Opinion needed for testing HV diode arrays.

[Jamie]

At work, we have irradiation units that comprise of a vessel
that contains a rectifier stack along with it's chorona cap rings
etc...
Basically, this unit is driven by a single triode (250Kwatts) to
create 100Khz into a transformer which is inside. This transformer
simply steps up, to supply a HV ac of 100Khz, that connects to a
voltage multiplier rectifier etc...

The official book, states that when testing the diode array modules,
simply apply 35 volts in the forward direction across the connections
and you should see ~ 50ma's or on the amp meter of your supply.

Question:
Since these units are subjected to much higher voltage, like around
the 30's,50's or more volts for each one, wouldn't you think it would be
more sensible to fault test these units closer to their operational
voltage for break down test?

Monday, we'll be taking a unit a part "once again", I've made
suggestions this time, to vear away a little from the trouble shooting
bible of these units and start inventing some new techniques, like,
using a HI-POT unit and subject these units to their actual operating
voltage..

Apparently, the low voltage and practical guide test suggested by the
manufacturer of these units are not sufficient for the problem we have
at the moment.

These diode array's are composed of 2.5 amp 1kv generic SI type
diodes, each having a snubber across each. I don't know the exact
count of each, But if I had to take a guess, 50 in each sounds about
right.
The manufacture does not suggest to test the single units inside,
they suggest using 35 volts on the over all that are in series.

This to me only tells me if one have blown open, how about those
that may have avalanched and there by, has inhibited it's voltage
handling ?
http://webpages.charter.net/jamie_5"

 

[Martin Riddle]

message |
| At work, we have irradiation units that comprise of a vessel
| that contains a rectifier stack along with it's chorona cap rings
| etc...
| Basically, this unit is driven by a single triode (250Kwatts) to
| create 100Khz into a transformer which is inside. This transformer
| simply steps up, to supply a HV ac of 100Khz, that connects to a
| voltage multiplier rectifier etc...
|
| The official book, states that when testing the diode array
modules,
| simply apply 35 volts in the forward direction across the connections
| and you should see ~ 50ma's or on the amp meter of your supply.
|
| Question:
| Since these units are subjected to much higher voltage, like
around
| the 30's,50's or more volts for each one, wouldn't you think it would
be
| more sensible to fault test these units closer to their operational
| voltage for break down test?
|
| Monday, we'll be taking a unit a part "once again", I've made
| suggestions this time, to vear away a little from the trouble shooting
| bible of these units and start inventing some new techniques, like,
| using a HI-POT unit and subject these units to their actual operating
| voltage..
|
| Apparently, the low voltage and practical guide test suggested by
the
| manufacturer of these units are not sufficient for the problem we have
| at the moment.
|
| These diode array's are composed of 2.5 amp 1kv generic SI type
| diodes, each having a snubber across each. I don't know the exact
| count of each, But if I had to take a guess, 50 in each sounds about
| right.
| The manufacture does not suggest to test the single units inside,
| they suggest using 35 volts on the over all that are in series.
|
| This to me only tells me if one have blown open, how about those
| that may have avalanched and there by, has inhibited it's voltage
| handling ?
| http://webpages.charter.net/jamie_5"
|

Forward/reverse resistance is a good indicator of HV diode health.
Also, low voltage operation should catch some problems.

Cheers

 

[whit3rd]

    At work, we have irradiation units that comprise of a vessel
that contains a rectifier stack ...
    The official book, states that when testing the diode array modules,
  simply apply 35 volts in the forward direction across the connections
and you should see ~ 50ma's or on the amp meter of your supply.

   Question:
      Since these units are subjected to much higher voltage, like around
the 30's,50's or more volts for each one, wouldn't you think it would be
more sensible to fault test these units closer to their operational
voltage for break down test?

Well, no; the break down is under REVERSE bias, at inconveniently
high voltages.

Probably a single unit short (of the dozens of rectifiers in the
stack)
will not disturb normal operation; if several short, the forward
current
will show a significant increase, so any important fault DOES show up
in your test. Capture the unit conduction readings in a log book,
and look for changes with time.

A failure by opening up of one rectifier (less likely) will obviously
show up clearly on the forward conduction test.

So, the forward conduction test should do the job for you. If there's
something subtle happening at extreme voltage, you can see the corona
at the fault point.

 

[Jamie]

Well, no; the break down is under REVERSE bias, at inconveniently
high voltages.

Probably a single unit short (of the dozens of rectifiers in the
stack)
will not disturb normal operation; if several short, the forward
current
will show a significant increase, so any important fault DOES show up
in your test. Capture the unit conduction readings in a log book,
and look for changes with time.

A failure by opening up of one rectifier (less likely) will obviously
show up clearly on the forward conduction test.

So, the forward conduction test should do the job for you. If there's
something subtle happening at extreme voltage, you can see the corona
at the fault point.

We will better know tomorrow.

Trying to resolve this issue also has the techs at the main head
quarters complexed at the moment.

Currently, we can bring the bus voltage on the triode up to ~ 5K
where we just start to see ~20-30Kv from the end of the main stack
coming back via the HVD (high voltage divider) string. If the bus
voltage to the triode is increase from this point, we either get an
arc at the Spark Gap on the plate (anode) xformer in the RF junction
box or, the HVD drops down to 0 volts and you see the grid voltage drop
down. If you lower the bus voltage back a little, it will magically
output it's 30Kv.

At this point, the anode current isn't even generating enough to
show on the display..

