Hi all,
I’ve been lurking for a while, but actually spending some time with a breadboard (rather than just reading about circuits and components) has prompted me to join and post. It’s probably too late for anyone to save my sanity, but if anyone cares to attempt salvage, read on.
I’m trying to learn how to work with JFETs. Thought I’d mostly understood the basics but am frustrated that the attached circuit is not behaving as hoped for (please excuse the smudges and rank amateurishness of diagram). I was hoping to use the circuit to measure the Vgs(off) of a few N-channel JFETs, by using the potentiometer to sweep away from the reference/ground (i.e. towards lug 3 on my diagram) until the ammeter just reads zero, then taking Vgs(off) from the voltmeter.
Unfortunately, while the ammeter will vary with the potentiometer, it never gets particularly close to zero. (The measured current also seems lowest in the middle of the pot’s sweep too, which I thought was odd. I’m using a 5K linear taper pot, if that matters.)
The left-hand battery has a voltage wide enough to encapsulate the datasheet’s range for Vgs(off). The right-hand battery is approximately triple the voltage of the left. I have also tried including a drain resistor and a source resistor, without any particular change.
Any thoughts? Feel free to drop a litany of all the stupid mistakes I’ve made. Thanks in advance!
(Can’t for the life of me get my image to show in preview; hoping it does in actual post…)
(Several edits later: success. Success that was not reflected in Preview mode…)