Is there any rule on how far a polarized tantalum cap can be used in reverse?

I have a case where it may need to work in reverse, but only up to the tens of mV. Maybe a worst-case pulse of 100mV, which would be an extremely rare occurance only lasting a couple of seconds.

I know a nonpolarized cap would be the first choice. I'm looking at this possibility since I might need a rather high capacitance and low leakage component for this app.

 

Here is a link of more than I really wanted to know about this, but an interesting read none the less:

http://nepp.nasa.gov/eeelinks/Februa...s_Behavior.htm

from my experience, and I'm sure yours, they generally don't like it long term or at high levels, get warm then kinda go bang. LOL A low reversing like you said probably would only hurt the capacitance long term, and make them more "leaky", providing the ambient temp is not already too high. I would think that this situation is fairly normal in power supply applications.

hope this helps

 

From my experience of tantalum capacitors they are extremely unreliable, they have been used in a number of domestic products over the years (TV's etc.) and are probably the most unreliable component ever fitted in TV's.

From a servicing point of view tantalum capacitors are the first thing to check when you see one - a blanket change of every single one (for electrolytics) is often a good idea.

Their usual failure mode is going short circuit, and apparently reverse power is particularly prone to causing this!.

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Tants are ok. They are aerospace approved (unlike electrolytic's) and to get that status they have to be pretty rugged.
They however cannot take reverse voltage.

Tants and Electrolytics are used in places where bulk storage is needed. You say it will see some 10's of mV for a few seconds??.

In what context are you using tants (timing cap/rail bulk capacitance)

 

Tantalums are Excellent Capacitors and Rarely Fail, IF USED PROPERLY. I use Ten of thousands of them. Never had a Failure yet.

A few millivolts reverse for very short periods should be OK.

Never use Tantalums for AC Coupling.

Take care..........Gary

 

OT, but I had to grin when I read this... how many times have you had repair notes that were a single line... "replace C1209, 10uF electrolytic...." LOL. Especially SMPS, TV verticals, etc.... I used to call it capacitor scheduled maintenance.

 

Certainly electrolytic capacitors are one of the main failures in domestic electronics, in fact I built an ESR meter to test them! - which is a wonderful bit of kit!.

BUT! - as I mentioned previously, in my experience tantalum capcitors are far worse - for a few years they were commonly used as HT decouplers on signal boards (colour decoders, IF, video amplifier etc.), they were so unreliable as to make a blanket change very worth while.

Imagine! - you have a PCB over a foot square, on this board are 30-40 tantalum capacitors, many of them on the same HT rail (near chips) - one, or more, of them is short circuit. It's often easier to just change them all for more reliable electrolytics - which are actually very reliable used in this way :lol: If you spend the time and find the faulty one, it will probably come back again a few months later with another one short circuit - although it's probably good for profits, it's not good for customer relations!.

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What's "HT"? High freq or high voltage or what?

In my experience electrolytics are the most unreliable thing on the board, particularly in stuff over 5-10 yrs old. Then again, you don't see 10 yr old tantalums for obvious reasons. But I understand generally their reliability is far greater, as demonstrated by established tests. Of course there may be apps in which tantalums won't work reliably. What's with the "HT"?

BTW, I have decided to go another way than the solution that would have put millivolts in reverse across the tantalum, so there's no reason to keep that question running.

 

HT , to my understanding is High Tension. Usually the B+ or rectified mains voltage, in the order of 150V and up. Also refers to the primary supply for the LOPT ( flyback transformer ).

Couldn't agree more on having an ESR meter!! The amount of time saved ( and hair pulling.. like I have any to spare ) is worth the price, or effort to build one. Mine is similar to the Dick Smith model, but from different plans.

 

HT is 'High Tension', originally used for the power rails in valve equipment it passed on to transistor equipment as well, and simply means the main power rail - even if it's only a 9V battery (so not very high!).

Again, in my experience, back in the 1980's, the life of tantalum capacitors was very short (and very short-circuit!) - perhaps they may have improved since then?, but you never seem to see used them anymore in domestic electronics.

__________________
PIC programmer software, and PIC Tutorials at:
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