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Testing N-JFET gate - source breakdown voltage (FAKE jfets!) (Successful finale)
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SilverSolder:
I did manage to source some genuine PN4391 N-Jfets.   These behave as expected in the test setup, blocking the drain-source current when a negative gate-source voltage is applied.  I haven't verified the breakdown voltage yet, that's the next step now that reality and expectations are in perfect harmony!

Interestingly, these JFETs show up as two diodes on the cheapy component testers.   I guess they "don't dare" to apply enough negative gate voltage to detect these devices (the cut-off is almost -15V).

The newer model does at least correctly indicate the Drain-Source resistance, which identifies the device adequately as a Jfet.  The older model is not able to tell it apart from a double diode.

Older model first:



Newer model:

edavid:

--- Quote from: SilverSolder on March 26, 2019, 12:05:29 am ---Interestingly, these JFETs show up as two diodes on the cheapy component testers.   I guess they "don't dare" to apply enough negative gate voltage to detect these devices (the cut-off is almost -15V).

--- End quote ---

The VP spec for the PN4391 is -4 to -10V.

The component tester has a 5V supply voltage, so it's not a matter of "don't dare", it's just impossible.

Zero999:
Get an expensive component tester. Mine has a small 12V battery and will no doubt test higher pinch-off voltage J-FETs with no problem, although I've not tried it, as I don't have any.
SilverSolder:

--- Quote from: edavid on March 26, 2019, 12:58:36 am ---The VP spec for the PN4391 is -4 to -10V.

The component tester has a 5V supply voltage, so it's not a matter of "don't dare", it's just impossible.

--- End quote ---

You might be able to do something with a capacitive voltage doubler?  Doesn't need to make the device much more complicated or expensive, or so it seems to me.



--- Quote from: Zero999 on March 26, 2019, 12:19:31 pm ---Get an expensive component tester. Mine has a small 12V battery and will no doubt test higher pinch-off voltage J-FETs with no problem, although I've not tried it, as I don't have any.

--- End quote ---

That sounds interesting - these things certainly are very convenient, for sure.  What component tester do you have there?

Zero999:

--- Quote from: SilverSolder on March 26, 2019, 01:48:31 pm ---That sounds interesting - these things certainly are very convenient, for sure.  What component tester do you have there?

--- End quote ---
It's the Peak DCA55. It uses an MN21 12V battery, but I don't know whether it actually puts out that higher voltage. For all I know, it could all run off an internal 5V regulator.
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