is it a problem with my transistor, or did I miswire it?

the voltage source is 5v.
Drain is connected to a LED which is connected to 5v-
Source is connected to 5v+
Gate is connected to a pushbutton, which is connected to 5v+

so, I'm testing out this J201, and this happens:

Video

Now, I'm not any kind of expert but... I've heard that J201 is "Unstable," but it shouldn't be THIS unstable... right?

 

The gate is floating when the button is not being pushed.

 

They are so sensitive and "unstable" they make good proximity sensors.

 

Welcome to AAC!

I've heard that J201 is "Unstable," but it shouldn't be THIS unstable... right?

Why would anyone say it was "unstable".

The gate is a reverse biased diode (e.g. high impedance). You're coupling line voltage to the gate with your hand.

A schematic is always preferred to a textual description:

 

Leaving a gate unterminated is a recipe for malfunctioning, as you have discovered already.
Additionally, an N-channel JFET requires a *negative voltage* to turn off. With zero VGS voltage, the channel current is maximum, what the datasheet calls IDSS.

 

One might ask why you rushed through the description of how the device works before hooking one up. That or you didn't bother to read the description in the firstplace. Either one is a problem you should attempt to fix. This sit has a wealth of introductory information. You should at least read it once before the next "experiment".

 

Welcome to AAC!

Why would anyone say it was "unstable".

The gate is a reverse biased diode (e.g. high impedance). You're coupling line voltage to the gate with your hand.

A schematic is always preferred to a textual description:
View attachment 314391

thanks but, i wasn't touching it

 

thanks but, i wasn't touching it

You don't have to touch it. Just capacitance effect will make it go crazy.

 

Leaving a gate unterminated is a recipe for malfunctioning, as you have discovered already.
Additionally, an N-channel JFET requires a *negative voltage* to turn off. With zero VGS voltage, the channel current is maximum, what the datasheet calls IDSS.

thanks

 

You don't have to touch it. Just capacitance effect will make it go crazy.

ok. well thanks again

 

In these examples below the J201 improves different types of stability.
The sensitive nature of FET low current switching and liquid crystal displays forced us to learn to work lower current.

Could be used as a current limiting diode or low current source where very small current is needed.
J201 low current source can improve performance where a resistor alone may tend to heat up, the J201 takes the heat allows the resistor to perform within a narrower tolerance range. Using a heat sink will further improve the current regulation. TO92 package are ok but less helpful for mounting.

In a resonant circuit that uses a varactor diode the frequency stability is improved by low current and voltage regulation.
It was the wide voltage range that gave voltage-controlled oscillators adjustable frequency range over simple crystal oscillators.