Back to Back Tantalum Caps

[John Larkin]

This may be interesting reading material:

http://www.epcos.com/inf/20/40/db/tec_03/01070134.pdf

Section 4.8 is oversimplified. For the back-to-back config with
diodes, small-signal AC (unbiased) series capacitance is indeed C/2,
but large-signal (when the diodes are being useful) it approaches C.

Nonlinear as hell.

John

 

[Pooh Bear]

The bias / bandwidth idea is one of my fool ideas. Ian Sinclair's book
is recommended by Bob Pease in his Troubleshooting book, so it has
been proofread by someone with some experience in the field.

When does it date from ?

I can barely remember when I last saw a tantalum cap !


Graham

 

[PaulCsouls]

When does it date from ?

I can barely remember when I last saw a tantalum cap !


Graham

Copyright 2001. I use tantalum caps all the time but just for power
supply decoupling.

Paul C

 

[Pooh Bear]

I use tantalum caps all the time but just for power supply decoupling.

Utter waste of money.

I hope you use current limiting Rs - the disposition of tantalums to self-ignite in
this aplication has long been discussed in this group.

My advice - tantalums are a waste of money and a source of unreliability.


Graham

 

[Rich Grise]

Utter waste of money.

I hope you use current limiting Rs - the disposition of tantalums to self-ignite in
this aplication has long been discussed in this group.

My advice - tantalums are a waste of money and a source of unreliability.

Oddly, I've never had an adverse experience with a tantalum cap. Then
again, I once fell into a gold mine of "surplus" parts - a company where I
consulted had just bought out another company, for a lump sum. Ergo,
everything in the old company's stock room was, essentially, free. There
were bins of LEDs, bins of transistors, bins of resistors, bins of caps,
even some exotic stuff like 10-turn precision pots, and optical
interruptors.

I did a lot of breadboarding in those days. I had a proto-board that I
populated with a 10 uF tantalum about every three inches along the power
buses. Admittedly, for the 100 uF at the board power entry point, I've
always used aluminum.

But I can't understand the bad rap Tantalum seems to have. I don't think
I've ever seen a tantalum cap fail. (of course, neither has Lassie, but
I _have_ seen stuff, like a selenium stack in a pressurized power supply
clearing the room when you open the unit...)

Cheers!
Rich

 

[keith]

Utter waste of money.

Nonsense. I'e used *thousands* of them. As long as you can get your
manufacturing under control and buy for a reliable supplier they're fine.
They're a *lot* better than the bogus aluminums that have been festering
in the supply chain.

I hope you use current limiting Rs - the disposition of tantalums to
self-ignite in this aplication has long been discussed in this group.

Sheesh, we haven't had that problem for 25 years. You must be an antique
who never grew up or are buying shit from who knows where.

My advice - tantalums are a waste of money and a source of
unreliability.

Nonsense. They're very usefull devices. One must be careful with them
but they're just fine. Hard to come by at times, but there is a reason
for that.

 

[Winfield Hill]

keith wrote...

They're a *lot* better than the bogus aluminums that have
been festering in the supply chain.

We know who makes the bogus aluminums; don't buy 'em.

 

[Spehro Pefhany]

Utter waste of money.

I hope you use current limiting Rs - the disposition of tantalums to self-ignite in
this aplication has long been discussed in this group.

They can be used reliably without the series R if the regulator has a
reasonable output current limit, and there are no "hot plug" issues
(like a BFC on the other side of a connector). Using them outside of
the manufacturer's recommendations (no series R and very high
potential source current) can lead to tears. Following the
manufacturer's recommendations to the letter prohibits their use in a
lot of bypass applications. Eg. "Do not use tantalum electrolytic
capacitors in power supply applications.".... "if you must".. (derate
by at least 3:1 in voltage.

My advice - tantalums are a waste of money and a source of unreliability.
Graham

Aluminums and ceramics can do a better job for less money in most
cases. The tantalums might be a better choice in some subminiature
portable applications (eg. cellphone handsets) because they are easier
to get and cheaper than the equivalent ceramics, but that's quickly
changing in favor of ceramics. And good riddance.


Best regards,
Spehro Pefhany

 

[Michael A. Terrell]

They can be used reliably without the series R if the regulator has a
reasonable output current limit, and there are no "hot plug" issues
(like a BFC on the other side of a connector). Using them outside of
the manufacturer's recommendations (no series R and very high
potential source current) can lead to tears. Following the
manufacturer's recommendations to the letter prohibits their use in a
lot of bypass applications. Eg. "Do not use tantalum electrolytic
capacitors in power supply applications.".... "if you must".. (derate
by at least 3:1 in voltage.

