• Author Author: robrenz
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Keithley 2450  Zener diode test

robrenz

I just had to try out the sweep functionality of the 2450 on a 12V zener.  Very easy to set up and the graph mode is very functional.The pan and zoom is a little slow and seems to have a glitch if you zoom in too far. Still very awesome.

 

As graphed with auto on X and Y axis Y is in linear mode

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As graphed with auto on X and Y axis Y is in Logarithmic mode. Notice Amperage is shown as all positive ??????

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Zoomed on the knee of the Linear Y graph

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Zoomed on the knee of the Logarithmic Y graph

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Zoomed on the forward conduction of the Logarithmic Y graph

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Time to get back to getting used to this unit and doing the review.

  • in reply to shabaz

    That quick test was done with the standard test leads from the front panel connections just to get familiar with the unit. So the first step is to re-do the test with triaxial cables from the rear so that the driven guard can minimize leakage currents in the cables. Also switch to 10NPLC and add some filtering. I also did not have the diode in a metal shield box during the test. It was not a glass body diode so I don't think that there were any photoelectric effects going on.  I have to make up some triaxial leads first.

  • in reply to robrenz

    Hi Rob,

     

    It is leakage current, but because of some offset inside the test tool, the measured current has the wrong polarity (which is why it is shown to flip at -7.xV rather than 0v). In a way you're trailblazing at testing out this, because it's rare to have the capability to go down to this incredible resolution. If it wasn't for the high-ish voltage it could even be dissimilar metals I suppose :-). I don't know what p.d. they could generate; some physics expert may know. These are just some ideas to help start to explain it, because I suspect it takes some different than normal thinking when the capability exists to measure such tiny values. I think there is a chance it may correct itself by increasing the time to maybe half a minute or so for the same number of steps (so that the box has time for a more accurate reading)  - not sure if that will help or not.

  • in reply to shabaz

    I don't think that is offset, it is the leakage current of the diode. Without something with this kind of resolution and accuracy it is easy to forget that they leak.  I did not time it but it seemed like about 10 seconds for the .1V steps from -20V to +1V.

  • in reply to robrenz

    Hi Rob,

     

    I know what you mean, actually it is folded, perhaps a different color could have shown that leg of the graph I suppose. However, most users will know that the direction is reversed, so I think it's acceptable. Incidentally there is a bit of an offset shown in the graph, but it is tiny (hundreds of pA) just going by the indication that direction was measured to be reversed at -7.x volts (but we know it must have been at zero volts), but I don't know if this can be reduced by changing the refresh rate (e.g. sweep over a minute or so, or just sweep over a smaller range) - no idea if this is a capability in the box or some filtering/averaging. How long did it take to take the measurements and generate the graph? Even at this level the results are impressive, but I have a feeling it is capable of higher accuracy since it seems to be able to represent these levels graphically.

  • in reply to shabaz

    I am saying that they should just fold the log scale over around Y zero and have logarithmic negative current shown properly. If I am interpreting the current log scale properly it should fold over at the -7.5 V where it touches the Y axis zero.  Without having already seen the linear version of the same data you have no idea that the current shown as positive to the left of -7.5 volts is really negative.   I consider this a firmware bug until someone educates me as to why I am wrong (which is very often)