On Wed, 21 Jan 2004 17:26:14 -0600, "Henry Kolesnik"
wrote:

Over the last few years I've acquired quite a few consumer electronincs pcbs
including TVs, VCRs, stereos, etc, so when I discovered that I needed a
tantalum to repair some test equipment I was going to salvage a tantalum. I
couldn't find one anywhere, so I assume they're too expensive or too
unrelaible for high end consumer electronics. A couple of the boards were
from my personal stuff purchased new. One example is a MGA Mitsubishi rear
projection TV that operated flawlessly for nearly 20 years of daily use.
Most of my test equipment comes from hamfests and is surplus after becoming
obsolete and non-operative in less than 20 years. That leads me to wonder
what the real story is behind tantalum capacitors. What do the experts have
to say?
tnx
hank wd5jfr


We often use surface-mount tantalums on high-density, high-cost
boards. They are very reliable (don't dry out like aluminums) if used
carefully, but high peak currents can ignite them, so they are
generally a bad idea for bypassing power rails.

Polymer aluminums (don't dry out) or polymer tantalums (don't explode)
seem like a good idea, but I haven't tried them yet.

I think multilayer ceramics are pushing 100 uF these days.

John

I have a Racal 9301A where a tantalum must have caught on fire because all
that was left was 2 leads, some crisp blackish ash and a little hardened
crust on the pcb where it burned. There's probaby 10 other tants on the
board and one or more are shorted but still intact and I'm trying to find
the bads one/ones with least effort without a schematic. The other unit is
a Wavetek 188-S1257 where a tantalum had a dead short but was intact. I
repalced it with an electrolytic. The cap is on a 15 volt rail where I
think it shorted and took out the regulator. Ireplace the regulator with
what I assumed was a good one out of a new box but it was bad and it put 23
volts on the rail that had a 20 volt rating but no more failed. Sometime I
have good luck.
73
hank wd5jfr
"John Larkin" wrote in
message ...
On Wed, 21 Jan 2004 17:26:14 -0600, "Henry Kolesnik"
wrote:

Over the last few years I've acquired quite a few consumer electronincs
pcbs
including TVs, VCRs, stereos, etc, so when I discovered that I needed a
tantalum to repair some test equipment I was going to salvage a tantalum.
I
couldn't find one anywhere, so I assume they're too expensive or too
unrelaible for high end consumer electronics. A couple of the boards
were
from my personal stuff purchased new. One example is a MGA Mitsubishi
rear
projection TV that operated flawlessly for nearly 20 years of daily use.
Most of my test equipment comes from hamfests and is surplus after
becoming
obsolete and non-operative in less than 20 years. That leads me to
wonder
what the real story is behind tantalum capacitors. What do the experts
have
to say?
tnx
hank wd5jfr


We often use surface-mount tantalums on high-density, high-cost
boards. They are very reliable (don't dry out like aluminums) if used
carefully, but high peak currents can ignite them, so they are
generally a bad idea for bypassing power rails.

Polymer aluminums (don't dry out) or polymer tantalums (don't explode)
seem like a good idea, but I haven't tried them yet.

I think multilayer ceramics are pushing 100 uF these days.

John

John Larkin wrote:

I think multilayer ceramics are pushing 100 uF these days.

Yes, but alas, only with zero volts across them. Capacitance
drops precipitously with DC bias. For a cap with Y5V dielectric,
at half the rated DC voltage, there's only 10% of the initial
capacitance left. Most manufacturers don't tell you.

"Jeroen" wrote in message
...
John Larkin wrote:

I think multilayer ceramics are pushing 100 uF these days.

Yes, but alas, only with zero volts across them. Capacitance
drops precipitously with DC bias. For a cap with Y5V dielectric,
at half the rated DC voltage, there's only 10% of the initial
capacitance left. Most manufacturers don't tell you.


I didn't think it was quite as bad as that. Also very temperature
dependent. These type of ceramics are also pyroelectric as well as being
piezoelectric!

In article , Fred
writes

"Jeroen" wrote in message
...
John Larkin wrote:

I think multilayer ceramics are pushing 100 uF these days.

Yes, but alas, only with zero volts across them. Capacitance
drops precipitously with DC bias. For a cap with Y5V dielectric,
at half the rated DC voltage, there's only 10% of the initial
capacitance left. Most manufacturers don't tell you.

The high k types vary to +-10% and +20-80% from memory if full temp is
allowed for.
High K doped with piezo material, they can be heard to click if hit with
a square wave.

I've seen more than one problem with high-value ceramics causing problems
during operational vibe tests because of microphonics. In our case it
usually seems to be the vibration causing capacitance change rather than
true piezoelectricity, but it happens in any case. The last one that I
remember we replaced the ceramic caps with back-to-back tantalums, in fact.
Worked like a charm.

"ddwyer" wrote in message
...
In article , Fred
writes

"Jeroen" wrote in message
...
John Larkin wrote:

I think multilayer ceramics are pushing 100 uF these days.

Yes, but alas, only with zero volts across them. Capacitance
drops precipitously with DC bias. For a cap with Y5V dielectric,
at half the rated DC voltage, there's only 10% of the initial
capacitance left. Most manufacturers don't tell you.

The high k types vary to +-10% and +20-80% from memory if full temp is
allowed for.
High K doped with piezo material, they can be heard to click if hit with
a square wave.

On Thu, 22 Jan 2004 08:54:58 +0100, Jeroen
wrote:

John Larkin wrote:

I think multilayer ceramics are pushing 100 uF these days.

Yes, but alas, only with zero volts across them. Capacitance
drops precipitously with DC bias. For a cap with Y5V dielectric,
at half the rated DC voltage, there's only 10% of the initial
capacitance left. Most manufacturers don't tell you.


Which opens up the possibility of using them as parametric amplifiers
or modulators. I have a paper somewhere that uses the nonlinearity of
ceramic caps to make a nonlinear transmission line - a shock line -
that sharpens the rising edge speed of kilovolt pulses.

John