This post will provide an easy solution for Bidirectional level translation using single N-channel MOSFET (NMOS). You may need level translator for interfacing variety of circuits. For signaling systems operating at different voltages level. More interesting thing is this is done by using single NMOS. In this particular example I am translating 3.3VDC signal to 5.0VDC signal and in other words from 5.0VDC to 3.3VDC.
In the following figure 1. You can see an Animation of how this level translator works. The red tracks shows higher voltage levels while greens tracks are shows ground. The arrows shows the direction of flowing current. You will find more details by scrolling down.
|Figure 1. Animation for Simulation.|
In the following figure 2. We have a simple signal level shifter or translator. As you see it is very simple by using only one NMOS and two resistors. The push buttons are just for simulating the design. You may connect these pins to your signaling pins. The most interesting thing about this circuit; it is bidirectional also. Now at this condition when no signal is pulled down you will get a corresponding voltage levels at its PINs as shown by voltmeters on both sides.
|Figure 2. No button is pressed.|
In the following figure 3. Let’s say I signaled this pin low this is done by pushing the push button for simulation. You may have noticed the right side voltmeter reading also goes LOW. And current is flowing through both the resistors towards the push button on left side as it is grounded. In technical language what I did is I switch ON the MOSFET by creating VGS > 0.
|Figure 3. Left side button is pressed.|
In the following figure 4. Now I want to test this circuit for its bidirectional feature. So for that when I signaled the right side pin to low. This is done by pushing button for simulation. You have noticed that the voltmeter on left side of the signal is also showing zero. Means that point also goes to low state. Now as you can see the arrows direction is towards right push button because all the current goes through this button. In technical language what happened in this case when I push the button the current flows through the resistor R1 keeping this drain point of NMOS low. So at very beginning the current flows through the diode of MOSFET. As the current starts flowing through diode the voltage at source started decreasing and hence a stage comes where VGS becomes less than zero and MOSFET is in ON state and current start flowing through the MOSFET rather than through Diode. This happened in a few microseconds.
|Figure 4. Right side button is pressed.|
In this example I take an example of 3.3VDC and 5.0VDC you may take any other voltage within their absolute rating of NMOS but remember don’t forget to connect the lower voltage at source side otherwise it will not work.
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