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Avalanche diodes?

G

Gloria West

Jan 1, 1970
0
I see that avalanche diodes have a "knee" in the reverse-bias region when you
look at the V-I characteristics.

What is the "knee" voltage? I see ratings of maximum reverse voltage and
current for the diode, but nowhere is it mentioned at what voltage the diode
starts to conduct in the reverse bias direction.

For example:

<http://www.datasheetcatalog.org/datasheet/vishay/86049.pdf>

Another Q:

If used to protect a thyristor, for example, is the avalanche diode installed
reverse polarity compared to a standard rectifier (ie, 1N400x)? (since the
avalanche diode is conducting current in the reverse direction and the
rectifier is conducting in the forward direction).

Another Q:

Why use an avalanche diode rather than a rectifier of similar max voltage &
current & recovery speed to protect a thyristor? Is it that the avalanche
diode will work less hard because it will not conduct below its operating
voltage (fairly high, I'm guessing), whereas the rectifier conducts above
~0.7v?

Thank you.
 
G

Gloria West

Jan 1, 1970
0
You should really do your own homework.
Phil Hobbs

Maybe you can fix this motor controller for me?

I love usent. It attracts the full spectrum. Even the bottom of the barrel
can respond to queries.

Cheers.
 
J

Jamie

Jan 1, 1970
0
Tim said:
Since Phil is one of the 'top of the barrel' folks on this group I guess
that places you, eh?
Top and bottom barrels?

Sounds like kraut to me! Yum!

You come to a newsgroup when school is starting and ask a question that
sounds like homework -- exactly what do you expect the response to be?

Jamie
 
P

Phil Allison

Jan 1, 1970
0
"Gloria West"
I see that avalanche diodes have a "knee" in the reverse-bias region when
you
look at the V-I characteristics.

What is the "knee" voltage? I see ratings of maximum reverse voltage and
current for the diode, but nowhere is it mentioned at what voltage the
diode
starts to conduct in the reverse bias direction.

For example:

<http://www.datasheetcatalog.org/datasheet/vishay/86049.pdf>


** That is a "controlled avalanche" rectifier diode - which merely
distinguishes it from standard types were reverse breakdown behaviours are
unspecified and to be avoided.

Such diodes have been around for decades and are useful where one wishes to
build up the max reverse voltage by putting several diodes in a series
string. Another use is mentioned in the link you quoted under "pulse
energy", where very small amounts of energy can be absorbed during reverse
breakdown.

It is not a form of zener diode and cannot be used as a voltage clamp and
hence there is no specification for reverse breakdown voltage.


Another Q:

If used to protect a thyristor, for example, is the avalanche diode
installed
reverse polarity compared to a standard rectifier (ie, 1N400x)? (since the
avalanche diode is conducting current in the reverse direction and the
rectifier is conducting in the forward direction).


** That is an extraordinarily naive question.

Another Q:

Why use an avalanche diode rather than a rectifier of similar max voltage
&
current & recovery speed to protect a thyristor? Is it that the avalanche
diode will work less hard because it will not conduct below its operating
voltage (fairly high, I'm guessing), whereas the rectifier conducts above
~0.7v?

** Even worse.


..... Phil
 
G

Gloria West

Jan 1, 1970
0
** That is an extraordinarily naive question.

So your point is.. "I know more than you do"?

Well done.
 
P

Phil Allison

Jan 1, 1970
0
"Gloria West"
So your point is.. "I know more than you do"?

** No.

You have made absurd assumptions.

And you over snip.

And you are bad mannered.

And I bet you are not female.



.... Phil
 
G

Gloria West

Jan 1, 1970
0
From what I've seen, avalanche rectifiers have waned in popularity - giving
ground to MOVs - which in their turn have stared to give way to SIDACs.

