Apparently I'm not that smart and can't turn an LED on in multisim

What am i doing wrong?

 

Hello,

The led most likely has blown.
Leds are current controlled devices.
You will have to use a resistor in series with the led to control the current.
Also what kind of mosfet do you use?
Standard mosfets will turn on at 10 Volts on the gate.
Logic gate mosfets will work as in your schamatic with the 5 Volts at the gate.

Bertus

 

Welcome to AAC!
I don't use Multisim, but it looks to me as though your probes show a 218uA LED current, so it is 'on' (but not very bright ). What FET are you using? If it's not a logic-level type it may need about 10V to turn it on properly.

 

Hey guys thanks for the reply

This is a generic "ideal" n chanel in multi sim

Its threshold voltage is set to zero in the parameters therefor it should turn on with anything across the gate to source
Also the LED don't blow in the simulator

They are ideal and can take infinite current obviously not in real life but just in the sim

Any other ideas?

No one has any ideas on this?

 

Hello,

Could you put a probe directly at the drain of the mosfet?
That way you can see if the mosfet is in conduction.
If the voltage is 5 Volts at the drain, the mosfet is not conducting.

Bertus

 

I'd try a 2n7000 FET model. That way you know its params (Vgs(th)~=2.1V typ). If all you say is true the LED should be ON but it isn't. Maybe having a Vth == 0V is throwing it off. Try setting it to 2V.

I'd suspect the FET. I'd also put a 240 ohm resistor in series with the LED.

 

Thanks for the ideas

I have tried all that. Im a reasonably experienced guy so i did try moving the threshold up just to see if it worked. I also have tried it with a resistor and not a resistor in with the LED.

I have also put a probe on the drain and there is a drop of about 1.5 volts across the LED leaving 3.5 at the drain which is plenty for VDS

Also i have tried exactly the 2n7000, funny you bring that guy up that is exactly what i tried.

Still the same issue. No light

Also just to go way way way back in the debugging i stuck a voltage source on the led to make sure it comes on and of course it does.

I have to be missing something simple here but can't see it

 

Hello,

With 1.5 volts accross the led, the led may still be off.
What kind of led did you select?
A white led will need more than 3 Volts.
A red or green led will need about 2 volts.

Bertus

 

like i said above its an ideal LED

so it should run on anything

 

I try with proteus and it's woring fine.



If I push the button, the current goes up to 407mA and the drain voltage is 1.5V.

Allen

 

I think you need PMOS.

 

I think you need PMOS.
View attachment 101161

Are you sure about that your diagram is correct ? Do you know why the voltage drop across the PMOS is 0.73V ? Also, have you tried turn off the LED by changing V2 to 0V ?

 

I think you need PMOS.
.............

No. Not it that circuit.
A PMOS will be on all the time due the the parasitic substrate diode.
To use a PMOS, it must be configured as a high-side switch, with the LED going to ground.