What is the correct Idss of jfet? - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th January 2011, 02:42 AM   #1
diyAudio Member
 
Sunsun22's Avatar
 
Join Date: Jul 2004
Location: Hong Kong
Default What is the correct Idss of jfet?

When I measure Idss, the current various for some time before it get stable. I should that the initial startup current as Idss or to the point when the current becomes stable?
  Reply With Quote
Old 9th January 2011, 05:22 AM   #2
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
Make sure you don't touch them with your fingers before measuring and all will be stable.
  Reply With Quote
Old 9th January 2011, 06:55 AM   #3
Calvin is offline Calvin  Germany
diyAudio Member
 
Calvin's Avatar
 
Join Date: Nov 2004
Location: close to Basel
Ji,

if You look at datasheets youŽll find, that several parameters among these the Idss vary with temperature, some varying over a quite wide range.
IŽd test preferrably under the voltage and current conditions the part will undergo later when built into the circuit. In any case give the JFET time to warm up. So You could read one set of parameters at turn on, when the JFET is still cold and another set when it has reached and stabilized at working temperature. In any case its a time consuming work to match JFETs.

jauu
Calvin
  Reply With Quote
Old 9th January 2011, 06:58 AM   #4
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
Sunsun,

If you are using your Curve Tracer, you should measure at steady state under working conditions, as Calvin suggested above.
You need to set both the working current range (which is not always Idss) and the Vds in the actual circuit.


Patrick
  Reply With Quote
Old 9th January 2011, 11:21 AM   #5
diyAudio Member
 
Sunsun22's Avatar
 
Join Date: Jul 2004
Location: Hong Kong
Thanks for the input. I am travelling in China that I will post details of my question when I returned Hong Kong later this week.

Sunsun
  Reply With Quote
Old 9th January 2011, 12:31 PM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
as said above the device Idss varies with junction temperature. That junction temperature is dependent on Ta, ambient temperature, Pd, dissipation at the junction and time since the dissipation last changed.
The standard Idss specified in the datasheets fixes the variables by testing at Tj=25degC and dissipation very short term so that Tj does not change.
We, poorly resourced amateurs, cannot usually replicate that.

If we need precise Id, then in-circuit testing is the best. Next best is modeling so that operating conditions are replicated as close as possible.
Worst is using fingers to place a device in a 3pin socket and applying an unknown voltage at unknown Ta for an unknown time. These measured Idss are almost useless, if precision is required for the circuit to operate properly.
However, many circuits are specifically designed to accept a range of device tolerances and still perform to specification. It is, in my view, bad design to require absolute precision in device parameters to get specification performance.
__________________
regards Andrew T.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
High Idss 2SK170GR? Sweetsweetback Parts 2 31st October 2010 11:40 PM
JFET question - Idss values JammyBStard Solid State 14 6th January 2010 01:54 PM
High Idss jFETs AndrewT Parts 24 30th October 2008 02:50 PM
Spice: how to set IDSS etc? bear Solid State 11 12th August 2005 02:20 PM
Ono - 2SK170, Which Idss? transducer Pass Labs 11 25th March 2003 10:29 PM


New To Site? Need Help?

All times are GMT. The time now is 01:24 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2