Back to Back Tantalum Caps

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

Paul C

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

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.

--
  Keith

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

--
 Thanks,
    - Win

self-ignite in

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.

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

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"it's the network..."                          "The Journey is the reward"
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Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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!

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Michael A. Terrell
Central Florida

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

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

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"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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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

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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

self-ignite in

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.

Yup.

Graham

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

Care to let me in on this snippet of info ?

Graham

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.

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

Isn't that the worst? Stuff that "almost" works. Problems that cause an immediate failure are less of an issue.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

I read in sci.electronics.design that John Larkin wrote (in ) about 'Back to Back Tantalum Caps', on Fri, 18 Feb 2005:

Sounds like the original Sum Yung Gai.

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
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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

Sure, you can build 'em, but you're not going to make any money building them. The reference designs aren't optimized. Often they'll be six-layer where only four is needed, etc.

--
  Keith