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Re: Electrolytic caps in series



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Whilst others have given their ideas as to the values (and how they are
determined)
to be used, ensure that the voltage rating of the resistors exceeds the voltages
expected across the capacitors, especially for the worst possible scenario,
ie if a capacitor fails for some reason.


Re: Electrolytic caps in series



"dmm"
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**  You can't be serious   ???

Worry about some 10 cent resistor failing AFTER a high voltage electro has
exploded ??

In any case, it would only do if its power ratings were exceeded.




............     Phil




Re: Electrolytic caps in series




 One day Phil Allison got dressed and committed to text

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Thanks folks for all the info, I think I have the gist of it. Whilst I have
not a great deal of theory I have a lot of exploded devices behind me :-)
I'm going along with Phil, that master of gentle explanation.
Cheers........... Rheilly



Re: Electrolytic caps in series



"Rheilly Phoull"
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**  Hey,  I'm doing real well this week  - that is the third "back handed"
compliment in a row  !!!


BTW

 Are you working on an old Fender guitar amp ??

 They are full of  20 or 22 uF  @  500 volt caps  -  sort of reddish brown
coloured ??




..............    Phil



Re: Electrolytic caps in series


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I think being favourably compared to Rod Speed probably doesn't quite count
as a compliment, forward- or back-handed.




Re: Electrolytic caps in series


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Here is a front-handed complement: you are right.

To design the balancing resistors, simply choose resistors that draw
more than the leakage current, to swamp out the variations. how much
more depends on how well the voltage needs to be balanced.

Thats all well and good, but leakage current increases with increasing
temperature, and the variation is extremely wide. So an effective
balancing resistor is a fairly low value, and gets hot.

Last time I did the calcs (Hitachi AIC caps), it was around 30k-40k. So
I thought "?!" and looked at some existing product. which used 470k.
which draw far less current than the *measured* leakage current of the
caps at room temperature. voltage measurements showed DC balance was not
great, and did not change when balancing resistors were removed. hardly
surprising really.

In addition, the balancing resistors affect only the DC voltage. AC
voltage sharing is governed entirely by capacitance ratios. This is very
important at power-on, when Vdc ramps from 0 to 100%, perhaps quickly.
Its easy to calculate the dV/dt needed to draw more current than the
balancing resistors, or the frequency at which Xcap < Rbalance. Any
faster than that, and Rbalance does nothing.

Cheers
Terry

Re: Electrolytic caps in series



"Terry Given"
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 **  Why even bother ????

  The two caps will sort it out between themselves  -  as long as there is a
30%+ voltage  margin.


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**  Correct  -  in many cases there is no need and no benefit at all.


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**  Some 56 uF,  400 volt caps ( WES  HSW  types) I tested today showed a
leakage of < 2 uA at 250 volts, at room temp.

 This increased to a mere 10 uA when quite hot.

 The appropriate ballast resistor value is therefore about 20 Mohms.

 Forget it  !!!!


 With 2 in series across 500 volts,  the middle point read 265 volts.

 IME  -  this is how most modern electro caps behave.





...............    Phil



Re: Electrolytic caps in series


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mentioned to answer the OPs original question/

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therein lies the rub. Sometimes thats not so easy to do. And a reduced
margin may mean cheaper caps.


You've probably already noticed that good cap manufacturers also have a
surge voltage rating. very handy, especially with regenerative loads.

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bloody good.

as the caps get bigger, leakage also increases - ISTR proportional to C.


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it also doesnt help that the data sheet values can be wildly
pessimistic. I know with Hitachi that the quoted lifetime figure is
actually the process -3 sigma point, and is about half the average lifetime.

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Cheers
Terry

Re: Electrolytic caps in series




 One day Phil Allison got dressed and committed to text

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Yup, thats the one. The coupling caps were all grey jobs (this is an asian
'knockoff') but as I recall the fenders were like that.
Funny how it kept going since all the caps were leaky and you could hear the
DC on the pots.

--
Regards ..... Rheilly Phoull



Re: Electrolytic caps in series



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Perfectly serious.


Caps can fail for may reasons, not necessarily by exploding, but that is one of
their more spectacular results. I remember many years ago having to clean the
guts of a Radford valve power amplifier whose main filter caps had let go. What
a mess.

A resistor could conceivably cause a fire if it isn't specified properly and the
correct
value, voltage, and power ratings and deratings aren't correctly calculated and
defined.
When playing with high voltages it would be prudent to spec the resistor to be
flameproof,
or at least to have a flame retardant coating.

If the leakage of one capacitor changed, the voltage across both bleed resistors
would change
as well, possibly exceeding their voltage rating.

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A standard MRS16 330K ohm 400mW metal film resistor across a 300 volt supply
would not exceed its power rating (273mW), but would exceed its voltage rating
of 200V.

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Re: Electrolytic caps in series



"Glenn Baddeley   =   top posting bullshit artist
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**  Correct  -  then you can expect to get close matching.


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**  For a cap to do as you suggest, it must be a faulty cap -  ie one that
is not able to meet maker's specs for max voltage or leakage at rated
voltage.

Ballast resistors are no help if a cap is faulty.


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**  Wrong  -  WW resistors are only readily available in values up to about
10 kohms  -  far too low a value for use in parallel with high voltage
electros.

Sustained high DC voltages make carbon or metal film resistors go high in
value or open circuit  -  even if used well below their power rating .
Composition resistors also go high value with time, heat and the application
of high DC voltages.

