Replace tantalum caps with aluminum?

[Al]

In article <vst6lv6qk16rc0@corp.supernews.com>,
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote:

"Boris Mohar" <borism@sympatico.ca> wrote in message
news:in4tsv0e1la66du9220t3m1dvikrv4idjq@4ax.com...

Are those HP tantalums bad? You would be surprised how long tantalums
last if
not abused.


This morning I measured their ESR, and they all seemed fine, though one or
two have a bunch of crystallization around the terminals.

Is that the definitive test? Is there a better test? (I suppose I should
also measure their capacitance, which I've not yet done.)

Crystallization? Hmmm. Some of the very old tantlums used sulphuric acid
as an electrolyte. They were not "dry" as the newer ones are. And
sometimes they happened to leak. They were really nasty when reverse
biased as they exploded and threw acid all over the place. Try some
litumus paper on the end of the anomalous ones.

If you need them, I have a supply of old military grade surplus
tantalums, axial leaded, from the 70's and could send some to you. Odd
values, though, like 8.2 uF @ 50V

Al

--
There's never enough time to do it right the first time.......

 

[Ken Finney]

"Al" <no.spam@here.com> wrote in message
news:no.spam-1D159D.09191104122003@news.verizon.net...

In article <vst6lv6qk16rc0@corp.supernews.com>,
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote:

"Boris Mohar" <borism@sympatico.ca> wrote in message
news:in4tsv0e1la66du9220t3m1dvikrv4idjq@4ax.com...

Are those HP tantalums bad? You would be surprised how long
tantalums
last if
not abused.


This morning I measured their ESR, and they all seemed fine, though one
or
two have a bunch of crystallization around the terminals.

Is that the definitive test? Is there a better test? (I suppose I
should
also measure their capacitance, which I've not yet done.)


Crystallization? Hmmm. Some of the very old tantlums used sulphuric acid
as an electrolyte. They were not "dry" as the newer ones are. And
sometimes they happened to leak. They were really nasty when reverse
biased as they exploded and threw acid all over the place. Try some
litumus paper on the end of the anomalous ones.

If you need them, I have a supply of old military grade surplus
tantalums, axial leaded, from the 70's and could send some to you. Odd
values, though, like 8.2 uF @ 50V

They still make wet slug tantalums that use sulfuric acid as the dielectric.
Not pretty when they explode.

 

[Ken Finney]

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:vst3oqkrmg9g7d@corp.supernews.com...

"Boris Mohar" <borism@sympatico.ca> wrote in message
news:46tssv0275t8ar40ofuvmaviugkhor9ct8@4ax.com...
So they probably put them in TV motherboards at 35C, and then the TV
dies
after 3 years and the owner buys a new one. The cap not only saves you
money, it *earns* you money!


Only if the customer is stupid enough to buy your brand again.

...Unless the customer figures that all the other brands suck just as
much.

Far as I can tell, practically everything out there these days is made to
the same low standards. It all seems to be a race to pinch every last
$0.01
out of the retail cost, regardless of whether it halves the useful life of
the device, makes it less usable, or what. Except for "audiophile" audio
gear, which is just as absurd in the opposite direction.

You didn't go far enough. Hardware manufacturers are actively working
with part manufacturers (not that either one will admit it publically) to
find
ways to make parts less reliable, even if it costs a little more. The goal
is
to not have any failures before 3 years (so they don't have extended
warranty costs), but as many failures as they can before 5 years.

 

[Walter Harley]

"Ken Finney" <kenneth.c.finney@boeing.com> wrote in message
news:HpE0Dp.8ps@news.boeing.com...

You didn't go far enough. Hardware manufacturers are actively working
with part manufacturers (not that either one will admit it publically) to
find
ways to make parts less reliable, even if it costs a little more. The
goal
is
to not have any failures before 3 years (so they don't have extended
warranty costs), but as many failures as they can before 5 years.

what's your basis for that assertion? Not that I'm challenging it, but I
wouldn't take it on faith, either...

 

[Boris Mohar]

On Thu, 4 Dec 2003 20:17:52 GMT, "Ken Finney" <kenneth.c.finney@boeing.com>
wrote:

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:vst3oqkrmg9g7d@corp.supernews.com...
"Boris Mohar" <borism@sympatico.ca> wrote in message
news:46tssv0275t8ar40ofuvmaviugkhor9ct8@4ax.com...
So they probably put them in TV motherboards at 35C, and then the TV
dies
after 3 years and the owner buys a new one. The cap not only saves you
money, it *earns* you money!


