How to decide if an ali can electrolytic can replace a tantalum?

[Chris Wilson]

I have an issue with an HP spectrum analyser from the late sixties. Trouble with tantalum caps is a known possibility. Is there a way to decide if a modern small aluminium can electrolytic is a suitable replacement for a tantalum? Thanks.

[tggzzz]

I have an issue with an HP spectrum analyser from the late sixties. Trouble with tantalum caps is a known possibility. Is there a way to decide if a modern small aluminium can electrolytic is a suitable replacement for a tantalum? Thanks.

It would help if you gave the value, rated voltage, applied voltage and - most importantly - the function of the capacitor (e.g. decoupling logic or decoupling RF etc)

[T3sl4co1l]

If it's only for power filtering / bypass, 10x value might even be appropriate.  But then you have to worry about total load on the supply and all that.

I'd check to verify if the caps are actually bad (test after removal is crappy, because replacement causes even more thermal stress..), and suggest replacing them with comparable parts.  They worked the first time (give or take), they'll work again.  You might use higher voltage ratings to help improve reliability further.

Tim

[David Hess]

My general rule of thumb based on ESR is that it takes an aluminum electrolytic capacitor of 4 times the value to replace a solid tantalum.  Tantalum capacitors have better high frequency performance however so this does not apply in all applications

[Chris Wilson]

Sorry for the late reply, that damned "work" thing got in the way! I will replace like for like, and not try and be clever substituting something from my parts shelf. Thanks for the explanations everyone! I have an ECR meter for testing.

[f5r5e5d]

as far as I know Ta caps function is pretty binary - they do or they don't, they fail as short circuits

its Al electors that fade, age, dry out, increase in ESR, leakage current...

[SeanB]

Only the dry one do that, the wet ones can fail if they use rubber seals ( the lower cost ones, still eye wateringly expensive but they are lower cost than the glass frit sealed units) as the electrolyte will dry out with time. the hermetically sealed ones will last until the case corrodes through, which is why they are the only capacitors that are space rated. Ok you have to tie them down and use a slightly flexible conformal coating to hold them to the board for vibration testing, otherwise the heavy units will break off at the leads, but they will last forever. Who knows, they probably will survive reentry as well until the solder melts.

[tautech]

Sorry for the late reply, that damned "work" thing got in the way! I will replace like for like, and not try and be clever substituting something from my parts shelf. Thanks for the explanations everyone! I have an ECR meter for testing.

Chris, Tants bead caps are much more reliable now than those of 40 years ago and although many of us loathe them, I will still use them for repairs where space is tight.
Normally they are used to "catch" any HF on a supply rail(you can identify this use) as opposed to "bulk" capacitance.
For reliability select a voltage rating 2-3 times that of the application is the recommendation these days.
Be careful with the polarization markings, Google if necessary.

For low uF values(1-10 uF) I now use multi-layer ceramic beaded caps as they have adequate speed for a similar application.

[IconicPCB]

Tantalums on rails make a bad decision as they can burst into flames under such conditions

[tautech]

Tantalums on rails make a bad decision as they can burst into flames under such conditions

Agreed, but used in this way for decades.

[tggzzz]

Tantalums on rails make a bad decision as they can burst into flames under such conditions

So do lithium-based batteries, but we use them in many different bits of equipment.

Everything is a tradeoff; there is no black-and-white, everything is grey.

[David Hess]

Tantalums on rails make a bad decision as they can burst into flames under such conditions.

So do lithium-based batteries, but we use them in many different bits of equipment.

I have not seen this happen in old instruments but having the embedded battery die and take the calibration constants with it is almost as bad.

[tggzzz]

Tantalums on rails make a bad decision as they can burst into flames under such conditions.

So do lithium-based batteries, but we use them in many different bits of equipment.

I have not seen this happen in old instruments

... it would be quite remarkable if you had!

but having the embedded battery die and take the calibration constants with it is almost as bad.

Ouch. Just so.