MOSFET's - Just curious

Thread Starter

Ford Prefect

Joined Jun 14, 2010
207
Hi guys,

I have been using MOSFET's for years and all the schematics that I see have the MOSFET's connected as shown in schematic 1 below and I have followed this method/rule/convention of connecting the MOSFET with the load between the Vcc and the MOSFET Drain.
But I am just curious and have not tried it, but can a circuit be connected as in schematic 2 with the load between the GND and the MOSFET source?
If it is possible to connect it as in schematic 2, why is it that all schematics are shown as in schematic 1 - or is it just a convention that people use?
MOSFET.jpg
 
Last edited:

bertus

Joined Apr 5, 2008
21,674
Hello,

For using a N-fet as a high side switch, you will need a high side driver.
The N-fet will only work when the gate voltage is higher than the source.

Bertus
 

Thread Starter

Ford Prefect

Joined Jun 14, 2010
207
For using a N-fet as a high side switch, you will need a high side driver.
The N-fet will only work when the gate voltage is higher than the source
.
Bertus
Hello Bertus, thanks for your answer. But a MOSFET is just and acts like a toggle switch, if used as in schematic 2 it is just providing the Vcc to the other side of the load which is connected to GND. - Or am I missing the point?
 
Last edited:

GopherT

Joined Nov 23, 2012
8,012
Hi guys,

I have been using MOSFET's for years and all the schematics that I see have the MOSFET's connected as shown in schematic 1 below and I have followed this method/rule/convention of connecting the MOSFET with the load between the Vcc and the MOSFET Drain.
But I am just curious and have not tried it, but can a circuit be connected as in schematic 2 with the load between the GND and the MOSFET source?
If it is possible to connect it as in schematic 2, why is it that all schematics are shown was in schematic 1 - or is it just a convention that people use?
It will work but, BUT,
If you have a standard MOSFET that needs the gate to be 10v above the source, the gate, in drawing #2 will need 22v if V+ for the relay is 12V
 
Last edited:

joewales44

Joined Oct 8, 2017
150
finally someone with the same question i had.
i had a hard time grasping this too.
just remember the gate must see a 'difference' in voltage from the source/drain, not just any signal.
most need 10 volt difference to fully turn on.
 

dl324

Joined Mar 30, 2015
13,518
But I am just curious and have not tried it, but can a circuit be connected as in schematic 2 with the load between the GND and the MOSFET source?
You should just try it. When you do, you'll find out why it has issues and a P MOSFET would normally be used to drive a relay high side.
 
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