Only if the customer is stupid enough to buy your brand again.

...Unless the customer figures that all the other brands suck just as
much.

Far as I can tell, practically everything out there these days is made to
the same low standards. It all seems to be a race to pinch every last
$0.01
out of the retail cost, regardless of whether it halves the useful life of
the device, makes it less usable, or what. Except for "audiophile" audio
gear, which is just as absurd in the opposite direction.


You didn't go far enough. Hardware manufacturers are actively working
with part manufacturers (not that either one will admit it publically) to
find
ways to make parts less reliable, even if it costs a little more. The goal
is
to not have any failures before 3 years (so they don't have extended
warranty costs), but as many failures as they can before 5 years.

Bullshit

--

Boris Mohar

 

[Ken Finney]

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:vsvbeohpcpf74f@corp.supernews.com...

"Ken Finney" <kenneth.c.finney@boeing.com> wrote in message
news:HpE0Dp.8ps@news.boeing.com...

You didn't go far enough. Hardware manufacturers are actively working
with part manufacturers (not that either one will admit it publically)
to
find
ways to make parts less reliable, even if it costs a little more. The
goal
is
to not have any failures before 3 years (so they don't have extended
warranty costs), but as many failures as they can before 5 years.



what's your basis for that assertion? Not that I'm challenging it, but I
wouldn't take it on faith, either...

In round numbers, the military/space/avionics market consumes about
1% of the integrated circuits produced, the automotive market
about 4%, and various flavors of telecom and consumer electronics
the other 95%. In general terms (and therefore open to much debate),
the parts made for the different markets 30 years ago were quite
different, but the parts made over the last 10 years or are quite similar.
The reason for this is the fantastic strides made in reliability of even the
cheapest parts made them "good enough" to use in a lot of applications
where previously, "high reliability" parts were required. But that
caused another problem for OEMs; commercial electronics have
become incredibly reliable. The OEMs initial response was
brilliant, they pushed "extended warranties" on consumers who
remembered problems they had had with previous generations
of hardware. Over time, consumers have become much more
leary of buying extended warranties that their new experience
showed they would never collect on.

The big problem for OEMs is how to get consumers to replace
hardware that doesn't fail. The computer and cell phone OEMs
have done very well in providing increased functionality to make
this a mute point, but other product sectors (e.g. TVs) haven't
(don't get me started on the "digital TV" shell game). The obvious
solution was to literally build in timers that would FORCE the
hardware to fail after a certain date, but the companies legal
departments didn't like that idea. The second solution was to
design in parts that would wear out. The beauty of this solution
was that in most cases the parts that will fail early are SLIGHTLY
less expensive, and often have slightly better performance. So
now, an OEM can win on both ends, a cheaper product up front,
and selling a replacement at an earlier date than they otherwise
would. The problem is that the automotive market needed the
long term reliability because the automotive environment ages
parts a lot faster than sitting on a desk does. The problem for
the part manufactures is that 95% of their customers want a
product that the other 5% can't tolerate.

What is happening today is that the differences between the
automotive grade parts and the military grade parts is becoming
almost nil, but that they are becoming more and more different
from consumer grade parts. Little things like passivation,
metalization thickness, current density, bond wire materials
and thickness, and ionic impurities in the plastic molding
compounds are all becoming different whereas they used
to be the same. The company line always is something like
"to reduce costs/to increase market share/to provide more
performance/etc" and in most cases those things are true,
but they aren't the whole truth. Also, since this is real
engineering, there are always tradeoffs. In many cases you
CAN'T get the increased performance without decreasing the
reliability or radically increase the cost. And none of this is evil,
it is capitalism, pure and simple.

 

[Al]

In article <8tcvsvk7g85gac91mr7a2cmevsse304hri@4ax.com>,
Boris Mohar <borism@sympatico.ca> wrote:

On Thu, 4 Dec 2003 20:17:52 GMT, "Ken Finney" <kenneth.c.finney@boeing.com
wrote:


"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:vst3oqkrmg9g7d@corp.supernews.com...
"Boris Mohar" <borism@sympatico.ca> wrote in message
news:46tssv0275t8ar40ofuvmaviugkhor9ct8@4ax.com...
So they probably put them in TV motherboards at 35C, and then the TV
dies
after 3 years and the owner buys a new one. The cap not only saves you
money, it *earns* you money!


