Robomart High Current DC Motor Driver Board

A typical digital logic output pin can only supply tens of MA of current. Even though they might require the same voltage levels, small external devices such as high-power LEDs, motors, speakers, light bulbs, buzzers, solenoids, and relays can require hundreds of MA. Larger devices might even need several amps. To control smaller devices that use DC, a transistor-based driver circuit can be used to boost the current to the levels needed for the device. When voltage and current levels are in the correct range, the transistor acts like a high-current switch controlled by the lower current digital logic signal.An electromechanical relay contains an electromagnetic coil (right side of image above) that moves a metal arm to make and break an electrical connection. Electromechanical relays can be used to switch high current and also AC devices. They provide electrical isolation between the control signal and the load and are relatively low cost. No common ground connection between the control signal and load is needed. A standard digital logic GPIO output pin does not supply enough current to drive a relay coil directly. When using logic signals to control a relay, a driver circuit must be used to boost the current needed to energize the relay?s electromagnetic coil. The load is switched on and off using the relay?s metal contacts that move when the coil is energized. Since the metal contacts actually touch, relays will have less of a voltage drop across the switch point than transistor circuits. They are sometimes used to switch regulated power supplies on and off. Relays tend to more resistant to failure caused by high voltage surges than semiconductor devices.Robosapiens makes a low-cost relay board shown above that contains both the relay and the required driver circuit built using a discrete transistor. The relay coils on this relay require around 6VDC. It is easier to drive relays like this that use a lower coil voltage. The relay can switch up to 220VAC at 20A using a logic signal for control, but the small PCB layout and screw terminals likely limit it to lower voltage and current levels to at least half of these ratings. A similar board is available from Robomart Widget.