I wasn't there when they did the initial test on all the components
in the vessel, i'm only going by what was told me.. I kind of told them
to bite the bullet and open it..

I was thinking that maybe some caps in side a diode module have
generated some low break down voltages which will not show on that
simple forward diode test laid out via the bible.
TOmorrow, we will be HI-POT testing the modules along with the
filament RF xformer for the beam tube.

http://webpages.charter.net/jamie_5"

 

[[email protected]]

  We will better know tomorrow.

   Trying to resolve this issue also has the techs at the main head
quarters complexed at the moment.

   Currently, we can bring the bus voltage on the triode up to ~ 5K
where we just start to see ~20-30Kv from the end of the main stack
coming back via the HVD (high voltage divider) string. If the bus
voltage to the triode is increase from this point, we either get an
arc at the Spark Gap on the plate (anode) xformer in the RF junction
box or, the HVD drops down to 0 volts and you see the grid voltage drop
down. If you lower the bus voltage back a little, it will magically
output it's 30Kv.

    At this point, the anode current isn't even generating enough to
show on the display..

   I wasn't there when they did the initial test on all the components
in the vessel, i'm only going by what was told me.. I kind of told them
to bite the bullet and open it..

   I was thinking that maybe some caps in side a diode module have
generated some low break down voltages which will not show on that
simple forward diode test laid out via the bible.
    TOmorrow, we will be HI-POT testing the modules along with the
  filament RF xformer for the beam tube.

http://webpages.charter.net/jamie_5"- Hide quoted text -

- Show quoted text -

The 35V relates to the forward voltage drop. In a typical diode, the
forward drop is roughly 0.7V. High voltage diodes are made by putting
many diodes in series. Thus, the forward voltage drop of a high
voltage diode can easily be 10's of volts. You can find information
on multiplier circuits like yours at http://www.highvoltageinfo.com

 

[Jamie]

The 35V relates to the forward voltage drop. In a typical diode, the
forward drop is roughly 0.7V. High voltage diodes are made by putting
many diodes in series. Thus, the forward voltage drop of a high
voltage diode can easily be 10's of volts. You can find information
on multiplier circuits like yours at http://www.highvoltageinfo.com

Today, 3 units were found to not comform with the operating designed
conditions.
Using the method laid out by the bible yields these units to be ok
how ever, applying 15k DC which is only a fraction of what they are
suppose to handle caused 3 units to fail.

So, we took it a step future and found not only do they break down
below the expected operating voltage, the capacitance of the failed
units has greatly increased over a working unit.. This, in it self is
most likely causing the LC tank which composes the rectifier stack of
D's to not resonate correctly..

So we summized that maybe some of the capacitors in the HV diode
modules are shorted.
I was told today that none of these units have ever been replaced
because they'ev never seen one fail.. This may explain why we have a
couple of other lines in house with poor efficiency even with a new
triode which runs ~$12k bucks.

It would be interesting to hear from others that work in this field
on this matter.


http://webpages.charter.net/jamie_5"

 

[Archimedes' Lever]

Today, 3 units were found to not comform with the operating designed
conditions.
Using the method laid out by the bible yields these units to be ok
how ever, applying 15k DC which is only a fraction of what they are
suppose to handle caused 3 units to fail.

So, we took it a step future and found not only do they break down
below the expected operating voltage, the capacitance of the failed
units has greatly increased over a working unit.. This, in it self is
most likely causing the LC tank which composes the rectifier stack of
D's to not resonate correctly..

So we summized that maybe some of the capacitors in the HV diode
modules are shorted.
I was told today that none of these units have ever been replaced
because they'ev never seen one fail.. This may explain why we have a
couple of other lines in house with poor efficiency even with a new
triode which runs ~$12k bucks.

It would be interesting to hear from others that work in this field
on this matter.


http://webpages.charter.net/jamie_5"

Always use the manufacturer's specs for testing those parts. At low
voltage, a faulty HV cap can appear as being perfectly fine. To HV,
however, they appear as a dead short from one plate to the other.

In other words, the individual diode element of a multiplier array do
not "see" the final voltage of the multiplier. The most they see will be
the multiplier drive signal voltage times 2.

The diodes of a multiplier rarely can handle the final voltage of the
output node. At least on the small circuit board scale stuff. We only
use the diode needed because they get very expensive, very fast.

Also, if capacitors are "built in" to your multiplier diodes, then they
are "multiplier stages". That means that it could be the caps that are
what has gone south.

 

[Jamie]

Always use the manufacturer's specs for testing those parts. At low
voltage, a faulty HV cap can appear as being perfectly fine. To HV,
however, they appear as a dead short from one plate to the other.

In other words, the individual diode element of a multiplier array do
not "see" the final voltage of the multiplier. The most they see will be
the multiplier drive signal voltage times 2.

The diodes of a multiplier rarely can handle the final voltage of the
output node. At least on the small circuit board scale stuff. We only
use the diode needed because they get very expensive, very fast.

Each diode array module is rated to handle 50KDC.

The over all stack is a Cockroft & Walton multiplier using
chorona ring dynodes for the capacitors in a SF6 gas chamber.

http://home.earthlink.net/~jimlux/hv/cw1.htm

Each diode module is an equal of each diode you see in the schematic.

the final net voltage can go up to 1.8 Mv.. The system can
actually do more but it's limited for safety reasons.

Also, if capacitors are "built in" to your multiplier diodes, then they
are "multiplier stages". That means that it could be the caps that are
what has gone south.

http://webpages.charter.net/jamie_5"