Driving using a single MOSFET

Logic-level outputs (such as the IO pins of a Basic STAMP or a PIC chip) have nowhere near enough current capacity to drive a motor directly. Typically they can source (i.e. connect to +5v) or sink (i.e. ground) up to 20mA without damage. You need to be able to use the logic-level output voltage (0v or 5v) to turn on/off a device that can handle 100mA of current, or more, up to 30A, depending on the motor being controlled.

There are many ways to do this. Conceptually simplest is to use a relay; some relays can be closed by 5V at 40mA or less. (The coil resistance would have to be 125 ohms or more.) Even 40mA is too much for many chips, however.

Most practical is to turn on/off a MOSFET. MOSFETs in their open state may have a resistance of 10^12 ohms, while in their closed state as little as 0.01 ohm - a remarkable dynamic range. Further, they are turned on by a "gate voltage" with essentially no current required. There is some gate capacitance, typically 5nF, but this won't require much current except at very high switching rates.

MOSFETs have three terminals, called Drain, Source, and Gate. If you are familiar with bipolar transisitors, an N-channel enhancement-mode MOSFET may be compared to an NPN transistor, where Drain-Source-Gate of the MOSFET are Collector-Emitter-Base of the NPN, respectively.

We'll use a MOSFET with the Source grounded, the Drain connected (through through the load) to a positive supply voltage, and a logic level input to the Gate, which will turn the Source-Drain connection on/off.

MOSFET Gate Voltage

MOSFET specifications indicate an "on voltage" applied to the Gate (with respect to Source), required to turn the Source-Drain connection on. The higher the Gate voltage, the lower will be the resistance of the Source-Drain connection. Graphs show the dependence of Drain-Source current on Drain-Source voltage and Gate-Source voltage. Often these are called just Drain current, Drain voltage, and Gate voltage.

For many MOSFETs, turning them on fully requires 7 to 10 volts at the Gate -- which is more than a standard logic level. The IRFU3708 is nice in that it requires relatively low Gate voltages, and conducts high Drain currents. (see graph)

MOSFET Circuit

The circuit diagram shows how to use the IRFU3708 to control a high-current load, such as a solenoid or a brushed DC motor. Note that the logic level output is 0 or +5 volts, but the +V applied to the load can be much higher than that; for instance you can control +24v applied to the load.

The diode is important for inductive loads such as motors, relay coils, solenoids, or electromagnets. Recall that once a current is flowing in an inductor, it has an "inertia" and will not easily stop. If the MOSFET is suddenly turned off (opened), the (downward) current flowing through the inductor will be able to continue briefly (upward) through the diode. Without the diode, the inductor could generate a high L*di/dt voltage and burn out the MOSFET.

What if you want to put a current in the opposite direction through the motor, to make it turn the opposite way?

With a single MOSFET, you can't. You can use an H-bridge, which contains four MOSFETs. Or, you can use a DPDT relay as a "reversing switch" to reverse the motor within the circuit. The coil of the relay will have to be driven by its own logic signal and MOSFET of course. The last circuit shows a DPDT relay arranged as a reversing switch.

My MOSFET gets hot / no longer works!

You might need to heat-sink it. (more coming)