Only if the customer is stupid enough to buy your brand again.

...Unless the customer figures that all the other brands suck just as
much.

Far as I can tell, practically everything out there these days is made to
the same low standards. It all seems to be a race to pinch every last
$0.01
out of the retail cost, regardless of whether it halves the useful life of
the device, makes it less usable, or what. Except for "audiophile" audio
gear, which is just as absurd in the opposite direction.


You didn't go far enough. Hardware manufacturers are actively working
with part manufacturers (not that either one will admit it publically) to
find
ways to make parts less reliable, even if it costs a little more. The goal
is
to not have any failures before 3 years (so they don't have extended
warranty costs), but as many failures as they can before 5 years.


Bullshit

I attended a IEEE Reliability Group meeting where this was shown to be a
fact. Most manufacturer's goal is to acheive a 3 yr MTTF. The is the
expected lifetime.

Al

--
There's never enough time to do it right the first time.......

 

[Walter Harley]

"Al" <no.spam@here.com> wrote in message
news:no.spam-3BBD38.09001105122003@news.verizon.net...

On Thu, 4 Dec 2003 20:17:52 GMT, "Ken Finney"
kenneth.c.finney@boeing.com
wrote: [...]
You didn't go far enough. Hardware manufacturers are actively working
with part manufacturers (not that either one will admit it publically)
to
find ways to make parts less reliable, even if it costs a little more.
The goal is to not have any failures before 3 years (so they don't
have extended warranty costs), but as many failures as they can before
5 years.

I attended a IEEE Reliability Group meeting where this was shown to be a
fact. Most manufacturer's goal is to acheive a 3 yr MTTF. The is the
expected lifetime.

But there's a far cry between "trying to achieve a 3 yr MTTF" and doing that
"even if it costs a little more."

It is well known that manufacturers aim for a certain system-wide MTTF. For
instance, there are stories of Henry Ford studying junked cars to see what
parts were still good, because those parts were by definition
overengineered: if some of the car had already worn out, but some of it
hadn't, he was wasting money on the good junked parts.

But that was with the goal of saving money. Then we get to "planned
obsolescence," that is, the effort to make sure something will fail in a
certain time so that customers will buy more. This still isn't quite the
same as paying more to have something fail; it's more a matter of
intentionally choosing the less reliable (but still presumed cheaper) parts,
across the board, to create an object that is disposable rather than
lasting. For instance, my HP654A test oscillator, the progenitor of this
thread, was built to last. My Neutrik Minirator, though a convenient,
powerful, and highly performant instrument, is made of plastic and is
completely nonmaintainable. It'll die before the HP does, even though it's
35 years younger.

Finally we get to Ken's premise, that manufacturers of equipment are
actually willing to work with parts makers to design, and pay extra for,
parts that fail sooner. I can't reject that claim, but I've never seen any
evidence of it from the designers I've spoken with (a small and biased
sample), and I'm skeptical.

Ken's response to my query didn't quite defend the premise; if I may
paraphrase his response, it was that for consumer parts, the prioritization
is cost, then performance, then reliability; so reliability is suffering.
That is different from saying that they are actively searching for ways to
reduce reliability even at higher cost.

-w

 

[Ken Finney]

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:bqqlm1$bq1$0@216.39.172.65...

"Al" <no.spam@here.com> wrote in message
news:no.spam-3BBD38.09001105122003@news.verizon.net...
On Thu, 4 Dec 2003 20:17:52 GMT, "Ken Finney"
kenneth.c.finney@boeing.com
wrote: [...]
You didn't go far enough. Hardware manufacturers are actively
working
with part manufacturers (not that either one will admit it
publically)
to
find ways to make parts less reliable, even if it costs a little
more.
The goal is to not have any failures before 3 years (so they don't
have extended warranty costs), but as many failures as they can
before
5 years.

I attended a IEEE Reliability Group meeting where this was shown to be a
fact. Most manufacturer's goal is to acheive a 3 yr MTTF. The is the
expected lifetime.


But there's a far cry between "trying to achieve a 3 yr MTTF" and doing
that
"even if it costs a little more."