MOVs generally have higher spike quenching capability than avalanche and are
less likely to fail S/C, they can however shatter and arc when damaged by
extreme surges.
Ian Field

Thanks, Ian, for the information. I did not know that MOVs can take the place
of avalanche dioces. I had a difficult time locating avalanche diodes locally
but MOVs I can find and have bought a few.

Re. sizing: the MOV should be sized such that the clamping voltage is above
the working voltage of the target device (that it's protecting) but less than
the failure voltage of the device, yes? And the balance between these two is
the trick?

Thanks for not playing the BS game that is so prevalent in usenet.

Cheers.
 
P

Phil Allison

Jan 1, 1970
0
"Gloria West"
Thanks, Ian, for the information. I did not know that MOVs can take the
place
of avalanche dioces.


** Well, that is because they cannot. MOVs are not diodes.

And you are still using the term ambiguously.

Try reading the part of my first post you rudely snipped.

Thanks for not playing the BS game that is so prevalent in usenet.


** The bullshit is all coming directly from you.



..... Phil
 
J

Jamie

Jan 1, 1970
0
Gloria said:
Thanks, Ian, for the information. I did not know that MOVs can take the place
of avalanche dioces. I had a difficult time locating avalanche diodes locally
but MOVs I can find and have bought a few.

Re. sizing: the MOV should be sized such that the clamping voltage is above
the working voltage of the target device (that it's protecting) but less than
the failure voltage of the device, yes? And the balance between these two is
the trick?

Thanks for not playing the BS game that is so prevalent in usenet.

Cheers.
I like using TVS diodes, the 1.5k series to start with. They make
them larger of course.

Mov's have a limited number cycles before they become permanently
clamped in the circuit or blow apart. I guess if you have enough energy
in the over load, both would destroy themselves how ever, I find that
TVS diodes recover more often and they are more compact.

Then again, maybe you should correct what is causing the issue in the
first play that is pushing you to this route.

Using MOV's is a good way of getting the equipment on the bench to
further investigate problems since they will eventually fail, and if
that being the case, it keeps the simple guys from replacing fuses or
resetting breakers. If things get that back, something is wrong.

Jamie
 
P

Phil Allison

Jan 1, 1970
0
"hrh1818"

For the devices you referenced there is a table for Electrical
Characteristics. It provides a value for the Breakdown Voltage, 1600
volts max This is the knee voltage. However for the devices you
referenced the typical value is not defined. The actual Breakdown
Voltage will be some place between VR, the repetitive reverse voltage
rating, and the max Breakdown voltage rating of 16000 volts. The
devices you referenced are designed to be used as rectifiers and not
in the avalanche mode.

** Correct.

Controlled avalanche rectifier diodes are NOT the same as "avalanche" diodes
use for transient suppression.

The OP has fallen for one of the perils of doing Google searches under a
single word.


..... Phil
 
K

Kaz Kylheku

Jan 1, 1970
0
I see that avalanche diodes have a "knee" in the reverse-bias region when you
look at the V-I characteristics.

What is the "knee" voltage? I see ratings of maximum reverse voltage and
current for the diode, but nowhere is it mentioned at what voltage the diode
starts to conduct in the reverse bias direction.

Hi Gloria,

After a diode starts to conduct, in either the forward or reverse
direction, any further increases in voltage are marked by huge increases
in current (a sharply rising curve on the current versus voltage graph).

So for all practical purposes, the knee voltage and the max
are the same. These V{sub}R values are basically the parameter you are
looking for. That's why they are listed for the various models of the
diode in the very first table in the datasheet: the basic info everyone
wants is up front.

If the avalanche diode has a VR of 200V, and you put, say, 210V across
it, it is toast. Diodes are used with some kind of current limiting
resistance (e.g. resistor). If a voltage in excess of the diode's
voltage is placed across this circuit consisting of the diode and the
resistor, then (approximately) the diode will drop its 200V, and the
resistor takes the rest. Current is determined by the resistor's
I = V/R. (A more accurate solution demands that we draw the load line:
the graph of the resistor current versus the diode's voltage, rather
than its own. Where the load line intersects the diode curve is the
operating point, or Q point, which tells us the voltage and current.)
Another Q:

If used to protect a thyristor, for example, is the avalanche diode installed
reverse polarity compared to a standard rectifier (ie, 1N400x)? (since the
avalanche diode is conducting current in the reverse direction and the
rectifier is conducting in the forward direction).

