mosfet question

so why is this:

different than this:

??

So I got a mosfet transistor and I was playing around with it and found the first schematic online. In my particular application, it would be impractical to run Vcc->load->Drain as opposed to Vcc->drain->source->load-gnd.

I tried switching the resistor and LED in the schematic to the second schematic and it was very dim. Seems backwards to me. Wouldnt the drain be the output of the mosfet?

N-channel MOSFET needs a voltage from G to S to turn on. In the top, S is 0V, G goes to 5V, so Vgs is 5V.

In the bottom, S goes to (Vcc - Vds) (very small) when G goes to 5V, so Vgs is very small.

Thank you!

Would another type of mosfet work in that way?

Use N-channel for top diagram, and P-channel for bottom diagram.

A common NPN transistor works with the second example. Voltage to the LED Will be about 4.3-4.4 volt.

Pelle

A PNP would work better in the 2nd example tho.

P-chanel works too.

Pelle

Yes, I mentioned that in #3. Need to ensure Logic Level MOSFETs are used when 5V power is used.