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The circuit on the right is what we are after (without the bottom res.) ..but we want to install a Nmos switch.
The reason for the 0.05 Ohm resistor there on the bottom right is because we want an Nmos switch to replace that resistor. We know that the value of this Nmos built-in resistor (Rds) is 0.05 Ohm as per its datasheet.
Now on the left circuit, the Nmos has indeed replaced that bottom right resistor, but it is not set at our chosen 0.05 Ohm as the Rds yet.
Let's set the Nmos Rds now in its settings:
1: Alter the VTO to be 3v ... that is our chosen on/off threshold crossing value ... off below 3v on above 3v
2: Alter any of the KP, W or L so that the drain terminal of the mosfet (not the arrow end) reads as 1mV
Why did I want the drain voltage to be 1mV ?
Because that will create the 0.05 Ohm Rds ... Look at the voltage across the bottom resistor on the right.
That 1mV seen in the wire at the far right can simply be used for the similar position drain wire voltage.. easy.
You have now set up a Nmos switch correctly.
It will switch on and off when the shown 5v gate volts passes the 3v position.
This Nmos is simulating a real mosfet which has a Rds value of 0.05 Ohm and a 3v Vth in its datasheet.
See how the LED is bright now with 20mA just like the circuit on the right.... not dim like the unaltered default.
If ever you want to set up a Nmos on EC to do full switching, but you don't want to bother about the Rds, then just use the easy settings as shown in the spare Nmos at the bottom left. ... The EC default Nmos is not set up for normal full switching, as you can see by the dim default LED.
Your turn:
Set the batteries both to be 24v each and the top resistors to be 1k Ohm each.
Set the bottom right resistor to be 0.08 Ohm, then make a note of the voltage across that resistor, as above.
Now set the Nmos to be 0.08 Ohm Rds by adjusting any W, L, or KP and adjust VTO to switch on/off at 2v
Extra if reqd:
For the Nmos circuit...
Set:
W=10mm
L=200nm
KP=200u/V^2
VTO=3v
Vdd=12v
Rd=500 Ohm
Vgs=5v
I=20mA seen
Vds=1mA seen
Those settings are just one set of a multitude that you could use to get a Nmos Rds= 0.05 Ohm
Now use this formula and see if you get 0.05 for the Rds by using those Nmos settings.... You should.
Rds = [KP(Vgs-Vth)(W/L)] ⁻¹
Advanced users may recognise Kn as 0.5KP(W/L) here, and so see that Kn is related to Rds
Later we will see how the effective Rds will alter beyond this datasheet minimum value in working situations.
Nmos simulation Guide for EC: http://everycircuit.com/circuit/6260747729633280
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