# Why doesn't ID current chance by Voltage on MOSFET

#### enesene

Joined Mar 3, 2020
29
I want to increase Id current value by voltage but it remains constant although i increase the voltage to make LED brighter. What's wrong with it.. or with me

#### Papabravo

Joined Feb 24, 2006
21,302
The resistor and the LED are limiting the current. The LED probably has a forward drop of about 2.2-2.4 volts. Apply Ohms Law to the drain circuit and what do you come up with?

#### ElectricSpidey

Joined Dec 2, 2017
2,880
Using a MOSFET as a switch is not a very good circuit for controlling brightness of an LED with voltage.

Try using a BJT in emitter follower mode instead.

#### Audioguru again

Joined Oct 21, 2019
6,782
You show a list of Vgg varying but your schematic does not have a Vgg.
I assume that Vgg is shown on your schematic as B1. Then at about 3V the 2N7000 is almost fully turned on. Look at the graph on its datasheet.
B1 voltage higher than 3V does not turn on the 2N7000 any more since it is already turned on.

#### Papabravo

Joined Feb 24, 2006
21,302
BTW -- why are you using an AC voltmeter to measure a battery voltage?

#### Audioguru again

Joined Oct 21, 2019
6,782
BTW -- why are you using an AC voltmeter to measure a battery voltage?
Good catch.

#### WBahn

Joined Mar 31, 2012
30,290
I want to increase Id current value by voltage but it remains constant although i increase the voltage to make LED brighter. What's wrong with it.. or with me

View attachment 202821
Let's run some numbers.

If you just put the 1 kΩ resistor across an 18 V source, you would get 18 mA, right? So you are pretty close to that.

Now consider that a red LED probably has a Vf of something like 2.2 V, so that drops the maximum voltage across the resistor to around 14.8 V, which would limit the current to around 14.8 mA even with transistor, which will drop as least a little bit.

So somewhere between 2 V and 3 V the transistor is going from full cutoff to fully on.

When in doubt, check the datasheet out: https://www.onsemi.com/pub/Collateral/2N7000-D.PDF

If we look at the graphs provided:

You can see that you are operating in the circled regions, so you would expect it to go from full off to full on over a span of perhaps 100 mV to 200 mV.