Best regards,
Spehro Pefhany

Another good rule: Don't let someone try to save money by changing
your 25 volt parts to 10 volts on the 12 volt rails. One place I worked
tried it and it wasn't pretty!

 

[Pooh Bear]

Nonsense. I'e used *thousands* of them. As long as you can get your
manufacturing under control and buy for a reliable supplier they're fine.
They're a *lot* better than the bogus aluminums that have been festering
in the supply chain.


Sheesh, we haven't had that problem for 25 years. You must be an antique
who never grew up or are buying shit from who knows where.


Nonsense. They're very usefull devices. One must be careful with them
but they're just fine. Hard to come by at times, but there is a reason
for that.

It's all about power supply voltage dV/dt at switch on.

Tantalums just don't like high dV/dt and the info is there on data sheets -
as it is also for MLCs.

I reckon you've just been lucky for some design reason.

It isn't *just* tantaums that burn up either in direct supply rail
decoupling. If you're only talking about 5V I can understand why you may not
have seen a problem perhaps.

It happens to be a subject I've had to investigate seriously and I would
*never* use tantalum - there's the cost for one - and consumer / semi-pro
gear has more than adequate lifetime with quality aluminium parts.


Graham

 

[Pooh Bear]

They can be used reliably without the series R if the regulator has a
reasonable output current limit, and there are no "hot plug" issues
(like a BFC on the other side of a connector). Using them outside of
the manufacturer's recommendations (no series R and very high
potential source current) can lead to tears. Following the
manufacturer's recommendations to the letter prohibits their use in a
lot of bypass applications. Eg. "Do not use tantalum electrolytic
capacitors in power supply applications.".... "if you must".. (derate
by at least 3:1 in voltage.

You got it exactly !

Funnily enough there are some old Nat Semi app notes that never seemed to get updated
that recommend tantalums for decoupling around 78xx and 79xx regulators. Sadly, ppl
still unwisely follow this advice.

Just use a modern aluminium type. The performance of modern aluminiums from reliable
manufacturers ( need not be expensive ) is really good these days.

Aluminums and ceramics can do a better job for less money in most
cases. The tantalums might be a better choice in some subminiature
portable applications (eg. cellphone handsets) because they are easier
to get and cheaper than the equivalent ceramics, but that's quickly
changing in favor of ceramics. And good riddance.

Yup.


Graham

 

[Pooh Bear]

A better method would be to put a bias resistor to a sufficiently large bias
voltage connected to the junction of the 2 capacitors. That way, both are
always
operating correctly biased.

I do exactly this in several current designs where the offset voltage from the
preceeding stage is of indeterminate polarity and may be up to around 0.5V.

I use a 1M resistor to the centre tap of the 2 x 100uF caps from one of the
supply rails.

Works a treat and costs less than a bi-polar cap.


Graham

 

[Pooh Bear]

keith wrote...

We know who makes the bogus aluminums; don't buy 'em.

Care to let me in on this snippet of info ?


Graham

 

[Spehro Pefhany]

Care to let me in on this snippet of info ?
Graham

Some (5 or 10) Taiwan capacitor manufacturers procured pirated
electrolyte for low-impedance caps as used on PC motherboards
(typically something like 1000uF/6.3V) from a company that created the
liquid from a formula stolen from a Japanese company. The formula was
not complete- it was missing some stabilizing agents, and the
resulting capacitors fail after only a year or so (they bulge up and
vent the electrolyte). This affected many tens of thousands of
motherboards (including two I had). A really miserable mess.
Motherboards are a cutthroat business.

It did not affect any of the capacitor companies in Taiwan that I've
dealt with. Taiwan is bad in this regard, but China is a lot worse.
Bad parts, counterfeit parts, counterfeit UL markings, etc. etc.


Best regards,
Spehro Pefhany

 

[John Larkin]

Some (5 or 10) Taiwan capacitor manufacturers procured pirated
electrolyte for low-impedance caps as used on PC motherboards
(typically something like 1000uF/6.3V) from a company that created the
liquid from a formula stolen from a Japanese company. The formula was
not complete- it was missing some stabilizing agents, and the
resulting capacitors fail after only a year or so (they bulge up and
vent the electrolyte). This affected many tens of thousands of
motherboards (including two I had). A really miserable mess.
Motherboards are a cutthroat business.