It is well known that manufacturers aim for a certain system-wide MTTF.
For
instance, there are stories of Henry Ford studying junked cars to see what
parts were still good, because those parts were by definition
overengineered: if some of the car had already worn out, but some of it
hadn't, he was wasting money on the good junked parts.

But that was with the goal of saving money. Then we get to "planned
obsolescence," that is, the effort to make sure something will fail in a
certain time so that customers will buy more. This still isn't quite the
same as paying more to have something fail; it's more a matter of
intentionally choosing the less reliable (but still presumed cheaper)
parts,
across the board, to create an object that is disposable rather than
lasting. For instance, my HP654A test oscillator, the progenitor of this
thread, was built to last. My Neutrik Minirator, though a convenient,
powerful, and highly performant instrument, is made of plastic and is
completely nonmaintainable. It'll die before the HP does, even though
it's
35 years younger.

Finally we get to Ken's premise, that manufacturers of equipment are
actually willing to work with parts makers to design, and pay extra for,
parts that fail sooner. I can't reject that claim, but I've never seen
any
evidence of it from the designers I've spoken with (a small and biased
sample), and I'm skeptical.

Ken's response to my query didn't quite defend the premise; if I may
paraphrase his response, it was that for consumer parts, the
prioritization
is cost, then performance, then reliability; so reliability is suffering.
That is different from saying that they are actively searching for ways to
reduce reliability even at higher cost.

I did say something that was a little wrong. I know of OEMs that have
approached parts manufacturers and said the 3 year/5 year thing.
I also know part manufacturers who have said "well, we could do
this", and that "this" did cost "more", but the "more" was on the order
of thousanths of a penny per part or less, so it is really in the noise.
Given the time value of money, I wouldn't believe anyone either that
said that OEMs were willing to spend X% more to obtain parts
that fail sooner.

 

[Walter Harley]

"Ken Finney" <kenneth.c.finney@boeing.com> wrote in message
news:HpG4vs.74n@news.boeing.com...

I did say something that was a little wrong. I know of OEMs that have
approached parts manufacturers and said the 3 year/5 year thing.
I also know part manufacturers who have said "well, we could do
this", and that "this" did cost "more", but the "more" was on the order
of thousanths of a penny per part or less, so it is really in the noise.
Given the time value of money, I wouldn't believe anyone either that
said that OEMs were willing to spend X% more to obtain parts
that fail sooner.

Thank heavens! A little bit of good news is welcome amidst all the rest of
the hoo-hah on this newsgroup. I'm glad to hear that my faith in humanity
is at least partly justified. Thanks

-w

 

[Gary Lecomte]

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message news:<vspni5c6bsge57@corp.supernews.com>...

I'm replacing the electrolytics in a nearly-40-year-old HP 654A test
oscillator. This is a 10Hz to 10MHz sinewave oscillator. I have the
service manual and schematics for it.

Many of the caps are specified in the service manual to be tantalum. Some
are quite large values, up to 390uF. The caps are all axial (cylinders with
leads at each end), which is a relatively uncommon package these days.

Can I replace these large-value tantalum caps with aluminum caps? I'm
thinking that in 1967, the ESR, tolerance, and stability of aluminum was
pretty lousy, but maybe now it's as good as tantalum was back then?

Thanks for any input.

I would extremely doubt that any of the tantalums are bad so why would
you replace them?
As well as low ESR they exibit much lower "leakage currents" and
generally better "dielectric absorbtion" compared to aluminum caps.
Even with todays technology. There are some good aluminum caps, but
they will cost as much or more than the tantalums. Not knowing where
these caps are used in the circuit, but these features could be
important.

In a lot of years experience, I have never seen a bad tantalum, when
use properly in a circuit. They are not good as coupling caps and
really hate reverse polarity, but in most other instances they are
very relilable.

Take care.......Gary

 

Ken Finney wrote:

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:vst3oqkrmg9g7d@corp.supernews.com...
"Boris Mohar" <borism@sympatico.ca> wrote in message
news:46tssv0275t8ar40ofuvmaviugkhor9ct8@4ax.com...
So they probably put them in TV motherboards at 35C, and then the TV
dies
after 3 years and the owner buys a new one. The cap not only saves you
money, it *earns* you money!


