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Old 22nd July 2008, 10:45 PM   #1
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Default Safe use of unknown small signal transistors

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?
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Old 22nd July 2008, 11:01 PM   #2
wg_ski is offline wg_ski  United States
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Keep it around 100mW and no more than 25V unless you test Vceo. Should be useable in just about any preamp, line stage, or as diff pairs in low power amps even with no information. You pretty much need a specialized device to get anything lower rated.
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Old 22nd July 2008, 11:53 PM   #3
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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.
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Old 23rd July 2008, 01:05 AM   #4
lineup is offline lineup  Sweden
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Quote:
Originally posted by mashaffer
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.

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!



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

Regards, Lineup
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File Type: png njfet_max_current_tester.png (2.9 KB, 139 views)
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Old 25th July 2008, 02:40 AM   #5
lineup is offline lineup  Sweden
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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
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Old 26th July 2008, 01:18 AM   #6
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Thanks for the input. Sorry I took so long to respond, a lot going on here.

I think I understand. Are you saying that the jfet limits its own drain current? So if I try to bias a 5mA fet to 10mA it will only pass 5mA anyway?
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Old 28th July 2008, 02:48 PM   #7
wg_ski is offline wg_ski  United States
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To get 10mA out of a JFET with a 5mA IDSS you would have to forward bias the gate. Limit the gate current to a fraction of a mA and it won't hurt anything, but you *will* get a lot of distortion because your current swing will be asymmetrical.
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Old 28th July 2008, 11:10 PM   #8
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Oh, OK. Similar to driving a pentode I guess except that raising the "plate" voltage has less effect?
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Old 29th July 2008, 12:35 AM   #9
lineup is offline lineup  Sweden
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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.
Quote:
.
http://www.ampslab.com/trans_2sj74bl.htm

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

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