Basically the whole point of using an avalanche or zener type diode is
to take advantage of its reverse breakdown behavior, so it is hooked up
such that a reverse voltage is applied.
Another Q:

Why use an avalanche diode rather than a rectifier of similar max voltage &
current & recovery speed to protect a thyristor?

I'm not sure that the comparison makes sense.

A rectifier is a circuit that uses one or more diodes to turn AC into
DC. It doesn't protect anything.

We should not say "rectifier" to mean "rectifier diode".

A rectifier diode doesn't protect anything either. Well, it protect some
part of a circuit from the ingress of a voltage of the wrong polarity!
(But if that voltage is too great, the rectifier diode will suffer
reverse breakdown, which, unlike for an avalanche diode, is probably
fatal for the diode.)

A rectifier does not protect from overvoltage. If a spike of excess
voltage comes into a rectifier, it generally goes out. (Unless it's a
half-wave rectifier, and the spike is on the suppressed half.)
diode will work less hard because it will not conduct below its operating
voltage (fairly high, I'm guessing), whereas the rectifier conducts above
~0.7v?

In rectifiers, the forward drop across the diodes does not go above that
0.7. Again, there is a load line there: the diodes supply current into
some load which drops most of the voltage. If the load resistance is
too small for the diode to handle, it will fry, but it will not attain,
say 1.5V.

The diodes conduct above 0.7V, but they never go far above that. If the
voltage in the circuit is much more than 0.7, something else has to pick
up the remainder of that voltage.
 
R

Rich Grise

Jan 1, 1970
0
Gloria said:
Thanks, Ian, for the information. I did not know that MOVs can take the
place of avalanche dioces. I had a difficult time locating avalanche
diodes locally but MOVs I can find and have bought a few.
Ah. After a little clarification, it seems that what you're actually
looking for is a Transzorb:
http://www.vishay.com/diodes/protection-tvs-esd/trans-zorb/

I once worked at a place where they had MOVs all over the place -
MOVs have a soft breakdown curve, and they deteriorate a little bit
with each transient they suppress. When the company muckety-mucks
discovered Transzorbs, they issued ECOs to throw away all the MOVs
and replace them with Transzorbs.

It worked.

Good Luck!
Rich
 
J

Jamie

Jan 1, 1970
0
Rich said:
Gloria West wrote:



Ah. After a little clarification, it seems that what you're actually
looking for is a Transzorb:
http://www.vishay.com/diodes/protection-tvs-esd/trans-zorb/

I once worked at a place where they had MOVs all over the place -
MOVs have a soft breakdown curve, and they deteriorate a little bit
with each transient they suppress. When the company muckety-mucks
discovered Transzorbs, they issued ECOs to throw away all the MOVs
and replace them with Transzorbs.

It worked.

Good Luck!
Rich
I've been using the 1.5k series for some time now. I use lots of 11V
bipolar types to protect sensitive 10 Volt inputs/outputs. We also have
some option modules that employ LM324 chips which seem to get hit a lot
in the applications we use them on. I place a TVS on the input and
output of those units and we no longer have issues with those, too.

I recently did home repair for one my friends Electric dog fence that
got hit by mother nature, that unit uses 2 48V BP types that saved the
day directly connected to the R.F. loop. That just goes to show they can
also be used in some basic low Freq R.F. applications.

Mov's are good in conditions where things are not suppose to happen.
TVS diodes are great in applications where you suspect something to
happen randomly.

