Hello:

I am having trouble with the following circuit in real life. V1 is a constant 3VDC power source in circuit below. 30 ohms resistor in cirucit is led light 3VDC takes 100ma. It should turn on the led light when V2 is less than 0.5V. It should turn off led light greater than 0.5VDC. Does anyone have any luck real life assembling the circuit and turning light on and off with circuit below?

 

Does anyone have any luck real life assembling the circuit and turning light on and off with circuit below?

I don't have a PH2625L mosfet but did work changing the value of R1 with the circuit below.
Keep in mind the mosfet was fully ON with V2 at appx -.5V but full OFF required V2 at -.8V

 

Vgs(th) is listed as 1.5V typical, so why do you think it should be on at 0.5V?
Why do you think your LED should light up at 3.0V?

 

If you want a well-defined switching point you should use a comparator circuit.

 

If you want a well-defined switching point you should use a comparator circuit.

How? Such low points? You have circuit

 

And negative gate voltages at that.

Notice that these are enhancement type FETs, not depletion mode.

Just in case it is helpful, Vth is usually the voltage that causes some minor but defined drain current to flow, not necessarily full on.

 

Why is V2 inverted in your post #1 schematic?

 

Why is V2 inverted in your post #1 schematic?

Givea negative voltage

 

Why do you want a negative voltage for a N-channel enhancement mode FET (as Dick noted)?

 

Why is V2 inverted in your post #1 schematic?

Negative voltage is at end of resistor. It is not applied to the gate.

You have better circuit then please show

 

What is the real-world source V2?

 

What is the real-world source V2?

Solar panel

 

We can keep guessing for a week, but that is not a good use of our time. Please tell us what you are trying to do.

ak

 

I just noticed the negative voltage V2 on the gate input.
This is an N-type enhancement mode MOSFET. You don't need a negative voltage on the gate.

 

I just noticed the negative voltage V2 on the gate input.
This is an N-type enhancement mode MOSFET. You don't need a negative voltage on the gate.

See post #10

 

See post #10

That still does not answer the question.

-0.5 V will turn off the MOSFET.
0.0 V will turn off the MOSFET.
+2 V will turn on the MOSFET.

 

That still does not answer the question.

-0.5 V will turn off the MOSFET.
0.0 V will turn off the MOSFET.
+2 V will turn on the MOSFET.

How? How is voltage at gate negative?

You have (3 - (-0.5))/(R1 + R2). Where you see negative voltage at gate?

 

Ok.
Vgs = 0.775 V
You need higher than that to turn on the MOSFET.
Also try higher V1 voltage.

 

Ok.
Vgs = 0.775 V
You need higher than that to turn on the MOSFET.
Also try higher V1 voltage.

Cant go v1 higher. It powers r3 which landscape lights 3vdc at 100ma

 

It powers r3 which landscape lights 3vdc at 100ma

LED lights should really be driven with a controlled current; not by a fixed voltage. The forward voltage of an LED varies with the current and, because of manufacturing tolerances, supplying 3V does not guarantee that two samples of nominally the same type of LED will result in both passing the same current.
Can you post a link to the lights you will be using?