Alternative to MOSFET transistors as a switch

Question

I am looking for a 3-legged component that can be used as a switch as an alternative to an NMOS transistor. My circuit currently looks the following: -

^{simulate this circuit – Schematic created using CircuitLab}

The NMOS is supposed to act as a "nearly perfect" switch. The switch is controlled by an Arduino that can output 0 or 5 V.

However, as seen in the circuit the input voltage is differential, hence the source of the NMOS is floating with respect to the Arduino. This causes the NMOS to behave undesirably as it is neither a fully closed switch or fully open switch (it doesn't enter the saturation region properly).

Is there an alternative component I can use instead of an NMOS that would solve this unwanted behavior, or am I faced with a serious design problem?

Answer 1

Put an opto-coupler ahead of the MOSFET. That way you only need to drive the LED side of the opto-coupler with the Arduino. You would then connect the switching side of the optocoupler to the MOSFET gate and the other side to the isolated 5 V supply. To get better drive and the correct logic you might also want to add an extra MOSFET driver or transistor between the opto-coupler and the gate. Last but not least, placing a high value resistor from the gate to source will help insure that the MOSFET stays off when it should be off.

Two typical examples using an opto-coupler:

Answer 2

You are attempting to obtain the impossible device that you asked about in this question

The problem is that the device you are describing is NOT a 3 terminal one - it is a 4 terminal one.
You have shown the "V_Arduino" voltage connecting to ground BUT are pretending that the ground connection does not constitute a terminal.
This is incorrect.

There are two terminals for the differential voltage.
There are two terminal for the control voltage.
You CANNOT have one connection "hang in the air" and pretend that it does not need to have a real world circuit involved.
The control circuit MUST have a reference point.

I explained in my two final substantial comments on your other question what you had to do and why.
I also noted that you needed to listen to what people tell you and understand it.
This is still required.

An opto coupler, suggested there and here, is one solution.
A discrete component solution is also possible BUT will need you to answer the questions asked on your other question.

Answer 3

If you are looking for a "nearly perfect" switch, then a relay should meet your requirements (it's an mechanical switch, excepted that it is moved by an electromagnet instead of my hand). So like a mechanical switch, you have "perfect" isolation (up to some breakdown voltage, often >1000V). You also have a real bidirectional switch (contrary to a MOSFET, that can't block reverse current).

The main drawbacks are :

• bulkier than a mosfet for equal current capability
• the electromagnet consumes current (usually in the 50mA-300mA range)
• because of the "high" current and inductive nature of the electromagnet, you need to add a Mosfet and a free-wheeling diode to drive the relay from the arduino (NB : if you do quick prototyping or a hobby project, you can use relay modules for arduino that already include all this stuff, so you can connect it directly to an arduino GPIO)
• it's more expensive than a mosfet

The other solution, as @Nedd suggested, is to use an optocoupler (or a digital isolator) and a NMOS

Answer 4

Using a 2N7000 is a poor choice. You need a logic level FET. IRLB8721 is an example, but might be overkill if you switch small currents.