Jamie
 
Ah. After a little clarification, it seems that what you're actually
looking for is a Transzorb:
http://www.vishay.com/diodes/protection-tvs-esd/trans-zorb/

Transorb is one trade name for a TVS diode. I use lots of 'em. They even
come in SC-70s, five per. ;-) They're especially useful if you can get some
impedance in front of them. Also useful after a polyfuse.
I once worked at a place where they had MOVs all over the place -
MOVs have a soft breakdown curve, and they deteriorate a little bit
with each transient they suppress. When the company muckety-mucks
discovered Transzorbs, they issued ECOs to throw away all the MOVs
and replace them with Transzorbs.

Different component for a different purpose. I don't think I'd use a TVS
diode on the AC entry. ;-)
 
R

Rich Grise

Jan 1, 1970
0
Transorb is one trade name for a TVS diode. I use lots of 'em. They even
come in SC-70s, five per. ;-) They're especially useful if you can get
some
impedance in front of them. Also useful after a polyfuse.


Different component for a different purpose. I don't think I'd use a TVS
diode on the AC entry. ;-)

Nah, these were mostly on inputs of sensors used in a HV ultra-high vacuum
environment with various electron guns, ion guns, x-ray sources, and
secondary electron analyzers and all kinds of kewl stuff. :)

http://en.wikipedia.org/wiki/Auger_electron_spectroscopy
http://www.google.com/search?hl=en&q=cylindrical mirror analyzer

I was an Engineering Tech, my first job out of the USAF in 1977;
I arranged to get myself fired after my boss quit and they hired
the Fourth Stooge to replace him. ;-)

Cheers!
Rich
 
Nah, these were mostly on inputs of sensors used in a HV ultra-high vacuum
environment with various electron guns, ion guns, x-ray sources, and
secondary electron analyzers and all kinds of kewl stuff. :)

http://en.wikipedia.org/wiki/Auger_electron_spectroscopy
http://www.google.com/search?hl=en&q=cylindrical mirror analyzer

No, I meant that TVS diodes and MOVs were two perfectly good components, in
the appropriate application. I agree, using MOVs where a TVS should be used,
as you implied was the case at the PPoE, is dumb. The opposite is also true
(TVSs across the mains).
I was an Engineering Tech, my first job out of the USAF in 1977;
I arranged to get myself fired after my boss quit and they hired
the Fourth Stooge to replace him. ;-)
There's a lot of that going around.
 
P

Phil Allison

Jan 1, 1970
0
"hrh1818"

What else is different and
makes one device an avalanche diode and the other a controlled
avalanche rectifier?


** Forget badly worded definitions - look at the specs.

There is a huge difference in the energy that can be absorbed during reverse
conduction.



..... Phil
 
J

Jamie

Jan 1, 1970
0
John said:
Thank you for your comments. Perhaps you can clear up some
confusion. I found this definition for controlled avalanche
rectifier. "A silicon rectifier in which carefully controlled,
nondestructive internal avalanche breakdown across the entire junction
area protects the junction surface, thereby eliminating local heating
that would impair or destroy the reverse blocking ability of the
rectifier." Whereas Wikipedia says "An avalanche diode is a diode
(usually made from silicon, but can be made from another
semiconductor) that is designed to go through avalanche breakdown at a
specified reverse bias voltage. The junction of an avalanche diode is
designed to prevent current concentration at hot spots, so that the
diode is undamaged by the breakdown."

These two definitions leave me confused about what is the real
difference between a controlled avalanche rectifier and an avalanche
diode. The only difference I have found so far is the reverse
breakdown voltage for an avalanche diode has tighter specifications
than a controlled avalanche rectifier. What else is different and
makes one device an avalanche diode and the other a controlled
avalanche rectifier?

Howard


If a diode has a tightly specified reverse breakdown voltage, like 5
or 10 per cent, and is intended to be used as such, we call it a zener
diode, even though the mechanism may technically be avalanche.