It did not affect any of the capacitor companies in Taiwan that I've
dealt with. Taiwan is bad in this regard, but China is a lot worse.
Bad parts, counterfeit parts, counterfeit UL markings, etc. etc.

I know a guy whose brother-in-law, a Vietnamese kid, dealt in hot
Pentium chips. Cops somehow got him on tape planning the hijacking of
a truck with a few megabucks of chips inside; part of the conversation
referred to killing the driver if he gave them any trouble. They had a
cop driving the truck, cops in the truck, cop cars holding back
traffic on Interstate 280 for a mile either way, and caught them in
the act. We won't be seeing that kid for a while.

John

 

[Spehro Pefhany]

I know a guy whose brother-in-law, a Vietnamese kid, dealt in hot
Pentium chips. Cops somehow got him on tape planning the hijacking of
a truck with a few megabucks of chips inside; part of the conversation
referred to killing the driver if he gave them any trouble. They had a
cop driving the truck, cops in the truck, cop cars holding back
traffic on Interstate 280 for a mile either way, and caught them in
the act. We won't be seeing that kid for a while.
John

I read about that one.

One of the first offshore makers of Bluetooth headsets (earsets?) made
a deal for something like 5 million dollars of chips with the
manufacturer (and negotiated a super-good price, of course). No big
deal, right? But this guy took delivery of the chips in one batch and
posed with a couple of bodyguards, the tycoon, and $5M US worth of
ICs. Made the papers (with photos) in Hong Kong, and was very well
timed as a publicity stunt. Instant credibility as a major supplier.


Best regards,
Spehro Pefhany

 

[Pooh Bear]

Some (5 or 10) Taiwan capacitor manufacturers procured pirated
electrolyte for low-impedance caps as used on PC motherboards
(typically something like 1000uF/6.3V) from a company that created the
liquid from a formula stolen from a Japanese company. The formula was
not complete- it was missing some stabilizing agents, and the
resulting capacitors fail after only a year or so (they bulge up and
vent the electrolyte). This affected many tens of thousands of
motherboards (including two I had). A really miserable mess.
Motherboards are a cutthroat business.

For sure.

Ages back I looked into the chipset business and was amazed that you could
basically replicate a 'reference design' offered by the chipset manufacturer on
a licence free basis.

It did not affect any of the capacitor companies in Taiwan that I've
dealt with. Taiwan is bad in this regard, but China is a lot worse.
Bad parts, counterfeit parts, counterfeit UL markings, etc. etc.

Heard of ( plenty of ) counterfeit semiconductors too !

Is anyone naming any names with regard to the dodgy electrolytics ? I don't use
anything under 16V ( with rare exceptions ). Am I ok ?


Graham

 

[Spehro Pefhany]

Is anyone naming any names with regard to the dodgy electrolytics ?

Yes, you can Google for a few names. That may not be all of them,
though.

I don't use anything under 16V ( with rare exceptions ). Am I ok ?

On new parts, you're probably okay. If they're not low-impedance
types, then I suspect you're also okay, even if they are older. Power
supply filter caps are worth being picky about, IMHO.


Best regards,
Spehro Pefhany

 

[John Woodgate]

I read in sci.electronics.design that John Larkin <jjSNIPlarkin@highTHIS
landPLEASEtechnology.XXX> wrote (in <2sed11ppcrc5jc5avu7hfj5bg38rpaihbm@
4ax.com>) about 'Back to Back Tantalum Caps', on Fri, 18 Feb 2005:

I know a guy whose brother-in-law, a Vietnamese kid, dealt in hot
Pentium chips. Cops somehow got him on tape planning the hijacking of a
truck with a few megabucks of chips inside; part of the conversation
referred to killing the driver if he gave them any trouble. They had a
cop driving the truck, cops in the truck, cop cars holding back traffic
on Interstate 280 for a mile either way, and caught them in the act. We
won't be seeing that kid for a while.

Sounds like the original Sum Yung Gai.

 

[Pooh Bear]

Yes, you can Google for a few names. That may not be all of them,
though.


On new parts, you're probably okay. If they're not low-impedance
types, then I suspect you're also okay, even if they are older. Power
supply filter caps are worth being picky about, IMHO.

Oh I am ! I expect our subcontractors either find me a total pain in the arse or
alternatively quite helpful for giving them a spec and recommended suppliers.

A recent prototype SMPS ( @ 130 kHz ) came in with standard reservoir caps on
the output side in place of low ESR btw. It actually seemed to work fine but we
had a moan all the same. Their substitution of 1N400x for UF400x was rather less
than sucessful though.


Cheers, Graham