NMOS in series with different Vds

Other Hardware General Electronics
1

Hi everyone!

I'm trying to wire two NMOS FQP33N10 in series to get an AND gate. I got it but the results are pretty weird:

circuit(1)

The lamp is a 9V LED, I know the suggested resistors for the arduino pin are 220 and 10k for the source-gate, but I don't have them.

I get the light bulb to turn on only when both the mosfet are on, which is what I was trying to get. The only thing is that Vds for the right mosfet is 6.9V and Vds for the left mosfet is 0V. The Vgs is 4.2V to both of them, so I guess the second mosfet goes in saturation because Vgs > Vth and there is a current coming from the source of the other mosfet.

The problem is: shouldn't Vds1 = Vds2 since the mosfets are in series? The first mosfet is getting all the voltage, so is the Rds(on) of the second mosfet 0? I tried to connect a diode like this:

circuit

The circuit still worked, but I expected to get at least the same Vds on both mosfets because now they would be in parallel (the diode is in reverse bias so the current doesn't flow through it from the right mosfet, making this an OR gate). However, the results are still the same.

Could anyone explain this, please? Probably it's a silly question, but I didn't find much on internet about it.

Edit: I remade the diagram, I'm sorry the preceding one is incorrect due to my inexperience with online diagram softwares:

circuit(3)

Before anyone else gets confused, the problem I am asking help for is why the Vds of the left mosfet is 0V while the Vds of the right mosfet is 6.9V. I probably shouldn't have used the term "AND gate". In the setting I'm proposing, both D9 and D10 are high and the LED lamp turns on.

2

The first problem is I don't know what to believe on your schematic, you have the positive of the battery at the bottom and labelled GND. The LED isn't drawn as an LED so I don't know which way round it is and the MOSFETs are sideways.

The standard way to draw a schematic is positive at the top, negative at the bottom, inputs on the left and outputs on the right. The long bar on a battery is positive. I can't follow what you have, maybe someone else can.

3

Yeah you're right, I kinda mixed up a couple of things in the schematic, I did it with an online software. In reality, the positive of the battery should be up and the MOSFETs with the drain on the right and the source on the left. The Led is with the positive connected to the battery and the negative to the drain of the right MOSFET.

4

It looks to me as though what you have made is in fact a single input NAND gate.

... or an inveter.

6

Hi everyone, I fixed the problem: the two Vds are now 0V when both the mosfets are on and between the drain of the right mosfet and the source of the left mosfet there is 6.9V, as expected from the theory. I'm going to close this thread, thanks to everyone.

7

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.