Electronic Switch Series

Welcome to the wonderful world of transistors!  There are many applications, but lets begin with the simplest.  An on/off switch.  In this series, we will use MOSFETs to turn things on/off with our computers.

  1. We begin by downloading MOSFET components to LT Spice.
  2. Next we simulate MOSFETs as switches in LT Spice.
  3. Than we control a physical LED with an NMOS switch.
  4. Coming Soon, a larger MOSFET is used to control my desk lamp!
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Simulating MOSFET Transistors

MOSFETs as Electronic Switches

We want to use transistors to power on/off devices remotely and/or automatically.  One useful tool at our disposal is the Metal Oxide Semi-Conductor Field Effect Transistor (MOSFET).  There are two primary types of MOSFETs that you should be aware of:

  1. P-Channel  (+)
  2. N-Channel (-)

Use a P-Channel MOSFET if you would like to switch a circuit before the load and with no power at its gate resulting in an ON switch.  Applying voltage to a P-Channel gate will disconnect the load.  Normally On

Use a N-Channel MOSFET to switch a circuit at its return or “common” wire. With no power to at its gate results in an OFF switch.  Applying voltage to a N-Channel gate will allow current to flow through the transistor.  Normally OFF

Hint: A simple way to simulate a load is by adding a resistor.

Now, how do we know how much voltage to apply to the gate?  Well.. our goal is to reach the saturation point of the transistor, where the maximum amount of current will flow through the Drain – Source junction:

NMOS Voltage - Current Profile

NMOS Voltage – Current Profile

Critical equation:  Vgs = Vds – Vt

Translation:  set Gate Voltage equal to Source to Drain Voltage minus Threshold Voltage (or higher)

Refer to component specification sheet for threshold voltage and other information.  EXAMPLE

Can’t find the LT Spice components used in this tutorial?  Please refer to my previous video for instructions on how to download and install them.

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