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Simple MOSFET Switch
 


 

Simple MOSFET Switch

 


Constructing a MOSFET switch is one of the basic level electronics projects and could be useful for beginners to get a hang of the various FET transistors. The aim of this project is to make control a heavy duty DC load by constructing a simple switch. MOSFET transistors are capable of handling a large voltage and current. It is because of this feature that it is being much sought after for connecting heavy loads to a circuit or a microcontroller.


Simple MOSFET Switch

This circuit allows a simple switch or a low voltage pulse (5V for example) to control a large dc load. There's a good explanation of MOSFET transistors and how to use them as a switch here. This is great for connecting a large load to a microcontroller or other logic circuit. Power MOSFET transistors are perfect for this application and can handle high voltage and current (100V, 77A for the NTP6411). This design would be able to power almost any load you can think of (probably even your car).

Either a switch or a pulse input can be used to activate this MOSFET transistor. Choose your input method by placing a jumper on the appropriate side of the INPUT header. The pulse input will probably be the most useful but a switch closure input was added to handle a specific project. This design was set up for use with 24V dc but it could be adapted to work with other voltages as well (tested fine with 15V dc as is). Just make sure the LED's get the correct amount of current and the input voltage stays within specs for the 7812 and the input voltage capacitor. It should also work with a different N-Channel power MOSFET as long as it's pin compatible with the NTP6411.

The heart of the circuit is the On Semiconductor NTP6411 power MOSFET. This transistor does all the hard work of turning on a huge load while drawing essentially no current from the input signal. A 12V regulator provides an input voltage if you're using the switch control. Otherwise, a low voltage pulse (between 5 and 20V) will turn on the transistor. A flyback diode is included to prevent voltage spikes from inductive loads. A load LED provides a visual indication of transistor activity (helpful when debugging). Screw terminals allow for making removable connections to relatively large wires. Dual power connectors allow for daisy chaining multiple circuits together. The layout was set up for panelizing multiple boards off the bottom edge because input/output connections are on the left and right sides. Heat sinking will only be required with large loads and extended periods of activity.

This design was load tested overnight (16 hours) with an industrial air solenoid valve (24V, 0.5A) and the MOSFET was cool to the touch without a heat sink. However, the valve body was quite hot after dissipating 12W continuously.

Clearly the on state resistance is very low when using the switch input. According to the datasheet the MOSFET will have a higher on state resistance when a small voltage (i.e. 5V pulse) is used so keep an eye on the circuit when using it in a new application.










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