electrolytic mystery

[Ross Herbert]

On Wed, 25 Aug 2004 11:51:12 GMT, Ross Herbert
<rherber1SPAMEX@bigpond.net.au> wrote:

|On 24 Aug 2004 16:59:09 GMT, "Walter Harley"
|<walterh@cafewalterNOSPAM.com> wrote:
|
||"Ross Herbert" <rherber1SPAMEX@bigpond.net.au> wrote in message
||news:g1sli01sgevq6mg4pjst2ftdo19lllj45k@4ax.com...
||> Have you checked that there is a physical connection between the
||> analog ground (AGND) and CHASSIS GND? If there isn't then the 48V from
||> the mixer input will be floating at the output socket of the mic amp
||> and the caps won't have that dc voltage impressed across them. You say
||> you have the circuit on your bench and operating but is that in its
||> built up form in the chassis or just the boards connected up on the
||> bench.
||>
||> Whatever is happening it can only be that either there is voltage
||> division (as in the SC circuit) or the 48V from the mixer input is
||> floating at the mic amp output. As far as I can see from the circuit
||> of the Rane that would only happen if the AGND and CHASSIS GND were
||> isolated (perhaps using a capacitor).
||>
||> What voltage do you measure;
||> a) across the blocking caps
||> b) from either side of the caps to both AGND and CHASSIS GND
||
||
||These questions were answered in my original posting.
|
|Sorry Walter, those references to the Rane circuits actually confused
|the issue...
||
||The circuit is a test circuit: a 48V bench supply, in series with a 6.8k
||resistor and a 15uF 25V capacitor. The voltage across the capacitor
||measures 48V. It's been running for about two days now; capacitor seems
||fine.
|
|Now I understand... You haven't actually got the Rane circuit on your
|bench but simply the series connection of 48V supply - 6k8 resistor -
|15uF cap. It is regrettable that the ascii sketch does not show what
|the test circuit looks like on my news reader (Agent). It is just a
|jumble of characters.
|
|here's what we have;
|
|| pos (48V) ----- 6k8 -------15u/25V------- neg (48V)
|
|Lets see what Rubycon has to say;
|
|Q1: What consequences are expected when the voltage exceeding the
|rated voltage is applied on an aluminum electrolytic capacitor?
|
|A1: On the anode foil of aluminum electrolytic capacitor, an oxide
|film capable of withstanding the rated voltage even if it is
|continuously applied at the maximum operating temperature.
|
|In the case when voltage higher than the withstand voltage of this
|oxide film (overvoltage) is applied, the anode foil of the aluminum
|electrolytic capacitor will form the oxide film equivalent to the
|applied voltage. Owing to the reaction, gases will be generated, thus
|leading to the pressure buildup in the capacitor. As the
|characteristics of capacitor, decrease in electrostatic capacity and
|increase in tangent of loss angle will be caused. The higher the
|applied voltage is and the higher the ambient temperature is, the more
|the gases are generated and the higher the internal pressure. This may
|sometimes lead to the phenomena such as swelling of sealing material
|(rubber packing) and further to activation of safety device (slipping
|out of rubber packing in the products with no safety device).
|Therefore, avoid the use of capacitor in the circuit where the voltage
|exceeding the rated voltage may be applied to it.
|
|The structural breakdown modes in case when overvoltage is applied are
|as follows:
|
|
|(1) Open
| The safety device is activated (or rubber packing slips out), and
|liquid electrolyte in the capacitor is flown out, thus leading to
|dryup and finally to open condition.
|(2) Short-circuiting
| If the voltage higher than the withstand voltage of anode foil, that
|of liquid electrolyte and that of separator paper is applied and it is
|no longer possible to keep insulation, dielectric breakdown will be
|caused, thus leading to short-circuiting.
|
|Note the words "even if it is continuously applied at the maximum
|operating temperature" in the first para of the answer.
|
|Because the 6k8 resistor limits the maximum possible current to 7mA
|there is no way that the capacitor can reach its maximum operating
|temperature or even get warm. Therefore no gases are produced to
|cause pressure buildup and without pressure buildup the cap will not
|self destruct. If there were no current limiting then the excessive
|voltage would be able to produce gases which would cause the
|characteristic " bang" we associate with applying overvoltage to an
|electrolytic cap.
|
|This means that Watson A. Name was precisely correct in his initial
|response.
||
|Rgds,
|
|Ross H

 

[Walter Harley]

"Ross Herbert" <rherber1SPAMEX@bigpond.net.au> wrote in message
newsdroi0h37pjt9a677l1hh92htvtpmkbk5h@4ax.com...
Thanks, Ross. I just went up and read all the rest of their tech notes and
FAQ. That's a good reference - much more helpful than what I had found on
my own.

