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Old 21st March 2011, 02:18 PM   #5941
AndrewT is online now AndrewT  Scotland
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Your ammeter in the supply feed also drops voltage. This results in a variable Vds as Idss changes.

You could swap the 100r to 10r or even 1r0 for a much lower voltage change with changing Idss.
A 199.9mVdc DMM scale will read to 0.1mV across 1r0 and show a resolution of 0.1mA which is good enough for rough batching of Idss.
When you have a batch that are within 0.5mA of Idss, you can increase the resistor to 10r or 20r for a finer resolution of the Idss. Far better than using an ammeter.
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Old 21st March 2011, 02:51 PM   #5942
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Quote:
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Your ammeter in the supply feed also drops voltage. This results in a variable Vds as Idss changes.
How much is likely to be dropped from an average quality DMM in mA mode?
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Old 21st March 2011, 02:56 PM   #5943
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measure it.
It will depend on what the manufacturer used for the shunt resistor.

It's the poor tolerance of that shunt resistor that requires the manufacturer to show different tolerances for the different A & V scales.
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Old 21st March 2011, 03:28 PM   #5944
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My 'good' calibrated one measures 2.2 Ohm resistance for my el cheapo one in mA series mode including the leads of course. So the cheap DMM will change VDS by 0.022V when in line with a 10mA IDSS mode JFET. Not bad in my book. The better one claims 0.15% accuracy in DCmA range. The economic one 0.8%.
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Old 21st March 2011, 06:31 PM   #5945
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Thanks AndrewT and Salas for the clarification. In Erno's test circuit, am I correct in assuming I need a bi-polar power supply, then? Or just supply 10v across the + and - points?
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Old 22nd March 2011, 08:24 AM   #5946
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Default Relativity

As I have the smaller PSU handy, I will just test Vp w/o my bipolar supply. I can never get absolute accuracy with my present bench tools. What is important is the relative test results I get from my batch of jfets.

I've done a tentative breadboard layout of the V1.2 shunt, attached. Not all parts are correctly sized yet. More of a general layout concept.

Thanks all for continued support.

-Kent
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File Type: gif shuntv1.2.gif (47.9 KB, 212 views)
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Old 22nd March 2011, 10:21 AM   #5947
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Quote:
Originally Posted by kstlfido View Post
Thanks AndrewT and Salas for the clarification. In Erno's test circuit, am I correct in assuming I need a bi-polar power supply, then? Or just supply 10v across the + and - points?
Use the left one you posted with 1M. It will do the same job with just a battery.
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Old 22nd March 2011, 10:33 AM   #5948
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Quote:
Originally Posted by kstlfido View Post
As I have the smaller PSU handy, I will just test Vp w/o my bipolar supply. I can never get absolute accuracy with my present bench tools. What is important is the relative test results I get from my batch of jfets.

I've done a tentative breadboard layout of the V1.2 shunt, attached. Not all parts are correctly sized yet. More of a general layout concept.

Thanks all for continued support.

-Kent
Just to make sure, this one looks like the V1.2R, right?
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Old 22nd March 2011, 08:14 PM   #5949
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Correct..I meant V1.2R.
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Old 23rd March 2011, 04:52 PM   #5950
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Question Subsonic filter

I'm looking for a subsonic filter to add, could you suggest any schematic?

Last edited by merlin el mago; 23rd March 2011 at 04:59 PM.
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