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Noise in electrolytic capacitors

I am trying to evaluate whether to use tantalum versus OS-CON aluminium electrolytics as the output decouplers for some linear postregulators after a (relatively low noise) switcher. This is for a high gain amplifier that will be operating down to audio frequencies. I've come across references to aluminium electrolytics being noisy, but no firm details (kind of odd as there are lots of details about ceramics' problems under bias, wet vs dry tantalums etc). There's also some references to "low noise" aluminium electrolytics for audio work, but I don't know whether to take them seriously as audiophools believe all kinds of weird stuff. So, can anyone advise if aluminium electrolytics - specifically low ESR solid electrolyte types - have some kind of noise problem? I know tants are NOT microphonic but I've not come across any info about electrolytics and microphony one way or another.

Assuming they do not, I favour them over tants because their ESR is lower, I get the impression that tants' ESR is poorer at low frequencies, and I can get electrolytics at higher voltages (I like to run caps at about double their rated voltage under the impression this improves their reliability). The load on these linear regs will be fairly constant, so ripple current will be low.

Thank you,

--
Nemo
Reply to
Nemo
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I've never encountered noise from electrolytics. Considering that self-discharge time constants are typically days or weeks, I can't imagine a serious noise mechanism. Charge one up and see if it makes noise.

The problem with tantalums is that they tend to detonate. Aluminums have bad ESRs at low temperatures and tend to dry out over time. The polymers seem OK so far.

John

Reply to
John Larkin

I tested ceramic, poly layer, tant and electrolytic as integrator capacitors in a PLL loop filter and I observed microphonics from all except the electrolytic.

Reply to
Andrew Holme

at half their rated voltage?

regards, Gerhard

Reply to
Gerhard Hoffmann

Oh, I sure did: pheeeooou ... *BOOM*

--
SCNR, Joerg

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Reply to
Joerg

But the cap isn't in a gain path, so I don't see this being an issue. I think you would need a combination of gain and high impedance to get a cap to sing.

I'm partial to oscons.

Reply to
miso

boy, you run a tant like that and STAND BACK; do not tell anybody at Homeland "Security"...

Reply to
Robert Baer

Ahh... meant I to write half their rated voltage. Well, it shows you were paying attention! 8)

Thank you for the advice. OS-CONs it is!

--
Nemo
Reply to
Nemo

John Larkin a écrit :

Oh, sure there is: that's leakage current instabilities. But I've found it significant only once, when doing my 200pV/rtHz preamplifier where it had to go down to 0.1Hz I had to resort to a bunch of bulky 2.2uF/250V polypro film caps.

BTW, caps microphony is only significant in "high impedance" circuits. More precisely, the cut off frequency is given by the circuit loading resistance x cap value.

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Thanks,
Fred.
Reply to
Fred Bartoli

dV/dT kills tantalums. If you use, say, an LM317 type reg, and bypass the adj pin to ground (which cuts noise, too) that can limit output slew to a safe value. If you use a reg with a slow-start option, that can protect tantalums.

I've never blown up a tantalum that was run at 1/3 rated voltage. If the gear has to run at, say, -20C or so, that's worth doing.

The polymer caps have such low ESR that some regs will get unstable with them as loads, especially switchers.

John

Reply to
John Larkin

United Chem-Con makes some nice polymer caps too.

John

Reply to
John Larkin

An electrolytic is trying really really hard to be a battery. There are self-EMFs there that are horribly temperature dependent.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
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Reply to
Phil Hobbs

So someone should actually measure the noise across a charged (or even discharged?) electrolytic cap, and maybe a polymer too. I'll try it next week if I get time.

John

Reply to
John Larkin

Nemo Inscribed thus:

^^^^^^

I'm glad I'm not in your lab. :-)

--
Best Regards:
                     Baron.
Reply to
Baron

I once got the can off one in the crotch, from across the lab. Makes yer eyes water ;-)

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)
Reply to
Fred Abse

I had a Tantalum blow up in my face one time. Fortunately I had my magnifiers down over my eyes.

(Client's board sent to me to evaluate to replace with a single chip. Who ever threw it together inserted Tantalums backwards, so a VCC to ground short _somewhere_. While I'm micro-probing down the rails to find the offending element, it blew.) ...Jim Thompson

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Reply to
Jim Thompson

Ouch, ouch ...

As a kid I was blissfully unaware of ESR and all those secondary "non-essentials". Fluorescents dimmed ... wait, there's no dimmer on that circuit ... *KABLOUIE* ... phsssooosh ... *POCK* ... phssss

Followed by some plaster raining down on me and a hissing and smoldering can on the carpet. It had sailed mere inches past my right eye and I sat there all shaking.

Had to spackle the crater in the ceiling, easy. Also had to get new carpet, from a discount store five miles away, using my bicycle to transport the large and heavy roll. Not so easy.

This was a serious capacitor, several hundred uF and a few hundred volts, directly fed by a 230V/16A mains line. In a Cockcroft-Walton tripler with numerous other caps feeding a kilowatt-size amplifier. In hindsight it's all pretty amazing that I survived my childhood.

--
Regards, Joerg

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Reply to
Joerg

What a vivid imagination!

Ban

Reply to
Ban

Sorry but your telepathic crystal ball let you down, like before.

It happened with a Siemens electrolytic no less. Amplifier with PL509 tubes, tripler from 230VAC to about 900VDC. To be exact, this one, made by yours truly:

formatting link 

--
Regards, Joerg

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Reply to
Joerg

I meant the vivid desciption of your exploding components, you really can differ between a failing MOSFET and passive parts, but nothing seems to beat a tantal. and by imagination was meant to be a special creative talent, unfortunately I worded it ambivalent. ;)

I was waiting for the crater in the ceiling tho. ciao Ban

Reply to
Ban

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