Noise in electrolytic capacitors

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

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.

at half their rated voltage?

regards, Gerhard

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

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.

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

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!

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.

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

United Chem-Con makes some nice polymer caps too.

John

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

Cheers

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

Nemo Inscribed thus:

^^^^^^

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

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

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

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.

What a vivid imagination!

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:

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