MOSFET – Attaching Load to Source on a MOSFET

ledmosfet

I need to control an LED with a N-channel mosfet, but need the cathode to be connected to ground. I learned to do it this way:

Textbook

but when I tried to connect it differently to keep the cathode of the LED grounded like this, there was a big voltage drop across the mosfet for some reason:

Imgur

Exactly why does this happen? Would I be correct in thinking that using a P-channel mosfet would serve my purpose?

Best Answer

Would I be correct in thinking that using a P-channel mosfet would serve my purpose?

Yes, you would be absolutely right.

Since MOSFET's are turned on by applying a voltage between Source and Gate, the diode in your second image acts as that very voltage drop that is stolen from your Source-Gate potential. That pretty much means, that the transistor is no longer turned on by the full battery voltage, but rather \$V_{bat}-V_{diode}\$.

What you are looking for is what's called a high-side switch. This usually requires a P-channel MOSFET.

schematic

simulate this circuit – Schematic created using CircuitLab