytic Capacitors between 1 and 470 ?F are recommended for this task.