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Old 4th May 2016, 02:59 PM   #1
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Default Zener diode tester

Hello all. I have a lot of zener diodes which is difficult to read their values from. From small to big metal ones. I want to build a zener tester. Would this be a good circuit ? It can test up to 50v and trigger between 5 and 15ma.
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Last edited by Vrystaat; 4th May 2016 at 03:59 PM.
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Old 4th May 2016, 03:09 PM   #2
Mooly is offline Mooly  United Kingdom
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It looks fine. C1 might be a little to small, even for your small currents. The LM317AHV (that's the high voltage version) has a minimum output current (in voltage mode) of around 15 ma worst case I think. I'm not sure how that relates to using it in current source mode.

There is also the TL783 High-voltage Adjustable Regulator which is functionally very similar but can withstand 125 volts differential.
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Old 4th May 2016, 03:19 PM   #3
Elvee is offline Elvee  Belgium
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I don't like the idea of relying on a 317 for current regulation: when you connect a zener, there will be a current spike, because the regulation loop is slow.

At the very least, you should include a series resistor (a few 100's ohm) to avoid frying or damaging zeners, but using just a transistor CCS looks like a much better option: you do not need a very high static accuracy for this kind of tester; a series resistor (a few 10's of ohm) would still be a good idea though.

Here is a tester I made that goes to 200V, but with a fixed 1mA test current:
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File Type: png ZenIt.png (56.6 KB, 149 views)
File Type: gif ZenSupp.GIF (9.1 KB, 146 views)
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Old 4th May 2016, 03:46 PM   #4
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Thanks for the feedback guy's. Elvee you mean to put the resistor in series with the zener under test to lower the current for lower power zeners I presume.
Will a 100V Vceo NPN transistor do ? Or something like a MPSA42. I see the origional is a 400V transistor .
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Last edited by Vrystaat; 4th May 2016 at 03:59 PM.
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Old 4th May 2016, 04:52 PM   #5
Frex is offline Frex  France
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Hello,

If you have an oscilloscope, you can easily plot the I=f(V) of the zener using
the X/Y mode and driving the zener with a AC secondary of transformer and
a series resistor + shunt resistor.
You can look here for example.

Frex
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Old 4th May 2016, 06:09 PM   #6
JMFahey is offline JMFahey  Argentina
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Quote:
the regulation loop is slow.

SLOW???????

Where do you see a time constant?
LM317 rules what current passes there and it's only reading voltage drop across a small value resistor.
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Old 4th May 2016, 06:45 PM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by JMFahey View Post

SLOW???????

Where do you see a time constant?
Like any regulator, it has an internal loop compensation.
In the absence of a zener, it tries to force the maximum output to comply with the regulation/closed loop condition, but since it is impossible, it is saturated and when a load appears, it has to first come out of saturation and then to integrate until it actually regulates.
There is thus a short current spike at the output.

Quote:
Originally Posted by Vrystaat View Post
Thanks for the feedback guy's. Elvee you mean to put the resistor in series with the zener under test to lower the current for lower power zeners I presume.
Will a 100V Vceo NPN transistor do ? Or something like a MPSA42. I see the origional is a 400V transistor .
Like this, but the CCS is a better option:
Attached Images
File Type: png Ztestmod.png (57.8 KB, 124 views)
File Type: png ZtestAlt.png (55.6 KB, 123 views)
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Old 4th May 2016, 07:56 PM   #8
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Thanks all !!
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Old 4th May 2016, 08:07 PM   #9
Elvee is offline Elvee  Belgium
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The "simple" CCS also has the advantage that you'll be able to test up to 75V zeners (avalanche in fact) with an 80V supply (and even more if your primary supply is higher)
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Old 4th May 2016, 08:45 PM   #10
Elvee is offline Elvee  Belgium
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I forgot to put a limiting resistor on the transistor CCS: as soon as there is more than one transistor involved in the loop, it is a wise precaution; for a more complex (and slower) system like a complete VR, it is more than just a precaution: the sim shows the behavior of such a reg when the load is brutally switched from zero to infinity.

Don't trust the fine details, it is just a sim, and the regulator modeled is not a HV (hence the 30V supply) and is a lower current, M-version.
The reality could be better or worse, but the general outlook is correct.
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File Type: png Zentest2.png (10.2 KB, 52 views)
File Type: png LM317M.png (58.0 KB, 45 views)
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