Puzzling diode...

[Jeff Liebermann]

On Fri, 8 Apr 2022 17:38:46 -0400, Bob Engelhardt
<BobEngelhardt@comcast.net> wrote:

On 4/8/2022 2:50 AM, Jeff Liebermann wrote:
...

Throw together
an I-V curve tracer and see what it looks like on an oscilloscope:
https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/articles-iv2.gif
https://circuitcellar.com/research-design-hub/create-your-own-i-v-curve-tracer/

I had Googled DIY curve tracer & got really elaborate circuits, so I
didn\'t pursue it.

The fancy I-V curve tracers will produce a family of curves that are
necessary to display transistor gain characteristics. You don\'t need
that for a simple diode.

Is the one you linked to really all you need? (AC & 2
resistors)?

It\'s the one I recommended, but please don\'t assume I like it.
It won\'t work for your diode anyway. The peak voltage from a 12VAC
xformer is about 17V peak. With that low an output voltage, you\'ll
never see the knee of the zener curve for a 79(?) volt zener. The
transformer needs a higher voltage. Above about 48VDC, things become
a bit dangerous. Remember, you have but one life to give for your
hobby (or profession).

You don\'t really need to run it on AC. All AC gives you is the
ability to see what\'s happening with both the forward and reverse
conduction curves in the same plot on the oscilloscope screen. If you
use a variable voltage DC power supply to \"draw\" the forward and
reverse curves independently, you get the same result. The circuit is
really simple. The diode is in series with a resistor to limit
forward current. X-axis (voltage) goes across the diode. Y-Axis
(current) goes across the series resistor. If your power supply has a
grounded negative lead, use a 4 trace scope and differential inputs to
keep the scope ground isolated.

Another way to do this is to use the sine wave output of an audio
generator. Connect a step up transformer to the audio output to get
up to maybe 90 volts peak. Keep the series current low (both forward
and reverse) because the transformer can\'t handle much DC current. You
should get a usable trace on the scope across the diode.

Doesn\'t that have the same problem that I had: once the diode breaks
down & conducts, voltage is dropped across the resistors and the voltage
across the diode drops, a lot?

Yep. That\'s the way a zener works. Just make sure that when the
zener conducts, the current through the zener is low enough to not
blow up the zener when it conducts. Mostly, that means use a large
value series resistor.


--
Jeff Liebermann jeffl@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Bob Engelhardt]

On 4/8/2022 8:34 PM, Phil Allison wrote:

bobenge...@gmail.com wrote:

==========================

I did another, with \"pure\" DC (instead of full wave) & limited the
current. Also measured the current as it approached breakdown. It
started at 46V (1ma), growing to 100ma at 62v, just before it broke down
at 64v.

** A 1W zener will not like 6.4 watts.

Then it went crazy: the voltage across it dropped to 7V (650ma) & 6V at
800ma. Is that the way a plain diode breaks down?

** That is failure, not breakdown.
The Si chip has melted.


...... Phil

But the thing is when I lowered the voltage below 40v, it blocked
current until the voltage was raised again. Starting at 46v & \"breaking
down\" at 64v. Repeatedly. It wouldn\'t do that if it was melted, would it?

 

[jfeng@my-deja.com]

Did you tell us the voltage drop versus a range of forward currents? Since this is for educational purposes, I would open one to see how it is constructed. I am guessing that there was no Si to melt, and it might have been made of Ge, especially if it has a point contact, or if it is a pellet with a spring-loaded contact, or if looks like one contact is \"soldered\" on (alloy junction made with indium).

 

[Phil Allison]

bobenge...@gmail.com wrote:
==========================

I did another, with \"pure\" DC (instead of full wave) & limited the
current. Also measured the current as it approached breakdown. It
started at 46V (1ma), growing to 100ma at 62v, just before it broke down
at 64v.

** A 1W zener will not like 6.4 watts.

Then it went crazy: the voltage across it dropped to 7V (650ma) & 6V at
800ma. Is that the way a plain diode breaks down?

** That is failure, not breakdown.
The Si chip has melted.


But the thing is when I lowered the voltage below 40v, it blocked
current until the voltage was raised again. Starting at 46v & \"breaking
down\" at 64v.

*** FSS finish your sentence.
Broke down to what ? 6V ?
That is what you posted earlier.

Zeners do not have 2 voltages.

FYI Reverse breakdown a regular diode is usually fatal.


...... Phil

 

[Phil Allison]

jfeng@my-deja.com wrote:
====================
> Did you tell us the voltage drop versus a range of forward currents?

** The OP claimed 1V at 3 amps and \"getting warm\".
Which is clearly bullshit.

3W dissipation in a small pack means it would be damn hot.


...... Phil

 

[legg]

On Sat, 9 Apr 2022 09:04:54 -0400, Bob Engelhardt
<BobEngelhardt@comcast.net> wrote:

On 4/8/2022 8:34 PM, Phil Allison wrote:
bobenge...@gmail.com wrote:

==========================

I did another, with \"pure\" DC (instead of full wave) & limited the
current. Also measured the current as it approached breakdown. It
started at 46V (1ma), growing to 100ma at 62v, just before it broke down
at 64v.