One comment; you'd said:

Because the 6k8 resistor limits the maximum possible current to 7mA
there is no way that the capacitor can reach its maximum operating
temperature or even get warm. Therefore no gases are produced to
cause pressure buildup and without pressure buildup the cap will not
self destruct.

But the Rubycon tech notes suggest that the gas formation is an
electrochemical process, a consequence of the formation of oxide layer from
electrolyte and foil, and will happen at any temperature, albeit with more
gas at higher temps. My read was that the main thing that current limiting
does is that it limits the rate of that chemical reaction. Presumably below
a certain amount of production, the gases either are reabsorbed by the
electrolyte, or dissipate out of the capacitor at a safe speed.

It sounds to me like the thing to watch out for would be DI circuits in
power amplifiers, which can get quite warm. Otherwise, since an increase in
ESR wouldn't matter in this application, and a slight decrease in
capacitance also wouldn't matter, the consequences of re-forming the cap
would probably not be noticed by most users.

I'm interested to try and find out how much capacitance is lost. The one
capacitor I tried showed no change at all; in fact, a slight increase. I
assume that was an aberration.

 

[Ross Herbert]

On 25 Aug 2004 17:36:08 GMT, "Walter Harley"
<walterh@cafewalterNOSPAM.com> wrote:

|"Ross Herbert" <rherber1SPAMEX@bigpond.net.au> wrote in message
|newsdroi0h37pjt9a677l1hh92htvtpmkbk5h@4ax.com...
|> Lets see what Rubycon has to say [...]
|
|Thanks, Ross. I just went up and read all the rest of their tech notes and
|FAQ. That's a good reference - much more helpful than what I had found on
|my own.
|
|One comment; you'd said:
|> Because the 6k8 resistor limits the maximum possible current to 7mA
|> there is no way that the capacitor can reach its maximum operating
|> temperature or even get warm. Therefore no gases are produced to
|> cause pressure buildup and without pressure buildup the cap will not
|> self destruct.
|
|But the Rubycon tech notes suggest that the gas formation is an
|electrochemical process, a consequence of the formation of oxide layer from
|electrolyte and foil, and will happen at any temperature, albeit with more
|gas at higher temps. My read was that the main thing that current limiting
|does is that it limits the rate of that chemical reaction. Presumably below
|a certain amount of production, the gases either are reabsorbed by the
|electrolyte, or dissipate out of the capacitor at a safe speed.
|
|It sounds to me like the thing to watch out for would be DI circuits in
|power amplifiers, which can get quite warm. Otherwise, since an increase in
|ESR wouldn't matter in this application, and a slight decrease in
|capacitance also wouldn't matter, the consequences of re-forming the cap
|would probably not be noticed by most users.
|
|I'm interested to try and find out how much capacitance is lost. The one
|capacitor I tried showed no change at all; in fact, a slight increase. I
|assume that was an aberration.
|

I must admit that the theory behind electrolytic capacitors is still a
bit of a mystery to me. However, the explanation given here
http://www.faradnet.com/deeley/chapt_02.htm seems the most plausible
to me.

In the para on Leakage Current it is shown that the less conductive
the electrolyte is, the lower the leakage current and that the oxide
layer thickness is automatically matched to the potential difference.
It seems that as V increases, the thickness of the oxide layer also
increases, hence the ability to withstand a greater field strength, at
least that's how I read it. Perhaps this point is somewhat cryptic in
the Rubycon info.

