Re: Opinion needed for testing HV diode arrays.

On Mon, 17 Nov 2008 18:48:53 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@xxxxxxxxxxx> wrote:

fred@xxxxxxxxxxxxxxxxxxx wrote:

On Nov 16, 4:55 pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@xxxxxxxxxxx> wrote:

whit3rd wrote:

On Nov 15, 5:34 pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@xxxxxxxxxxx> wrote:

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.

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

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.
.

Relevant Pages

  • Re: Opinion needed for testing HV diode arrays.
    ... So we summized that maybe some of the capacitors in the HV diode modules are shorted. ... voltage, a faulty HV cap can appear as being perfectly fine. ... the individual diode element of a multiplier array do ...
    (sci.electronics.design)
  • Re: More fridges
    ... volt equipment switches between the two at 18 volts or thereabouts. ... It might be worth trying a power diode in series with the positive ... This will drop the voltage to the fridge by something bwteen 0.6 ... diodes) to switch diode in and out. ...
    (uk.rec.waterways)
  • Re: Opinion needed for testing HV diode arrays.
    ... The official book, states that when testing the diode array modules, ... Since these units are subjected to much higher voltage, ... the 30's,50's or more volts for each one, wouldn't you think it would be ... in it self is most likely causing the LC tank which composes the rectifier stack of D's to not resonate correctly.. ...
    (sci.electronics.design)
  • Re: Where to get SD Card reader
    ... for the lesser voltage drop per diode", that is a bit like a friend ... the voltage over the diodes dropped. ... MicroSD card) a drop of 0.05 volts is a lot, and 0.1 volts a full step on ... is when using a CMOS switch as a voltage shifter. ...
    (comp.arch.embedded)
  • Re: MRC Ampack upgrade: Attn electronics froods
    ... transistor as possible and a reverse oriented diode across the output ... if your output voltage doesn't go ... fully to zero then a diode in series with the output. ... wave rectifier mode with 120 sinusoidal pulses per second, ...
    (rec.models.railroad)
Loading