Simple MOSFET Question

I'm trying to understand MOSFETs, but I don't understand what I am seeing with my simple test circuit.

I have a FDP8440 logic level N-Channel MOSFET. Drain connected to 12V. Source connected to an LED, a resistor, and GND in series. I am switching the Gate between 12V and Ground to turn the MOSFET on/off. Trying to KISS, I'm leaving out the pull-up or pull-down resistor for now.

Why does my meter measure 12V at the Drain, but 10V at the Source? MOSFETs are "current controlled" and I have zero resistance between the 12V and the Gate. Is there not enough current in the load (LED) for it to make it up to 12V? Or, is there some sort of voltage drop associated with MOSFETs (like BJT?).

I'm confused...

Thanks for any words of wisdom.

You are using the MOSFET as a source follower. Vgs must be a couple of volts for the transistor to be ON. Connect the source to ground, and the LED/resistor to +12V. Then you will only get a few millivolts across the MOSFET.
BTW, MOSFETs are voltage controlled.

Thanks Ron. FETs voltage controlled...transistors are current controlled... I know this, not sure what I was thinking when posting, but I'm admittedly a bit confused right now.

Interesting, what you are saying. The load between 12V and Drain, Source to ground. It's backwards from what I was thinking. Clearly I have more studying to do. Hopefully this will get me going in the right direction. One small success at a time.

Think of what happens when you put the load between source and ground, and the gate is at vcc. You probably know that most NMOS transistors are enhancement mode devices, which means they are off when Vgs=0. If the source voltage went to vcc (or very close), the device would be off. So - the source voltage will be less than the gate. The actual Vgs will depend on the load resistance. The lower the load resistance is, the higher Vgs will be, because higher Vgs is required for higher Ids.

Okay, I think I'm following you. The difference in voltage between the 12V and what I measured at Source is basically the voltage the FET required to turn on. Being a logic level FET, that's between 1V and 3V (according to the datasheet). Interesting that it even functioned...

In my corrected experiment, the FET is essentially operating as a switch and controlling the circuit by interrupting the path to ground. I suspect a P channel MOSFET would be connected like my original experiment and would control the circuit by interrupting the 12V power source. Now to find myself a P channel MOSFET to confirm.

Baby steps...

Here is a very good guide to MOSFETs:

https://www.electro-tech-online.com/custompdfs/2010/05/slup169.pdf

krich said:

Okay, I think I'm following you. The difference in voltage between the 12V and what I measured at Source is basically the voltage the FET required to turn on. Being a logic level FET, that's between 1V and 3V (according to the datasheet). Interesting that it even functioned...

Click to expand...

You got it, but I don't understand why you said it was interesting that it even functioned.

In my corrected experiment, the FET is essentially operating as a switch and controlling the circuit by interrupting the path to ground. I suspect a P channel MOSFET would be connected like my original experiment and would control the circuit by interrupting the 12V power source. Now to find myself a P channel MOSFET to confirm.

Baby steps...

Click to expand...

You got it again. Keep in mind, though, that whereas 12V (or less) on the gate turns on the NMOS and 0V turns it off, 12V (or whatever the source voltage is) on the gate of a PMOS will turn it off, whereas 0V (or possibly higher) will turn it on.

Roff said:

You got it, but I don't understand why you said it was interesting that it even functioned.

Click to expand...

Well, I'm sure it's my lack of understanding of the technology beyond a simple on/off scenario. I'm trying, though. As you can probably tell, digital logic is easiest for me to grasp and that is due to my software background. The concept of something not being fully on or fully off (the analog world) is a challenge for me. On the bright side, in my studies I can see that even digital electronics are simply analog with techniques applied to them to make them appear digital (pull ups, pull downs, schmitt triggers, etc.)

I appreciate forums like this one and contributors like you Roff, because among other things, it fills a gap for us beginners who want to know, are not afraid to experiment, and because we have not been formally educated on these things, find that much of the "self help" material on the internet to be WAAYY over our collective heads. The link misterT posted is a good example of that, although much of it made sense to me, so I've kept it and will read it in a few months to see if I've progressed.

The project I'm working on, and that you've unwittingly helped me with, is a wirelessly controlled scoreboard for my daughter's softball teams. I'm planning on writing an iPhone app to control it while we're at the ball parks. Take a peek.

Ramblings of an E-geek Blog

Perhaps this will help explain what is going on: X-axis is VG as it ramps from -2 to 15V. Y-axis are the voltages at A,B,C, and D.