The point is simple -  you are better off without ballast resistors across
electros unless they are 100 % reliable.





...........    Phil







Re: Electrolytic caps in series


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and the more caps in parallel, the better the matching (variations tend
to cancel out)

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LHS of bell curve. not terribly likely, but if you use 1,000,000 caps
per year, you *will* see some.

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bingo. lots of people seem to like throwing money at equipment so that
"in the event of a failure, such-and-such happens." In the event of a
failure, the costs associated with returning the equipment for repair
are often far in excess of the actual replacement parts cost, so why bother.

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depends on your definition of "readily available". From a hobbyist
perspective, yes. For a real company, no.

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whats the mechanism?

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yep.


Re: Electrolytic caps in series



"Terry Given"    =  kiwi bullshit artist
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 **  What an asinine and pointless comment  -   faulty caps get removed from
gear at the factory.

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**  What an asinine and pointless comment  -   faulty caps get removed from
gear at the factory.


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**  What an asinine and pointless comment  -   faulty caps get removed from
gear at the factory.


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  **  Same one everyone else uses   -  fuckhead.


 From a hobbyist
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**  What an asinine and pointless comment..


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 **    Just trolling   -  or you need to know  ??


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  **  What a pointless comment.



............    Phil




Re: Electrolytic caps in series


Hi Phil,

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Maybe, but there is a low % of failure rate that will be encountered
eventually, especially after 20+ years. I would rather "insure" for that
possibility that risk a catastrophic failure and expensive repair.

If one cap has 1mA leakage and another cap has 2mA leakage,
the additional voltage across one of the caps may lead to its
premature failure.

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It depends on the type of fault. If it is an excessive DC leakage
current problem, the ballast resistors actually help by reducing
the degree of voltage variation across each cap due to variation
in leakage.

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eg. Design for 450 V across each cap, using 10W resistors.

R = E^2 / P = 450 x 450 / 7.5 = 27 KOhm. These are available
in 10W, or use two 12K or 15K 5W in series.
I = E / R = 17mA approx. This would protect an excess leakage
fault of 5 - 10 mA.

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Sorry, I don't agree. Every text book and article I have ever seen uses
resistors. eg. The ARRL handbook represents decades of HAM
experience, and their HV power supplies for valve transmitters
always use them.

Cheers,
Glenn.
(If you personally insult me again you will not receive any further
replies from me,
effectively halting this interesting objective discussion, which also
benefits
other people on the newsgroup)

Phil Allison wrote:
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Re: Electrolytic caps in series



"Glenn Baddeley   **  STOP  TOP  POSTING  !!!!!!!

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 ** What does  "maybe " mean  ???

 Do you see the point or not  ??

I'll post it again:  " If neither elector cap is faulty -   then nothing bad
can happen when wired in series and used at around 70% of rated voltage.  "

So far  -  you have simply not addressed this at all.

You have yet to post any sort of case.



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**  Totally begs the question.


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**  That is a whole bunch of  "ifs"  -   there.

 The caps in question have tested at less than 10 uA of leakage at 70 % of
rated voltage.

 You are  IGNORING  facts and just making up numbers that suit you.



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**  I carefully described the type of fault.

      Go read my post again  !!!!!!



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**  Shame how the 47 uF caps in question only have  2 to 10 uA of leakage.

 Shame how 17 mA of extra current is not acceptable in a valve amp or many
other situations where such electros are used.

 Shame how the 7.5 watts of heat from the resistors would damage the
electros if placed near them.

 Shame how high value WW resistors are notorious for failing open when
subjected to continuous, high DC voltages.

 Shame what a load of crapology you post.


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**  Huh  - so it is OK by you if one resistor fails open and the other then
forces an electro overvoltage ???

 Do you realise you just totally contradicted yourself   ???


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**  Ok  -  so you rely on rote learning and the superstitions of others.

     How intelligent   -  NOT !!



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 **  You are posting mindless tripe  -   Glenn.

 Worse,  you INSIST on top posting  -   the method only used by fools and
wankers.


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**  Confusing the matter with irrational drivel is not my idea of
"objective".




  ...........     Phil







Re: Electrolytic caps in series


Hi Phil,

OK, you've tempted me to post again.  You are quite entitled to
your personal opinion, and I to mine. Most of your points are
quite valid and arguable, but I do see my point of view as
still plausible. Would anyone else care to throw in their
personal opinion or experiences?

Glenn.

Phil Allison wrote:

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Re: Electrolytic caps in series


$1@mws-stat-syd.cdn.telstra.com.au:

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Been involved in designs that used both approaches. Went away from the no
resistor approach because techs used to get a shock from picking up the
electronics module, needed to bleed away that charge (700V). Never saw any
failures of caps in either situation, with a 20 year service life of the
product. Never paid much attention to this 'problem' before this.

Re: Electrolytic caps in series



"Geoff C"
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**  The possible need for a bleed R is a separate issue to the ballast
resistor one.

The OP wanted to replace a single 500 volt cap with two in series  -  there
was no bleed R in his case.



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**   Ditto.




............    Phil








Re: Electrolytic caps in series




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  ( snip drivel)

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  ( snip more crazy stuff)


**  So they don't just do weird things to sheep in NZ ........




.............    Phil




Re: Electrolytic caps in series



"Glenn Baddeley"   =   still bloody top posting  !!!


**  STOP   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


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**  So how do you know it really is a good point  ??





..........    Phil






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