** A 1W zener will not like 6.4 watts.

Then it went crazy: the voltage across it dropped to 7V (650ma) & 6V at
800ma. Is that the way a plain diode breaks down?

** That is failure, not breakdown.
The Si chip has melted.


...... Phil


But the thing is when I lowered the voltage below 40v, it blocked
current until the voltage was raised again. Starting at 46v & \"breaking
down\" at 64v. Repeatedly. It wouldn\'t do that if it was melted, would it?

If you\'re judging the breakdown voltage by the temperature of
the zener, you are unlikely to get an accurate. measurement.

If you have meter and can turn your AC test signal into DC,
with a diode and capacitor, you\'ll get a better idea.

The aim in nondestructive testing is to burn up the series
current limiter first, because it\'s cheaper and there are
probably more of them, lying around.

V^2 / R .

RL

 

[Bob Engelhardt]

On 4/9/2022 8:57 PM, Phil Allison wrote:

jfeng@my-deja.com wrote:
====================
Did you tell us the voltage drop versus a range of forward currents?

** The OP claimed 1V at 3 amps and \"getting warm\".
Which is clearly bullshit.

3W dissipation in a small pack means it would be damn hot.

OK, it was too hot to touch. What I was trying to say was that it
wasn\'t so hot as to be destroyed. It seemed like it could run \"forever\"
at that current.

 

[Bob Engelhardt]

On 4/9/2022 8:51 PM, Phil Allison wrote:

bobenge...@gmail.com wrote:
==========================

But the thing is when I lowered the voltage below 40v, it blocked
current until the voltage was raised again. Starting at 46v & \"breaking
down\" at 64v.

*** FSS finish your sentence.
Broke down to what ? 6V ?
That is what you posted earlier.

It looks like something was lost along the way and you didn\'t get the
last part of my sentence that read \"...\'breaking down\' at 64v.\" See above.

Zeners do not have 2 voltages.
FYI Reverse breakdown a regular diode is usually fatal.

This isn\'t marked as a zener & at breakdown: the voltage isn\'t fixed. I
don\'t think it is a zener. Nor does it seem like a plain diode from its
crazy behavior at breakdown. I.e., the breakdown not being fatal.

 

[legg]

On Sun, 10 Apr 2022 10:50:25 -0400, Bob Engelhardt
<BobEngelhardt@comcast.net> wrote:

On 4/9/2022 8:57 PM, Phil Allison wrote:
jfeng@my-deja.com wrote:
====================
Did you tell us the voltage drop versus a range of forward currents?

** The OP claimed 1V at 3 amps and \"getting warm\".
Which is clearly bullshit.

3W dissipation in a small pack means it would be damn hot.

OK, it was too hot to touch. What I was trying to say was that it
wasn\'t so hot as to be destroyed. It seemed like it could run \"forever\"
at that current.

Zener tolerance test current levels, above 48V are usually less
than 10mA.

RL

 

[Phil Allison]

bobenge...@gmail.com wrote:
==========================

But the thing is when I lowered the voltage below 40v, it blocked
current until the voltage was raised again. Starting at 46v & \"breaking
down\" at 64v.

*** FSS finish your sentence.
Broke down to what ? 6V ?
That is what you posted earlier.

It looks like something was lost along the way and you didn\'t get the
last part of my sentence that read \"...\'breaking down\' at 64v.\" See above.

** So your \"breakdown\" = start conducting current ?

Zeners do not have 2 voltages.
FYI Reverse breakdown a regular diode is usually fatal.

This isn\'t marked as a zener & at breakdown: the voltage isn\'t fixed.

** WTF are you on about?

Zeners do not drop suddenly in voltage when conducting.

Nor does it seem like a plain diode from its
crazy behavior at breakdown. I.e., the breakdown not being fatal.

** So you say.


...... Phil

 

[Bob Engelhardt]

On 4/10/2022 5:33 PM, Phil Allison wrote:

bobenge...@gmail.com wrote:
==========================
But the thing is when I lowered the voltage below 40v, it blocked
current until the voltage was raised again. Starting at 46v & \"breaking
down\" at 64v.

*** FSS finish your sentence.
Broke down to what ? 6V ?
That is what you posted earlier.

It looks like something was lost along the way and you didn\'t get the
last part of my sentence that read \"...\'breaking down\' at 64v.\" See above.

** So your \"breakdown\" = start conducting current ?

Zeners do not have 2 voltages.
FYI Reverse breakdown a regular diode is usually fatal.

This isn\'t marked as a zener & at breakdown: the voltage isn\'t fixed.

** WTF are you on about?

Zeners do not drop suddenly in voltage when conducting.

Nor does it seem like a plain diode from its
crazy behavior at breakdown. I.e., the breakdown not being fatal.

** So you say.


..... Phil

Never mind.