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Old 1st June 2014, 08:27 PM   #21
BigE is offline BigE  Canada
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No problem, I really appreciate the help. The numbers are so very far out, I did not trust them either, add to that my inexperience, and you have a recipe for uncertainty.

I don't have quite the part right values for the circuit, but I will try a few things and definitely keep you posted!

Thank you very much!
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Old 1st June 2014, 10:32 PM   #22
juma is offline juma  Germany
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Never mind the accuracy of parts values - R5 can be anything from 220R to 2k and P1 can be up to 5k (preferably multiturn pot). It'll do for the purpose of the test.
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Old 2nd June 2014, 12:38 AM   #23
BigE is offline BigE  Canada
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Hello again!

This is strange.... using pin 1,2,3 = S,G,D as per Toshiba....

Pin 1 = floating
Pin 2 = to 3K05 Ohm resistor,
Pin 3 = to Ammeter.

Applying -10V to Pin 2 resistor gives 2.91 mA out of the drain.

Pin 1 = floating
Pin 2 = to 470 Ohm resistor,
Pin 3 = to Ammeter.

Applying -10V to Pin 2 resistor gives 17.91 mA out of the drain.

Pin 1 = floating
Pin 2 = bare,
Pin 3 = through Ammeter.

Applying -10V to Pin 2 gives 122 mA out of the drain.

Attaching the floating pin 1 to -10V directly or through 470 ohm resistor does nothing at all!
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Old 2nd June 2014, 01:07 AM   #24
BigE is offline BigE  Canada
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These are close to the same currents that are delivered across the resistors alone....
however, the jFET is not shorted out. DMM reads OL when measuring ohms between gate and source or gate and drain....
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Old 2nd June 2014, 01:27 AM   #25
BigE is offline BigE  Canada
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A diode tester beeps between gate and drain.

I think this is a bjt!
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Old 2nd June 2014, 08:01 AM   #26
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by BigE View Post
A diode tester beeps between gate and drain.

I think this is a bjt!
The gate to drain and gate to source channels do behave like a diode, that is why the meter beeps. If you've pulled over a 100ma through the device then it may have been impaired even if it seems to function OK at a basic level.

For the majority of JFET's, the drain and source are not defined, you can use either pin and call it drain or source, it doesn't matter. The device is symmetrical.
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Old 2nd June 2014, 01:30 PM   #27
BigE is offline BigE  Canada
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This "jFET" is not symmetrical with respect to source and drain.

It has never functioned ok. The 2sk117's and 2sk170's I have tested show reasonable Idss.

Must find an hfe tester....
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Old 2nd June 2014, 01:34 PM   #28
AndrewT is offline AndrewT  Scotland
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Although I never recommend the hFE function built into many DMM, they are good at identifying BJTs. It confirms the pin out as well as PNP NPN.

I have a 4pin 0.1" socket with 4 wires that plugs into the 4holes of the hFE function.
This allows me to check To126 and To220 devices which are too big to fit the DMM.

When will we see a FET identifying function on our DMM?
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Old 2nd June 2014, 02:45 PM   #29
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by BigE View Post
This "jFET" is not symmetrical with respect to source and drain.

It has never functioned ok. The 2sk117's and 2sk170's I have tested show reasonable Idss.

Must find an hfe tester....
It sounds suspect then. A good basic method of testing is to use an LED and resistor as a load and then vary the gate voltage to see how it performs and whether it cuts off OK. A FET will go wild with a floating gate, even as you just approach it with a finger, a bjt will not.
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Old 2nd June 2014, 03:20 PM   #30
BigE is offline BigE  Canada
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When -10V is attached to the Source and the Gate floats, no current flows.

When the Gate is attached to the Source and -10V is applied, then 122 ma flows. One would think that is due to Vgs = 0 and the jFET is very powerful.

However, when the -10V is removed from the Source, 122ma still flows trough the Gate.
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