Why is my MOSFET switch not behaving as expected?

In summary, the speaker has a circuit with a BSS138 MOSFET on a protoboard. When 3.3V is applied to the gate resistor, the MOSFET turns on and the output shows 3mV. When the gate resistor is removed, the pull-down resistor turns the gate off and the output shows 2.7V instead of the expected 3.3V. The speaker is questioning whether this voltage drop is expected and if their expectations for the MOSFET are incorrect. The other person suggests using a milliammeter to test the current and see if the FET is faulty.
  • #1
saad87
85
0
I have the following circuit wired up on a protoboard.

PZlmB.png


The datasheet for the BSS138 MOSFET is here. I'm puzzled regarding what I'm seeing happen with this circuit - when I apply 3.3V at the gate resistor, the MOSFET turns on fully and I see 3mV at the output. This, of course, is expected.

However, if I remove 3.3V from the gate resistor, the pull-down resistor turns the gate off. I expected to see approximately 3.3V at the output, yet I only see 2.7V. If I replace the 3.3V on R1 with 5V, the output shows 4V. In other words, a volt is being dropped on R1 when the MOSFET is off. Is this expected? Somehow, I expected the MOSFET to have an immensely high resistance when off and hence expected approximately 5V being dropped on it when it's off.

Are my expectations wrong for this MOSFET?
 
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  • #2
What is the Output tied to . . some kind of load?
 
  • #3
One way this could happen is if you used an analog voltmeter to do the measurements.

If it drew about 1 mA while taking the measurements, you would get voltage readings like this..

It could also just be a faulty FET.

One easy test would be to put a milliammeter in series with the supply and see if you can cut off the current when the gate voltage is dropped to zero.
 

1. What is a MOSFET switch and how does it work?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) switch is an electronic component that acts as a switch by controlling the flow of electric current between its source and drain terminals. It can be turned on or off by applying a voltage to its gate terminal, which creates an electric field that controls the flow of electrons between the source and drain.

2. What are the advantages of using a MOSFET switch?

MOSFET switches have several advantages over other types of switches, including high switching speed, low power consumption, and low on-resistance. They also have a high input impedance, making them compatible with a wide range of input signals.

3. What are some common applications of MOSFET switches?

MOSFET switches are commonly used in power supplies, motor control circuits, electronic relays, and other electronic devices that require high-speed switching with minimal power loss. They are also used in audio amplifiers, LED drivers, and voltage regulators.

4. What are the potential issues with using a MOSFET switch?

One potential issue with MOSFET switches is that they can be sensitive to electrostatic discharge (ESD), which can damage the device. They also have a limited maximum voltage and current rating, so care must be taken to ensure that these limits are not exceeded.

5. How do I choose the right MOSFET switch for my application?

The key factors to consider when choosing a MOSFET switch are the required voltage and current ratings, switching speed, and on-resistance. Additionally, the type of load being switched (inductive vs. resistive) and the desired level of ESD protection should also be taken into account.

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