current limiter with 2N3820

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drumanart

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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
 

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It's nicely stated in the forum rules:
Only you know what "doesn't work" means
Click to expand...

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.
 

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.
 

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