A "controlled avalanche rectifier" has a specified minimum reverse
breakdown voltage but usually no max. It's a rectifier that's designed
to dissipate some amount of reverse-bias power without being damaged.

The rectifier diode is, well, intended to be used as a rectifier. The
zener is not. [1]

John

[1] I screwed up and used an LM1117 to regulate 3.3 volts down to 2.5
for an FPGA Vcc_aux supply. My bad. Turns out that a 7.5 volt MELF
zener, use in the forward direction, drops almost exactly 0.8 volts at
our operating current, and solders beautifully into the 1117
footprint. So that's what we do now.

ftp://jjlarkin.lmi.net/Diode.jpg
Wouldn't it be safer to place a TL431 on the output to insure it does
not move above that 2.5V ? I think the little extra drift can be
suppressed with the 431 with out any issues.


Jamie
 
John said:
"hrh1818"




For example:

<http://www.datasheetcatalog.org/datasheet/vishay/86049.pdf>

For the devices you referenced there is a table for Electrical
Characteristics. It provides a value for the Breakdown Voltage, 1600
volts max This is the knee voltage. However for the devices you
referenced the typical value is not defined. The actual Breakdown
Voltage will be some place between VR, the repetitive reverse voltage
rating, and the max Breakdown voltage rating of 16000 volts. The
devices you referenced are designed to be used as rectifiers and not
in the avalanche mode.

** Correct.

Controlled avalanche rectifier diodes are NOT the same as "avalanche" diodes
use for transient suppression.

The OP has fallen for one of the perils of doing Google searches under a
single word.

.... Phil

Thank you for your comments. Perhaps you can clear up some
confusion. I found this definition for controlled avalanche
rectifier. "A silicon rectifier in which carefully controlled,
nondestructive internal avalanche breakdown across the entire junction
area protects the junction surface, thereby eliminating local heating
that would impair or destroy the reverse blocking ability of the
rectifier." Whereas Wikipedia says "An avalanche diode is a diode
(usually made from silicon, but can be made from another
semiconductor) that is designed to go through avalanche breakdown at a
specified reverse bias voltage. The junction of an avalanche diode is
designed to prevent current concentration at hot spots, so that the
diode is undamaged by the breakdown."

These two definitions leave me confused about what is the real
difference between a controlled avalanche rectifier and an avalanche
diode. The only difference I have found so far is the reverse
breakdown voltage for an avalanche diode has tighter specifications
than a controlled avalanche rectifier. What else is different and
makes one device an avalanche diode and the other a controlled
avalanche rectifier?

Howard


If a diode has a tightly specified reverse breakdown voltage, like 5
or 10 per cent, and is intended to be used as such, we call it a zener
diode, even though the mechanism may technically be avalanche.

A "controlled avalanche rectifier" has a specified minimum reverse
breakdown voltage but usually no max. It's a rectifier that's designed
to dissipate some amount of reverse-bias power without being damaged.

The rectifier diode is, well, intended to be used as a rectifier. The
zener is not. [1]

John

[1] I screwed up and used an LM1117 to regulate 3.3 volts down to 2.5
for an FPGA Vcc_aux supply. My bad. Turns out that a 7.5 volt MELF
zener, use in the forward direction, drops almost exactly 0.8 volts at
our operating current, and solders beautifully into the 1117
footprint. So that's what we do now.

ftp://jjlarkin.lmi.net/Diode.jpg
Wouldn't it be safer to place a TL431 on the output to insure it does
not move above that 2.5V ? I think the little extra drift can be
suppressed with the 431 with out any issues.

If I was going to add a bunch of new parts, I could just use a real
LDO and a couple of programming resistors.

....or a fixed LDO, and forget the programming resistors. ;-)
The FPGA isn't very picky about the Vcc_aux voltage, so the funny
zener trick is safe. And it's sure not going to oscillate.

Always a tradeoff but there is usually plenty of margin in the LPUL (as long
as you're not trying to drive 3.3V logic or something equally dumb).
 

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