The current through the LED actually changes dramatically when you short the transistor. ![Roll Eyes ::)](https://www.eevblog.com/forum/Smileys/default/rolleyes.gif)
Also the terminology with transistors is the opposite of relays and switches - when a transistor is open, it conducts current, when it's closed, it doesn't.
Sorry my terminology is wrong, what I meant were open=nothing goes trough, extremely high resistance.
Here's a little modification to hopefully make it clear: http://tinyurl.com/yaz4oeo2
I added some ripple on top of that DC voltage. If you close the switch and short out the JFET the current follows that ripple. (it's a lot higher too)
But with the switch open the JFET is in circuit and it creates a nice and steady DC current, despite the ripple. That's the reason for it. It acts like a current sink, keeping the current constant even with varying voltages.
Wow thanks, what a great illustration, thanks for that, it learned me a lot!! :-)
It looks like the JFET do act as a throttle to keep the current constant.
I guess you have misunderstanding about different FETs.
N-channel mosfet as you know typicaly has a gate threshold voltage somewhere from +2 to +5V. That means if gate voltage relative to source is above that, the mosfet starts to turn on.
Now N-channel JFET has a threshold voltage (actually it is more common to say pinch-off voltage) in the negative region. Like for example -4V to -1V relative to source. So JFET is conducting and is in fact almost fully on when gate is at the same potencial as source. To turn it off, you need to drive gate to lets say -4V (4V below source potential).
In this circuit there is voltage drop resistor on the source. So when current increases, the source is becoming more and more positive. But gate stays at zero. So gate is becoming negative relative to source. As gate becoming more and more negative, JFET starts closing, so current decreases. So it effectively regulates (balances) current.
Oh yes I see, I just thought that "connected to earth" clamped it to ground current and Voltage, but I can see it's not what is happening.
But do the JFET's model / data not have a lot to do with the amount of current it allows to go trough?
If so, how do I select the right JFET? It may be a big difference if you need 200mA or 5A?
Just as Manul and I said:
you don't understand the operation of a JFET. Remove the 1.5 kohm resistor and the switch and do your disco-simulation again.
A JFET lets through a constant current from drain to source depending on the gate voltage.
Shorting the JFET gate to source is what's known as a "current regulator diode". The variable resistor in the source-ground limb allows you to regulate this current.
Now, if you're doing a simulation using a MOSFET, it'll never work. Period.
Try simulating with a 2SK170 (a very famous N-JFET). This should give you ~12 mA if the variable source-ground resistance is zero. The current will drop as you increase the resistance.
After you've done that little task, we can discuss how to completely cut off the current (your dubious 1.5 kohm resistor).
No you are right, I did not understand the circuit right. :-) Did I say MOSFET earlier? If so, am I sorry, sinse yes a MOSFET, BJT and JFET is nothing like each other.
My reason for the 1.5k is I was recommended to use it, if I would let it be a precision dimmer, down to zero.
It's thanks to you guys that I learn so much, I am super grateful!! :-)