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How to identify fake JFETs?
How to identify fake JFETs?
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Old 16th June 2019, 12:57 AM   #71
808eengr is offline 808eengr
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I don't doubt that there are fake parts all over the place out there. Was just trying to find out why the LCR-T4 tester was giving different results.

...Apparently my testing error is quite obvious. If anyone could point out what the obvious testing error is it would go a long way in making the criticism constructive.
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Old 16th June 2019, 09:32 PM   #72
PRR is offline PRR  United States
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Quote:
Originally Posted by scott wurcer View Post
I don't understand the continued mis-understanding here you CAN'T test and N channel JFET as an NPN transistor.
It's well known that these little totally automated testers do OK for 99% of parts, but sometimes go down the wrong path.

The tester does what you would do with unknown part and orientation: poke some juice at it and try to interpret it as some class of device, then measure like it was that class.

No, the user can't give the tester a clue.

These things were written by Programmers. Any transistor-man would know that hFE=99K is bogus, even 2K is dubious; but the programmers didn't know this.

I went into the notes and code once, it's all open-source. But too complicated for me. The algorithms and decision-points have been changed over the years, they are better. Maybe the one he has uses an older code; maybe so few people use high-Gm JFETs that this screw-up has not bubbled to the top of the bug-list.

IAC it is trivial to rig a battery and two meters (one is less convenient) and measure JFETs the right way.
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Old 17th June 2019, 07:24 AM   #73
DualTriode is offline DualTriode  United States
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Yes, that is more than a bit harsh.

I too purchased a cheap $20.00 transistor tester it also incorrectly identified a bag of JFETs as NPN transistors.

The cheap $20.00 transistor tester was intended to do a quick screening. In this case it did not work out so well.

Thanks DT
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Old 17th June 2019, 01:34 PM   #74
adason is offline adason  United States
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I too have that cheap tester, and I find it very useful. It never happened to me that it would identify jfet as npn transistor.
I use this guidelines (Fig 9) to construct curves to match jfets:
Biasing JFETS

The tester has limitations, but for quick Idss selections its ok. Plus it measures caps, resistor, diods, you name it. Not bad for $20.
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Old 17th June 2019, 02:17 PM   #75
Zung is offline Zung  Switzerland
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It's more like $10 shipped: for that price, no complain, but it doesn't do FETs; for these, I use EUVL's jig. Next, I'll add Vp as per PMA.
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Old 17th June 2019, 02:44 PM   #76
Trollet is offline Trollet  Sweden
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How to identify fake JFETs?
I have a DCA-75 PRO and it works with Most component, but sometimes it fails with Lateral fets.
I don't match them with the DCA anyway, but had to check if it worked.
Hfe test for small signal BJT's seems to work fine with resonable tolerance.

Figge
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Old 17th June 2019, 05:51 PM   #77
w5jag is offline w5jag  United States
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I love the convenience of the Mega328 tester.

Mine has also mistakenly auto detected JFET's as bipolar transistors.

They are terrific considering the cost, but some user intuition is still required. It has never gotten an EBC pinout wrong on a bipolar device, and when it detects a JFET, it gets that right also.
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Old 17th June 2019, 11:15 PM   #78
808eengr is offline 808eengr
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Seeing how many of the diy circuits measure values at certain steps only gets me thinking... maybe it would be nice to have an Arduino based curve tracer. Could always start with a simple breakout/hat, and can be written in C.

Matlab simulink has lots of interesting features. Last semester there was a guy in one of my project classes using Matlab Simulink and an Arduino DUE as as DAQ for a marine thruster. The sample rate is low but seems like there is potential for much more analysis once the data is in Matlab.

That said the cheap tester is convenient for identifying most parts plug it in push the button. Beyond that it seems like there might be room for another solution once the device type and pinout are known and one is trying to find the device parameters.

C code easily generates a Matlab best fit line graph, it was one of my lab homework problems last semester. Maybe I'll break down and get an arduino after all. And try to figure out to make the code generate excel best fit line.
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Old 18th June 2019, 01:55 AM   #79
davidsrsb is offline davidsrsb  Malaysia
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Most cheap tracers sense source current by voltage drop measurement across a source resistor. When Vgs is very low as in laterals and high Gm jfets, this does not work unless there is a gain stage after a low value sense resistor before the ADC
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Old 20th June 2019, 03:41 PM   #80
stereohifi is offline stereohifi  Canada
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About thoses? I haven't tested or tried them!
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