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Electrolytics

  • Thread starter Dirk Bruere at NeoPax
  • Start date
D

Dirk Bruere at NeoPax

Jan 1, 1970
0
Something I never really thought about until just now.
When electrolytics are used for AC coupling in (say) audio circuits I
assume they survive because AC isn't polarising?

--
Dirk

http://www.onetribe.me.uk - The UK's only occult talk show
Presented by Dirk Bruere and Marc Power on ResonanceFM 104.4
http://www.resonancefm.com
 
H

Homer J Simpson

Jan 1, 1970
0
Something I never really thought about until just now.
When electrolytics are used for AC coupling in (say) audio circuits I
assume they survive because AC isn't polarising?

If there's a DC bias (and there often is) you wire them in that way.
 
H

Homer J Simpson

Jan 1, 1970
0
True, but the voltage swings both ways even with bias.

Should still be OK. They won't explode on a slight negative for less than
50% of the time. But it would be better to bias them into the correct
setting.
 
D

Dirk Bruere at NeoPax

Jan 1, 1970
0
Homer said:
Should still be OK. They won't explode on a slight negative for less than
50% of the time. But it would be better to bias them into the correct
setting.

I was just casually wondering whether modern electrolytics are fairly
immune to AC, since I have seen some AC electrolytics advertised.

--
Dirk

http://www.onetribe.me.uk - The UK's only occult talk show
Presented by Dirk Bruere and Marc Power on ResonanceFM 104.4
http://www.resonancefm.com
 
D

DaveM

Jan 1, 1970
0
Dirk Bruere at NeoPax said:
True, but the voltage swings both ways even with bias.

--
Dirk

http://www.onetribe.me.uk - The UK's only occult talk show
Presented by Dirk Bruere and Marc Power on ResonanceFM 104.4
http://www.resonancefm.com



The electrolytics survive because the DC voltage impressed across the capacitor
is much greater than the peak-to-peak levels of the AC being impressed. If you
have a capacitor between a 20 VDC source and a 5 VDC source, the DC voltage seen
by the capacitor is 15 VDC. Now, suppose the AC signal is 1VAC. the capacitor
never becomes reverse biased, therefore, it stays polarized in the manner it was
designed to be.
If, however, the AC signal voltage is greater than the DC voltage across the
capacitor, then you need to use a non-polarized unit, or a non-electrolytic
unit.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer to the end, the faster it goes.
 
P

Phil Allison

Jan 1, 1970
0
"Dirk Bruere at NeoPax"
Something I never really thought about until just now.
When electrolytics are used for AC coupling in (say) audio circuits I
assume they survive because AC isn't polarising?


** Obviously, AC current is not polarising.

The electro cap value needs to be large enough so very little SIGNAL voltage
appears across it - maybe 100mV rms at the lowest operating frequency.




......... Phil
 
P

Phil Allison

Jan 1, 1970
0
"DaveM"
The electrolytics survive because the DC voltage impressed across the capacitor
is much greater than the peak-to-peak levels of the AC being impressed.


** It will "survive" and function fine long as any reverse polarity
imposed is limited to a low voltage - ie under 1 volt.

Be even more careful with tantalum electros - as reverse current flow
permanently damages them. If this is a possibility, then parallel any with
a diode for protection.

If, however, the AC signal voltage is greater than the DC voltage across
the capacitor, then you need to use a non-polarized unit, or a
non-electrolytic unit.


** Better get this clear - in a correctly designed circuit, the AC voltage
ACROSS a polarised electro is normally FAR less than the AC signal voltage
it couples to the load.

IOW do not use a polarised electro to create a low frequency roll off in a
circuit with no polarising voltage for the cap.


....... Phil
 
R

Robert Baer

Jan 1, 1970
0
Dirk said:
I was just casually wondering whether modern electrolytics are fairly
immune to AC, since I have seen some AC electrolytics advertised.
Usually, an "AC electrolytic" is a non-polarized electrolytic, or it
was derated by 50% to 75% on voltage.
 
E

Eeyore

Jan 1, 1970
0
Dirk said:
Something I never really thought about until just now.
When electrolytics are used for AC coupling in (say) audio circuits I
assume they survive because AC isn't polarising?

You mean in zero-bias AC coupling of course. For a very long time, audio
circuits were prediominantly and many still are single supply.

But basically yes. Also, aluminium electrolytics will happily tolerate a small
reverse voltage. Don't use tantalums for this, I gather they're less happy about
it.

Graham
 
E

Eeyore

Jan 1, 1970
0
Homer said:
Should still be OK. They won't explode on a slight negative for less than
50% of the time. But it would be better to bias them into the correct
setting.

It turns out (I've measured them) that if you keep the reverse voltage below
about -100mV, you don't even see any non-linearity. Beyond that, yes they start
to add distortion.

They're not going to 'explode' for small signal coupling anyway since we're
talking typically about a mA or less of current. In loudspeaker crossover
filters where criminals use electrolytics they're the back-to-back non-polarised
type.

Graham
 
E

Eeyore

Jan 1, 1970
0
Dirk said:
I was just casually wondering whether modern electrolytics are fairly
immune to AC, since I have seen some AC electrolytics advertised.

Motor start ? Those will be 'back-to-back' types.

Graham
 
E

Eeyore

Jan 1, 1970
0
DaveM said:
If, however, the AC signal voltage is greater than the DC voltage across the
capacitor, then you need to use a non-polarized unit, or a non-electrolytic
unit.

Not true. But you need to be careful.

Graham
 
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