Only if the customer is stupid enough to buy your brand again.

...Unless the customer figures that all the other brands suck just as
much.

Far as I can tell, practically everything out there these days is made to
the same low standards. It all seems to be a race to pinch every last
$0.01
out of the retail cost, regardless of whether it halves the useful life of
the device, makes it less usable, or what. Except for "audiophile" audio
gear, which is just as absurd in the opposite direction.


You didn't go far enough. Hardware manufacturers are actively working
with part manufacturers (not that either one will admit it publically) to
find
ways to make parts less reliable, even if it costs a little more. The goal
is
to not have any failures before 3 years (so they don't have extended
warranty costs), but as many failures as they can before 5 years.

But there is no need for the manufacturers to do that. Simply
design the thing (whatever) for lower reliability. e.g. If a
resistor has to dissipate .475 watts, good practice would be
to use a 1 watt (or more) unit. To lower the reliability, use
a 1/2 watt unit. Specify a 16 V electrolytic in a 12 volt
circuit instead of a 25, 35 or 50 volt cap. Don't use ESR
caps. Use smaller heat sinks. Essentially, use the minimum
spec'ed parts to do the job, and leave out parts that are for
reliability only.

I think the economics automatically drives this, and there
is no need to purposely lower the designed in reliability.
The dictate is to make it cheaper. That pressures the
engineer toward cheaper parts and leaving out stuff that
is not absolutely necessary. Protective diodes - we don't
need no stinkin' protective diodes. And so forth.

From the parts manufacturer's viewpoint, the same thing
happens. The clients are clamoring for a lower priced
xyz component. If you can make the component with 10%
less material, you can reduce the cost, retain the
client, and maybe increase the profit margin. So what
if the life expectancy of the component is reduced as
a result, as long as it still lasts for ~3 years.

 

[Walter Harley]

"Gary Lecomte" <chemelec@hotmail.com> wrote in message
news:ca76d1a9.0312061403.6c5100e6@posting.google.com...

In a lot of years experience, I have never seen a bad tantalum, when
use properly in a circuit. They are not good as coupling caps and
really hate reverse polarity, but in most other instances they are
very relilable.

What makes them not good for coupling?

 

[Al]

In article <br0gb0$tmi$0@216.39.172.65>,
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote:

"Gary Lecomte" <chemelec@hotmail.com> wrote in message
news:ca76d1a9.0312061403.6c5100e6@posting.google.com...
In a lot of years experience, I have never seen a bad tantalum, when
use properly in a circuit. They are not good as coupling caps and
really hate reverse polarity, but in most other instances they are
very relilable.


What makes them not good for coupling?

The always need a forward bias. You have to ride the AC on a DC bias.
This is not always acheivable.

Al

--
There's never enough time to do it right the first time.......

 

[Frank Miles]

In article <no.spam-FB7539.09242908122003@news.verizon.net>,
Al <no.spam@here.com> wrote:

In article <br0gb0$tmi$0@216.39.172.65>,
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote:

"Gary Lecomte" <chemelec@hotmail.com> wrote in message
news:ca76d1a9.0312061403.6c5100e6@posting.google.com...
In a lot of years experience, I have never seen a bad tantalum, when
use properly in a circuit. They are not good as coupling caps and
really hate reverse polarity, but in most other instances they are
very relilable.

What makes them not good for coupling?

The always need a forward bias. You have to ride the AC on a DC bias.
This is not always acheivable.

Are you saying that tantalum electrolytics have more severe bias requirements
in coupling applications than aluminum electrolytics? (My experience
is that they are less severe). Granted-- neither will be as tolerant
as a true nonpolar capacitor.

-frank
--

 

[Al]

In article <br276a$hue$1@nntp6.u.washington.edu>,
fpm@u.washington.edu (Frank Miles) wrote:

In article <no.spam-FB7539.09242908122003@news.verizon.net>,
Al <no.spam@here.com> wrote:
In article <br0gb0$tmi$0@216.39.172.65>,
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote:

"Gary Lecomte" <chemelec@hotmail.com> wrote in message
news:ca76d1a9.0312061403.6c5100e6@posting.google.com...
In a lot of years experience, I have never seen a bad tantalum, when
use properly in a circuit. They are not good as coupling caps and
really hate reverse polarity, but in most other instances they are
very relilable.

