current limiter with 2N3820

[drumanart]

Hello.

I'm try to make a simple current limiter circuit using a P-FET, a 2N3820.
With some FET's the circuit works, with other not.
With the values used at the attached schematics the current limits about 4,5mA.
But as I mentioned above some FET work other's wont. (I use only the 2N3820)

Thanks Martin

 

[FvM]

It's nicely stated in the forum rules:

Only you know what "doesn't work" means

To guess about a possible problem though: FETs have a large type variation of threshold voltage. If the threshold voltage is a bit higher than the value resulting in 4.5 mA regulation, the voltage drop at the 1.2k resistors becomes too large to maintain constant current. Get rid of the resistor.

 

[Audioguru]

The maximum current from some 2N3820 Jfets is only 0.3mA, others have 15mA. The range is 50 to 1.
Some are cutoff with a gate-source voltage of 7.9V and others have 0.3V.