The MOSFET power output portion of the circuits used in various projects on this site serves to increase their power handling capacity.
MOSFET (Metal Oxide Semiconductor Field Effect Transistor) – A voltage controlled transistor for amplification and switching.
The circuits in the 555 timer video series use an enhancement mode N-channel type power MOSFET, which is controlled by a positive voltage at the gate. Enhancement mode MOSFETS are off when there is no voltage at the gate. The transistor will be either fully on or fully off for most of the time in this application. When it is off, the transistor’s resistance is very high, so only a very small amount of leakage current will pass through it. When it is on, the resistance is very low. Because the MOSFET will be in these two states for the majority of the time, very little power is consumed, so minimal heat in generated. However, if the current required by the load is high enough, then sufficient heat will be generated due to the resistance of the transistor when it is on that heat will become a issue. Heat sinks and multiple MOSFETs connected in parallel can be used to increase power handling.
The diode in the MOSFET portion of the circuit provides a path for the current resulting from the voltage spikes that occur when the power to an inductive load is switched from on to off. A diode used in this capacity is called a flyback or snubber diode. The maximum forward current rating for the diode should be high enough to handle the full current of the load, which it will be momentarily subject to each time the MOSFET shuts of the power to the load. The maximum repetitive reverse voltage for the diode needs to be as high as the voltage across the load. The diode is only needed across the load when that load is inductive like with an electric motor.
Gate resistor – MOSFET gates have capacitance, and require charging which results in a delay between when the voltage is applied and when the transistor is fully on. MOSFETs also have a small amount of inductance which can react with the capacitance as unwanted oscillations. These oscillations can be controlled by way of connecting a resistance to the gate. This lowers the Q and dampens the oscillations. The resistance also increases the amount of time it takes for the capacitance to charge and discharge, so it increases the amount of time it takes for the transistor to turn on and off. Without an appropriate gate resistor, the oscillations can cause the transistor to turn back on when it should be in the off state. The oscillations can also damage the MOSFET.