Electrolytic caps in series

"Rheilly Phoull"



 **  Forget it  -  just use caps that have a large margin in excess of the
needed voltage.

 Eg  -  two 350 volt types applied to a 500 volt supply.

 The caps will very soon reach a mutual, acceptable agreement on what
precise voltage suits their individual taste !!






..............   Phil

 One day Phil Allison got dressed and committed to text



Thanks Phil

On Wed, 22 Jun 2005 21:47:41 +1000, "Phil Allison"


 in this case a resistor is good for a bleeder :) thats about all i
ever use them for

"The Real Andy"
  Phil  Allison



**  What type do you use that can stand 250 volt DC indefinitely ??

High value resistors, of all types,  seem to have a very high failure (
failing open) rate after a few years when subjected to continuous DC above
about 100 volts.




..........    Phil

any ideas as to the mechanism? come across any decent app notes?

I presume that as high value Rs the peak power dissipation would still
be pretty low - I've seen lots of low-value resistors fail after being
abused in this way, eg 0603 10R 12V gatedrive resistor.

what about electromigration? wouldnt that tend to bridge out the
serpentine cuts, thus reducing R

what about VR37s, they are designed for very high voltage pulses. we
used pairs of 470k VR37 resistors in series for DC bus and line voltage
sensing, +/-300V - 500V about earth so 150V-250V per resistor. never had
a single failure over perhaps 10 years and 40,000 products.

We also used 100k PR02's across each half of the DC bus cap bank, so
about 0.9W continuous at 600V/2 and 1.6W at 800V/2(short term during
regen). dont know about failures there though, as power electronics
failures tend to trash the entire area, so its quite possible. the
dissipation is a bit high at 800V, but luckily the nearby electrolytics
will provide some cooling :)

At one stage I tried damping an EMI filter with 1R PR02s in series with
the 1uF caps. If turned on at an appropriately high line voltage (which
one phase always is in 3-phase systems) the PR02 emitted a bright flash
of light and went open circuit. probably because (400V*1.4)^2/1R = 320kW
peak pulse power. We found that by observing the flash...wtf?! A carbon
composition resistor ate the peak pulse though.

Cheers
Terry

"Terry Given"



** The resistors that fail are operating within published specs.

     Ergo  -  the "mechanism" is bad manufacture.

     Not much mention of that in any  "app notes'.







 **  What  -   without the right Visa ??






 **  Values start at  1M ohm   -  useless.





...........   Phil

I too suspect this is largely the reason.

I once had a batch of 0.1% resistors from vishay that were off by up to
25%. it was a ups DC balancing circuit, and it didnt like it at all.
Only 20-30 units were affected.

vishay were great, wrote me a nice report. basically contaminated
material wrecked part of a batch, but the effect was triggered by
high(ish) temperatures so they all measured ok at the factory, and
through our genrad pcb tester, but once the unit heated up to 50C or so,
they drifted like crazy.

in general my experience is that nice cheap components are quite often
unreliable shit, and the only way to tell is to wait a year or two. e.g.
every company I've ever worked for has had a cheap ceramic bypass
capacitor horror story to tell.





lol. other than its existence, I dont know much about electromigration,
but it appears these guys do:

http://www.theo-phys.uni-essen.de/tp/forsch/krug.html

whaddya know, its kind of analogous to crack propagation, so certainly
could cause open circuits. AIUI its an issue at IC level because the
small dimensions result in high E field strengths. The same would occur
in a serpentine resistor at high voltage.



I have 100pcs 470k sitting in front of me.

1998 PA08B databook:

VR25 100k - 15M 1,5,10% E12/E24  1,600Vdc peak
VR37 100k - 33M 1,5%    E24/E96  3,500Vdc peak
VR68 100k - 68M 1,5%    E24/E96 10,000Vdc peak
PR02 0R33 - 1M  1,5%    E24/E96    500Vdc peak




Cheers
Terry

"Terry Given"
 Phil Allison wrote:






  **  My sources

1.   Farnell in One cat.

2.  Philips General cat  1978.






 **  I believe you  -  but that is not the simple answer.


The simple one is to use a few cheap  0.5 or 0.75 watt  MF resistors in
series  -  so each cops less than 100 volts and runs cool.




...........  Phil

I "inherited" a full set of philips 2000 databooks (many are 1998). I
regret throwing out so many databooks in the past :(



absolutely, and with smt its trivial to put down as many as required
(although per-part power dissipation also drops). The overall footprint
is not dissimilar, but the inductance is a whole lot lower, as is the
capacitance. designing to cope with single component failures is a lot
easier (eg 6 aint much more than 5, but 2 is a lot more than 1) too.

Cheers
Terry

Did the same thing with a 500V DC distributed supply system. Was much
easier to simply use a few SMD resistors in series as the ballast
resistors, rather than having to source high voltage ones. Too bad
there wasn't the same choice on the caps, volume realestate was at a
premium (more so than PCB realestate) so we had to run tight margins
and had little selection available when it came to the cap vs voltage
vs volume vs height tradeoff.

Dave :)

    Electros have a fairly wide value tolerance and corresponding
leakage current variation, it's considered good practise to have
a resistor across each capacitor so that a series divider chain
is formed that swamps out the effect of leakage currents. If not
then the individual voltages across the caps can be imbalanced
to the point where the operating voltage of one of the electros
is exceeded: I have seen it happen and the effect does exist.

  In Morgan Jones' book "Valve Amplifiers" he has a good description
of what's involved but I've loaned my copy to a friend so I can't
give an exact quote but there was some rule of thumb.

This guy has it:
http://www.jproc.ca/ve3fab/sercaps.html

Regards
Mark Harriss

"Mark Harriss"




 **  In fact it never does.





**  Utter crap  -  if the actual max operating voltage is approached, cap
leakage current dramatically increases.

When there is a series pair of similar caps, the current flow MUST be the
same through both caps  -  so the one with lower leakage immediately takes
up the additional voltage. There is no problem at all as long as there is a
total voltage margin of 30 %.





**  Mark Harrarse is a wet behind the ears, 20 year old,  Qld fascist prick
with zero experience.






..........   Phil

  1. Please note that the localised "Phil Allison Effect" means
that electronics behave completely differently in Phil's
workshop than to the rest of universe.

  2. Also note that Phil is in disagreement with most respected
texts on the subject.

  3. Also note that taking Phil's advice will cause problems
down the track as the electrolytics age and dry out.

"Mark Harriss"


**  Just another anonymous, gutless, neo-fascist, know nothing pile of sub
human crap hiding under a rock somewhere in Brisbane.


  Joh would have loved him.




 .............     Phil

  With a resister correctly installed the voltage wont be problematic as
excess is bleed so some smothing effect is evident


  correct I refer to ARRL 1995 / filtration and bleed resistors (dual
purpose)

  wrong toaster boy , its std practise on all hv valve tx gear I have
seen the last 35 years , Im certain now you are doing a roddles
misinformation post

"Uncle-Fester"


** Utterly incomprehensible & anencephalic with no bright ideas.






.............   Phil

Sounds suspiciously like you're describing yourself there Toaster Boi,
except the last part which should read ".....with not even any dull ideas."

Inability to raise even the slightest meaningful rebuttal containing
even a smidgen of proof noted .

"Uncle-Fester"



 ** I supplied a proof for my recommendation  -  which BTW is based on long
experience of dealing with high voltage elctros.

You simply ignored it.

 Piss off.




............   Phil