Electrolytic caps in series

[Rheilly Phoull]

One day Phil Allison got dressed and committed to text

"dmm"

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.


** 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

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

 

[Phil Allison]

"Rheilly Phoull"

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.

** 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

 

[dmm]

On Fri, 24 Jun 2005 21:25:34 +1000, "Phil Allison" <philallison@tpg.com.au> wrote:

"dmm"

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.


** You can't be serious ???

Perfectly serious.

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

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.

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

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.

>............ Phil

 

[Poxy]

Phil Allison wrote:

"Rheilly Phoull"

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.



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

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

 

[Terry Given]

Phil Allison wrote:

"Rheilly Phoull"

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.




** 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

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

 

[Phil Allison]

"dmm"
"Phil Allison"

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.


** You can't be serious ???

Perfectly serious.

** More fool you - dickhead.

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

(snip irrelevances)

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.

** Hmmm - so I see you actually have no fucking idea what particular
failure mode the voltage rating refers to.

( So you pulled in all those extraneous ones to cover that fact. )

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.

** Obviously a Sea Food lover here - red herrings galore is a favourite.

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

** Hmmm - so I see you actually have no fucking idea what particular
failure mode the voltage rating refers to.

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


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.

** Just for fun - how about YOU tell all of US what failure happens
( and after what time span ) when a resistor is run somewhat beyond its
rated voltage BUT well within its power rating.

Then explain how a high voltage cap can fail ( either short or open) BUT
and the gear keeps working for a significant time so is left switched on.

Then explain why this matters to anyone ??




............. Phil

 

[Phil Allison]

"Terry Given"

Phil Allison wrote:
"Rheilly Phoull"

Thanks folks for all the info, I think I have the gist of it. Whilst I
havenot 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.


** 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 ??


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.

** Why even bother ????

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

how much more depends on how well the voltage needs to be balanced.

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

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.

** 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

 

[KLR]

On Fri, 24 Jun 2005 20:32:29 +1000, dmm
<dmmilne_REMOVE_@ozemail.com.au> wrote:

On Wed, 22 Jun 2005 19:39:08 +0800, "Rheilly Phoull" <Rheilly@bigpong.com> wrote:

G'day All
What are the rules or method to calculate the values of balancing resistors
when using electros in series ??

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.

I dont see why there is such a fuss about this really.

If you have 2 caps in series, or if you have a single cap of the same
value (as the series combination) across the rail why is the failure
of ONE cap in a series pair some major problem, or somehow worse than
the single capacitor failing in the same way and deserving of a lot of
fuss and bother ?

Either way, depending on the way it fails, (eventually either goes
open circuit or short) you are going to have either an almighty bang,
or a hell of a lot of hum on the rail that the cap is filtering.
Either way the cap(s) is fucked

I would just use caps in series, as long as they are same value and
voltage rating everything should be fine. I wouldnt go mixing values
and WOULD have a rating on EACH cap significantly higher than 50% of
the rail voltage.

I have done it before too on a few occasions, with no problems at all.
Both on high and low voltage rails.

 

[Terry Given]

Phil Allison wrote:

"Terry Given"

Phil Allison wrote:

"Rheilly Phoull"


Thanks folks for all the info, I think I have the gist of it. Whilst I
havenot 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.


** 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 ??


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.

mentioned to answer the OPs original question/

** Why even bother ????

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

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.

how much more depends on how well the voltage needs to be balanced.



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



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.



** 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.

bloody good.

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

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.

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.

............... Phil

Cheers
Terry

 

Oh dear! Here comes the language again And up until now he was making some
very intelligent contributions to an interesting thread - which proves he can do
it.

Phil Allison wrote:

** More fool you - dickhead.

** Hmmm - so I see you actually have no fucking idea what particular
failure mode the voltage rating refers to.

This masterpiece of understatement said twice for some reason (timeless literary
quality?). Again one should lok past it to see real ability, but it's not
shining through - it is well hidden and must be dug out

> ............ Phil

 

[Rheilly Phoull]

One day Phil Allison got dressed and committed to text

"Rheilly Phoull"

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.