What makes them not good for coupling?

The always need a forward bias. You have to ride the AC on a DC bias.
This is not always acheivable.

Are you saying that tantalum electrolytics have more severe bias requirements
in coupling applications than aluminum electrolytics? (My experience
is that they are less severe). Granted-- neither will be as tolerant
as a true nonpolar capacitor.

-frank
--

Tantalums may tolerate about 0.5 V reverse bia whereas an aluminum
electrolytic may tolerate up to 1.5 V reverse bias. In audio coupling
applications this can make the diff.

Al

--
There's never enough time to do it right the first time.......

 

[Jim Adney]

On Mon, 08 Dec 2003 16:51:02 GMT Al <no.spam@here.com> wrote:

In article <br276a$hue$1@nntp6.u.washington.edu>,
fpm@u.washington.edu (Frank Miles) wrote:

In article <no.spam-FB7539.09242908122003@news.verizon.net>,
Al <no.spam@here.com> wrote:
In article <br0gb0$tmi$0@216.39.172.65>,
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote:

"Gary Lecomte" <chemelec@hotmail.com> wrote in message
news:ca76d1a9.0312061403.6c5100e6@posting.google.com...
In a lot of years experience, I have never seen a bad tantalum, when
use properly in a circuit. They are not good as coupling caps and
really hate reverse polarity, but in most other instances they are
very relilable.

What makes them not good for coupling?

The always need a forward bias. You have to ride the AC on a DC bias.
This is not always acheivable.

Are you saying that tantalum electrolytics have more severe bias requirements
in coupling applications than aluminum electrolytics? (My experience
is that they are less severe). Granted-- neither will be as tolerant
as a true nonpolar capacitor.

Tantalums may tolerate about 0.5 V reverse bia whereas an aluminum
electrolytic may tolerate up to 1.5 V reverse bias. In audio coupling
applications this can make the diff.

I believe that this is correct. The manufacturers' literature will
back this up.

OTOH, if there was no bias in the circuit, a coupling capacitor
wouldn't be necessary. We could just use direct coupling.

My impression has always been that capacitive coupling was always used
precisely BECAUSE there was a DC bias present.

I've used lots of tantalum capacitors as coupling capacitors to
replace coupling caps that were originally alum electrolytics. The
reliability of the devices I've done this in has always gone up as a
result of this.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------

 

[Walter Harley]

"Jim Adney" <jadney@vwtype3.org> wrote in message
news:tckatv0b4rggihkhvrjsidf06e5nao51v0@4ax.com...

My impression has always been that capacitive coupling was always used
precisely BECAUSE there was a DC bias present.

Sometimes you don't know what the bias will be; you just presume there could
be one. For instance, at the input to an amplifier. Could be some DC
riding on the input; you don't want to amplify it; so you block it. Or
output; FWIW, the HP654A test oscillator that started this thread has two
4000uF, 3V nonpolar caps between the output amp and the attenuators.

Amusing factoid: many "balanced outputs" in audio devices have blocking
caps. Because the presumed load impedance is 600 ohms, the caps need to be
high value, so they're electrolytics, usually rated at somewhere near the
supply rail voltage. But it's common to plug balanced outputs into mixer
mic inputs, which often have phantom power present. Phantom power is
usually 48V behind a 6.81k resistor. Not all 16v caps will hold up well, if
you connect them to that. Bang.

 

[Jim Adney]

On Tue, 9 Dec 2003 20:17:57 -0800 "Walter Harley"
<walterh@cafewalterNOSPAM.com> wrote:

"Jim Adney" <jadney@vwtype3.org> wrote in message
news:tckatv0b4rggihkhvrjsidf06e5nao51v0@4ax.com...
My impression has always been that capacitive coupling was always used
precisely BECAUSE there was a DC bias present.

Sometimes you don't know what the bias will be; you just presume there could
be one. For instance, at the input to an amplifier. Could be some DC
riding on the input; you don't want to amplify it; so you block it. Or
output; FWIW, the HP654A test oscillator that started this thread has two
4000uF, 3V nonpolar caps between the output amp and the attenuators.

That's a good point. I had only been thinking about coupling caps
between stages WITHIN a device. Your examples are for blocking caps at
the interfaces. I'd think such caps would need to be non-polar so that
they could handle I/O offsets of either polarity.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------