Measuring V across a JFET

Thread Starter

SamR

Joined Mar 19, 2019
2,604
Trying to measure VDS across this JFET and all I see is zero. Can you not measure the V across it? I can measure across R1 + J1 or across R1 but cannot measure across the 2N5458 by itself. Is there actually 0V and no internal resistance at all? I wouldn't think so but that is what I am seeing. 0.0000VDC across J1 and 5.9949VDC across R1. R1 actually measures 101.9Ω. I am tweaking the V2 power supply to measure exactly -2.00VDC at the Gate.

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Thread Starter

SamR

Joined Mar 19, 2019
2,604
OK, I went back to it again and now I have a V from the D pin to ground. So VDS. Not sure just what I may have changed but now I am measuring the voltage. Had to be a wiring problem with the meter leads. Had me scratching my head for a while.
 

Bordodynov

Joined May 20, 2015
2,662
A minus added to the minus gives a plus. Since you're using the LTspice, look at the voltage on the gate. You've shifted the pn junction in a forward direction!
 

Thread Starter

SamR

Joined Mar 19, 2019
2,604
Was actually doing it on a breadboard and just using LTS to illustrate it. Had to have been a wiring issue with the meter leads but now working. Kind of a rat's nest of wires with 2 power supplies and 3 meters attached.
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EDIT: Corrected the double negative 2 volts on LTS so there is -2V on the gate and it correlates closely to what I am actually measuring now when I run the simulation.
 
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MisterBill2

Joined Jan 23, 2018
7,050
The one question to always ask about data: "Is this reasonable" will often lead to finding an error of some kind. I have meter probes that seem to only make contact at the points, although it looks like the rest of the probe tip is bare metal. That can lead to a bit of confusion .
 

Thread Starter

SamR

Joined Mar 19, 2019
2,604
That's what had me scratching my head, it wasn't reasonable. I had a PN junction that typically has a lot of R. In fact when I measured the FWD bias of the bare JFET it was ~3MΩ. But not being familiar with them in their ON state and conducting there was this niggling doubt that it would be like trying to measure the V across a wire since I was getting 0V. I knew it was ON and conducting as I could measure V across RD. But, as usual, it was apparently a wiring issue that I finally resolved after working on it multiple times over the day for quite a while. Ah well, frustrating but at least it gave me something to do and think about.
 
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ci139

Joined Jul 11, 2016
1,696
just using LTS to illustrate it
\(\unicode{x1f44d}\) it's what it's primary purpose is - i assume
a wiring issue with the meter
the copper inside the meter leads can ?? wear-off ?? actually snap open (but at certain bend it still gives a contact-enough to fool you think they're ok)
+ the range switch can end up in a 'bad' position (both cases familiar identified)
 

Delta prime

Joined Nov 15, 2019
511
Had to be a wiring problem with the meter leads. Had me scratching my head for a while.
Hello there :) Who doesn't, but you're allowed . When we resolve other people's questions about circuits of their..."own design" they think we do it by magic. But we know it's not magic to the magician.;)
 
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