** 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

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

 

[GM]

nospam@novirus.com.au wrote:

Oh dear! Here comes the language again And up until now he was making some
very intelligent contributions to an interesting thread - which proves he can do
it.

Phil Allison wrote:


** More fool you - dickhead.


** Hmmm - so I see you actually have no fucking idea what particular
failure mode the voltage rating refers to.


This masterpiece of understatement said twice for some reason (timeless literary
quality?). Again one should lok past it to see real ability, but it's not
shining through - it is well hidden and must be dug out


............ Phil

What ? I think Phil hit the nail on the head with his first post in the
thread - Why bother with resistors as they are absolutely not required.
He then explained his reasoning for this in later posts, after most had
ignored the first one. Give us a break nospam.

 

[Phil Allison]

"Glenn Baddeley = top posting bullshit artist

The generally accepted practise is to always use the same capacitance
and voltage ratings of HV elec caps. They should even be the same
manufacturer and type.

** Correct - then you can expect to get close matching.

If there are no voltage sharing resistors and one of the capacitors has
a DC leakage current which is much greater than the other caps, it will
put a high voltage stress on the other caps, and they may go BANG.

** 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.

For HV caps, which are usually rated in the range 350 - 550 volts, the
voltage sharing resistors whould be at least 1 Watt types, to have
sufficent voltage rating. 2W, 5W and 10W are also fine (carbon or wire
wound).

** 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

 

[Glenn Baddeley]

Folks,

The generally accepted practise is to always use the same capacitance
and voltage ratings of HV elec caps. They should even be the same
manufacturer and type.

If there are no voltage sharing resistors and one of the capacitors has
a DC leakage current which is much greater than the other caps, it will
put a high voltage stress on the other caps, and they may go BANG.

For HV caps, which are usually rated in the range 350 - 550 volts, the
voltage sharing resistors whould be at least 1 Watt types, to have
sufficent voltage rating. 2W, 5W and 10W are also fine (carbon or wire
wound). The resistance value should be such that at least a few mA of
current is flowing and about 50% to 75% of the wattage rating is used
(for long term reliability). Higher power resistors improve the voltage
regulation a bit, but they produce more heat and waste some of the
power transformer capability (could be an issue if its running close to
its full rating).

eg. Design for 450 V across each cap, using 2W resistors.

R = E^2 / P = 450 x 450 / 1.5 = 135000. Use 150K resistors,
(I = E / R = 3mA). 120K would also be OK (1.7W approx dissipation).

HTH,
Glenn.

I would just use caps in series, as long as they are same value and
voltage rating everything should be fine. I wouldnt go mixing values
and WOULD have a rating on EACH cap significantly higher than 50% of
the rail voltage.

KLR wrote:

On Fri, 24 Jun 2005 20:32:29 +1000, dmm
dmmilne_REMOVE_@ozemail.com.au> wrote:

On Wed, 22 Jun 2005 19:39:08 +0800, "Rheilly Phoull"
Rheilly@bigpong.com> wrote:

G'day All
What are the rules or method to calculate the values of balancing
resistors when using electros in series ??

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.


I dont see why there is such a fuss about this really.

If you have 2 caps in series, or if you have a single cap of the same
value (as the series combination) across the rail why is the failure
of ONE cap in a series pair some major problem, or somehow worse than
the single capacitor failing in the same way and deserving of a lot of
fuss and bother ?

Either way, depending on the way it fails, (eventually either goes
open circuit or short) you are going to have either an almighty bang,
or a hell of a lot of hum on the rail that the cap is filtering.
Either way the cap(s) is fucked

I would just use caps in series, as long as they are same value and
voltage rating everything should be fine. I wouldnt go mixing values
and WOULD have a rating on EACH cap significantly higher than 50% of
the rail voltage.

I have done it before too on a few occasions, with no problems at all.
Both on high and low voltage rails.

 

[Terry Given]

Phil Allison wrote:

"Glenn Baddeley = top posting bullshit artist

The generally accepted practise is to always use the same capacitance
and voltage ratings of HV elec caps. They should even be the same
manufacturer and type.



** Correct - then you can expect to get close matching.

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

If there are no voltage sharing resistors and one of the capacitors has
a DC leakage current which is much greater than the other caps, it will
put a high voltage stress on the other caps, and they may go BANG.



** 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.

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

Ballast resistors are no help if a cap is faulty.

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.

For HV caps, which are usually rated in the range 350 - 550 volts, the
voltage sharing resistors whould be at least 1 Watt types, to have
sufficent voltage rating. 2W, 5W and 10W are also fine (carbon or wire
wound).



** 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.

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

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.

whats the mechanism?

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

yep.

........... Phil

 

[Phil Allison]

"Terry Given" = kiwi bullshit artist

Phil Allison wrote:

If there are no voltage sharing resistors and one of the capacitors has
a DC leakage current which is much greater than the other caps, it will
put a high voltage stress on the other caps, and they may go BANG.


** 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.


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

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

Ballast resistors are no help if a cap is faulty.

bingo. lots of people seem to like throwing money at equipment so that "in
the event of a failure, such-and-such happens."

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

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.

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

For HV caps, which are usually rated in the range 350 - 550 volts, the
voltage sharing resistors whould be at least 1 Watt types, to have
sufficent voltage rating. 2W, 5W and 10W are also fine (carbon or wire
wound).


** 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.

depends on your definition of "readily available".

** Same one everyone else uses - fuckhead.


From a hobbyist

perspective, yes. For a real company, no.

** What an asinine and pointless comment..

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.

whats the mechanism?

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

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

yep.

** What a pointless comment.



............. Phil

 

[Phil Allison]

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

Phil Allison

has a DC leakage current which is much greater than the other caps,
it will put a high voltage stress on the other caps, and they may
go BANG.

** 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.

Maybe,

** 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.

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.

** Totally begs the question.

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.

** 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.

Ballast resistors are no help if a cap is faulty.

It depends on the type of fault.

** I carefully described the type of fault.

Go read my post again !!!!!!

** 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.

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.

** 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.

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

Sorry, I don't agree.

** 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 ???

Every text book and article I have ever seen uses
resistors.

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

How intelligent - NOT !!

(If you personally insult me again you will not receive any further
replies from me,

** You are posting mindless tripe - Glenn.

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

effectively halting this interesting objective discussion,

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




........... Phil

 

[Glenn Baddeley]

Hi Phil,

"Glenn Baddeley = top posting bullshit artist

has a DC leakage current which is much greater than the other caps,
it will put a high voltage stress on the other caps, and they may
go BANG.
** 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.

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.

Ballast resistors are no help if a cap is faulty.

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.

** 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.

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.

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

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:

"Glenn Baddeley = top posting bullshit artist

The generally accepted practise is to always use the same
capacitance and voltage ratings of HV elec caps. They should even
be the same manufacturer and type.


** Correct - then you can expect to get close matching.


If there are no voltage sharing resistors and one of the capacitors
has a DC leakage current which is much greater than the other caps,
it will put a high voltage stress on the other caps, and they may
go BANG.


** 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.


For HV caps, which are usually rated in the range 350 - 550 volts,
the voltage sharing resistors whould be at least 1 Watt types, to
have sufficent voltage rating. 2W, 5W and 10W are also fine (carbon
or wire wound).


** 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

 

[Geoff C]

"Glenn Baddeley" <glenn.baddeley@team.telstra.com> wrote in news:d9t3p4$pb7
$1@mws-stat-syd.cdn.telstra.com.au:

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.

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.

 

[Phil Allison]

"Geoff C"

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).

** 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.

Never saw any
failures of caps in either situation, with a 20 year service life of the
product.

** Ditto.




............. Phil