Faradnet says:

Effect of Temperature on Breakdown Voltage

As the breakdown voltage or potential is a function of the anode film
thickness and conductivity of the electrolyte; and the conductivity of
the electrolyte varies with the temperature, it must hold true that
the voltage breakdown is also a function of temperature. Increases in
temperature cause increases in electrolyte ionization with resultant
increase in electronic emission from the electrolyte. This lowers the
potential required to rupture or puncture the dielectric or oxide
film. Thus, an increase in temperature results in a lowering of the
breakdown voltage and a decrease in temperature causes an increase in
the voltage breakdown of any specific dry electrolytic capacitor
structure. No graphic illustration of this effect of temperature
change is shown as there are other factors which also concern the
breakdown voltage. Reference is particularly made to the type of
separator material employed. These various other factors will be
mentioned again in later paragraphs.

When a static DC potential is impressed across the capacitor the
leakage current will be minimal and as long as the potential is below
the "breakdown" voltage, the capacitor should be able to withstand a
quite high voltage probably double its normal rated voltage. Where the
potential is varying the current through the cap will also vary and
the temperature will increase and thus lower the breakdown voltage.

Anyway, I leave the full reading of the information on Faradnet to you
and hope you can gain some further understanding of what is happening.

Ross H

 

[Franc Zabkar]

On 25 Aug 2004 07:43:38 GMT, "Walter Harley"
<walterh@cafewalterNOSPAM.com> put finger to keyboard and composed:

"Dave VanHorn" <dvanhorn@cedar.net> wrote in message
news:Kf-dnWdvM6IgarbcRVn-rA@comcast.com...

The source is current limited, and I think you'll find that the majority
of the caps, in this situation either take it, or become slightly leaky.

It dosen't "blow up" because the current is limited by the 6.8k
The 48V is only twice the elevtrolytic's working rating.

Bench it, it's only a few components.
I've seen a lot of things in music circuits, that are pretty questionable
technically, but I guess they must sound good, and/or be blessed by the
high preists of audio.


As I mentioned in my initial post, I have benched it. Leakage was a
fraction of a microamp.

My present idea on this is that the voltage rating must be taken in
conjunction with the temperature rating: since both temp and voltage
increase leakage, a cap rated 85C and 25V is good for more than 25V if you
keep the temp well under 85C. And probably vice versa, too.

If I get a chance, I'll try the experiment in a temp-controlled oven, and
see if the cap that fared well at 48V 25C fares as well at 48V and, say,
65C. Need to build a temp-controlled oven, first - I don't think my wife
would be happy with me trying the experiment in our kitchen oven.

I believe this reverse-biased-cap scenario always (?) occurs when
connecting two pieces of AV equipment, eg TV and VCR, DVD and
amplifier, etc.

C C
--||--o --> o--||--
+ - - +
device #1 device #2
output input

Actually, this arrangement looks like the NP substitution trick I
alluded to elsewhere in this thread.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

[Ross Herbert]

On Thu, 26 Aug 2004 02:36:18 GMT, Ross Herbert
<rherber1SPAMEX@bigpond.net.au> wrote:
I should have put the Fadanet text in quotes....

|I must admit that the theory behind electrolytic capacitors is still a
|bit of a mystery to me. However, the explanation given here
|http://www.faradnet.com/deeley/chapt_02.htm seems the most plausible
|to me.
|
|In the para on Leakage Current it is shown that the less conductive
|the electrolyte is, the lower the leakage current and that the oxide
|layer thickness is automatically matched to the potential difference.
|It seems that as V increases, the thickness of the oxide layer also
|increases, hence the ability to withstand a greater field strength, at
|least that's how I read it. Perhaps this point is somewhat cryptic in
|the Rubycon info.
|
|Faradnet says:
|
QUOTE
Effect of Temperature on Breakdown Voltage

As the breakdown voltage or potential is a function of the anode film
thickness and conductivity of the electrolyte; and the conductivity of
the electrolyte varies with the temperature, it must hold true that
the voltage breakdown is also a function of temperature. Increases in
temperature cause increases in electrolyte ionization with resultant
increase in electronic emission from the electrolyte. This lowers the
potential required to rupture or puncture the dielectric or oxide
film. Thus, an increase in temperature results in a lowering of the
breakdown voltage and a decrease in temperature causes an increase in
the voltage breakdown of any specific dry electrolytic capacitor
structure. No graphic illustration of this effect of temperature
change is shown as there are other factors which also concern the
breakdown voltage. Reference is particularly made to the type of
separator material employed. These various other factors will be
mentioned again in later paragraphs.
UNQUOTE

 

[Walter Harley]

"Ross Herbert" <rherber1SPAMEX@bigpond.net.au> wrote in message
news:hldqi0ho73rcmi9emih3odi3oa3jverj2h@4ax.com...

