Safe use of unknown small signal transistors

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I have salvaged a lot of parts from junked solid state organs and the like. Often times parts are either unmarked or marked with internal part numbers only making it difficult to get a data sheet for them. Often the type of part can be determined from schematics or even connection information on the PCB itself (D,S, G, E, B, C etc) but nothing else.

With power devices this is obviously a big problem but I wonder if there is a general rule for typical current, dissipation and voltage for small signal devices. In other words is it possible to accurately say that 90% of the time a small signal JFET can handle at least X volts d-s with Y mA source current and Z mW dissipation?
 
mashaffer said:
Thanks wg_ski, what would you guess on s-d current on jfet? Should I keep it to a couple of mA?
I am looking at a SF which I would like to bias up to about 10mA if possible.

Different JFET types and different exemplars within a type have a value called MAX IDSS.
For a 2SK170BL this is something like 8-12 mA.
For a 2SK170GR calssified it may be like 4-6 mA.

This is the max current this JFET can run, when Volt Gate-Source (VGS) is = 0 Volt.
So, you can not normally run too much current in a JFET.
It is limited to that max IDSS value.
Of course you can run too much power making it burn
by apply high voltage across Drain-Source.
Power = Volt x milliAmpere

See my attached Simple NJFET Tester.
The main headache is to find out the G, D, S pins.
Then you attached Gate + Source to ground
and Drain to + 5 Volt.
:cool:
As I have chosen 100 Ohm for measurment circuit,
this will automatically limit to max 50 mA and max power will be =
5 Volt x 50 mA = 250 milliwatt ( 0.25 Watt )
Which is 99% a safe value!

:cool:

For example, read the 2SK170 datasheet about MAX IDSS .. etc. etc.
http://www.ampslab.com/PDF/2sk170.pdf

Regards, Lineup
 

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Of course there are more advanced JFET testers.
There are also real Projects for Building JFET testers
that can do N & P and several tests, by just using a switch
while testing devices.

A forum search about
matching - testing - JFET
... might render some useful stuff

Regards
 
maschaffer

You should NOT force JFET to deliver more than IDSS current.
This is the upper limit of current we should use for audio.
And my little tester circuit is what you use to see what is IDSS
of any N-JFET transistor you have.

There are some JFET with 50 and even 100 mA IDSS.
But for audio buffers and input stages
you can count Hi-Fi jfet has got max like 10-15 mA.

My posted link to 2SK170 datasheet,
shows the ruling standard low noise JFET used for today's Hi-Fi applications.

2SK170 has got one Complementary P-JFET sister:
.. but this P-JFET is getting harder and harder to find .. as production has stopped = Obsolete.

This following is an extreme good bargain offer for Audio JFET.
Of course, if not United States, you have to add some oversea's money.
.
http://www.ampslab.com/trans_2sj74bl.htm
:cool:
2SJ74BL Toshiba Low Noise Audio JFET P-Channel $20.00 for 20 pieces

2SK170BL Toshiba Low Noise Audio JFET N-Channel $20.00 for 20 pieces

:cool:
 
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