Thus, an increase in temperature results in a lowering of the
breakdown voltage [...]

Seems pretty clear. What works on the bench is not promised to work inside
a hot amplifier

Thanks for finding and quoting that.

 

[Ross Herbert]

On 27 Aug 2004 05:04:16 GMT, "Walter Harley"
<walterh@cafewalterNOSPAM.com> wrote:

|"Ross Herbert" <rherber1SPAMEX@bigpond.net.au> wrote in message
|news:hldqi0ho73rcmi9emih3odi3oa3jverj2h@4ax.com...
|> [...] Thus, an increase in temperature results in a lowering of the
|> breakdown voltage [...]
|
|Seems pretty clear. What works on the bench is not promised to work inside
|a hot amplifier

I suppose you could say that.

I would venture to say that if you had a continuously varying DC
voltage applied in your test circuit instead of a static 48V DC, the
cap would start to get warm and it would eventually break down.
Typically, the SV rating of your 15uF/ 25V cap will be around 32V so
if you applied a continuously varying DC voltage of between say 35 and
50V, I would guess that you might see some action.

Try it and see if this happens.

In a real life situation the internal temperature of the equipment
would also contribute to reducing the breakdown voltage to somewhere
around the SV rating, although possibly a bit higher than this value.
It might not happen immediately either.

 

[Dave VanHorn]

Jim Williams published a very nice equation to predict electrolytic failure.
Temperature, voltage, ripple current, many terms affect their lifetime.

--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR

 

[Walter Harley]

"Dave VanHorn" <dvanhorn@cedar.net> wrote in message
news:76KdnRS8wYuyobLcRVn-qA@comcast.com...

Jim Williams published a very nice equation to predict electrolytic
failure.
Temperature, voltage, ripple current, many terms affect their lifetime.

Got a reference for where I might find that? Google didn't turn anything
up.

 

[Dave VanHorn]

Hmm. My copy is out on loan, and I don't remember the title offhand..

I'm pretty sure it's this one.
http://www.amazon.com/exec/obidos/tg/detail/-/0750670622/qid=1093645686/sr=1-4/ref=sr_1_4/103-2372900-8017423?v=glance&s=books
--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR

 

[Walter Harley]

"Dave VanHorn" <dvanhorn@cedar.net> wrote in message
news:tfydnQS5i-cxKLLcRVn-vg@comcast.com...

Hmm. My copy is out on loan, and I don't remember the title offhand..

I'm pretty sure it's this one.
http://www.amazon.com/exec/obidos/tg/detail/-/0750670622/qid=1093645686/sr=1-4/ref=sr_1_4/103-2372900-8017423?v=glance&s=books

Doesn't seem to be. I have the book; just took a peek and didn't see
anything in the index nor in any of Jim's articles in there. That book is a
compilation of articles by various authors. I didn't look through all the
articles, but I don't remember seeing it last time I read it.

-walter

 

[Dave VanHorn]

It's definitely in one of his.
It's an expansion of the half life for +10C, and adds in voltage, ripple
current Vs ripple current rating, and a few more terms.

I used it in a printer design, where I was operating from a 40W wall-wart,
supplying 400W pulses to a printhead, at maximum energy transfer rate. It
operates like a photoflash, the boost switcher charges up the output caps,
and when it comes into regulation, it triggers an int on the processor that
starts the next burn pulse.

The interesting part, was that I ended up using three cheap caps, because
when I ran the calclations, they beat out all the expensive single caps, and
by far, were (in this case) the least BANG for the buck.

One case had a relatively expensive "low ESR" cap, predicted to fail in
about 5 minutes!

That design has been in production now since '94, and cap failures are
essentially zero, so I guess it was valid.


--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR