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Old 7th March 2011, 10:10 PM   #1
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Default polyprop caps, metalized or foil?

I've heard that metalized polypropylene capacitors aren't as good as foil and polyprop caps. Something to do with residual charge not draining off as quick due perhaps from resistance of the metalization. I wonder if the diff is significant? Does one sound better than the other?
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Old 7th March 2011, 10:13 PM   #2
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The ESR is slightly higher for metallized, as is max ripple current. For coupling use, the difference is insignificant. For high ripple current applications, the foil cap will be better.
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Old 8th March 2011, 01:06 AM   #3
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The ESR is slightly higher for metallized, as is max ripple current. For coupling use, the difference is insignificant. For high ripple current applications, the foil cap will be better.
Is that because of the resistance of the metalized layer vs the thicker foil? Just curious.

G
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Old 8th March 2011, 01:13 AM   #4
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Exactly right. Metallization is thinner and "island-y."
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Old 8th March 2011, 01:51 AM   #5
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I don't recall the specifics of polypropylene, but in mylar the metallized film is preferred in line V applications because the arc in local breakdown in the dielectric can "clear" by eroding the thin metallization in the vicinity of the arc

with foil electrodes a dielectric breakdown, not limited by external impedance, will destroy the Cap

in fact , some "metallized film" polypropylene use double sided metallized mylar as the electrodes

for low esr and high SRF you really want "extended foil" construction where the foil edges are shorted by endcaps/spray metallization/ultrasonic welding
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Old 8th March 2011, 02:20 AM   #6
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I thought dielectric absorbtion was the key parameter. How does metallized compare to foil? The original post seems to have asked same question in a different way.
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Old 8th March 2011, 02:21 AM   #7
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DA is important if you're making timers. The metallization vs foil is pretty irrelevant for that- it affects ESR.
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Old 8th March 2011, 02:31 AM   #8
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Are you aware distortion from capacitors is due to dielectric absorbtion phenomenon? Does metalliazed or foil have lower DA?
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Old 8th March 2011, 02:35 AM   #9
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DA is important if you're making timers. The metallization vs foil is pretty irrelevant for that- it affects ESR.
More curiosity. Would the 'islandy' nature of the film mimic DA?

I think the poor conductivity of the coating on CRTs causes them to 'recover' a charge after shorting them out. I always thought the discharge causes a voltage gradient around the anode connection because of the resistance caused slow discharge. The 'recovery' is just the charge migrating through the high resistance coating to equalize the charge. Doesn't this mimic DA?

G
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Old 8th March 2011, 02:43 AM   #10
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Originally Posted by stratus46 View Post
More curiosity. Would the 'islandy' nature of the film mimic DA?

I think the poor conductivity of the coating on CRTs causes them to 'recover' a charge after shorting them out. I always thought the discharge causes a voltage gradient around the anode connection because of the resistance caused slow discharge. The 'recovery' is just the charge migrating through the high resistance coating to equalize the charge. Doesn't this mimic DA?G
Resistance, either internal or external, is not the same as DA. Distortion analysis will reveal the difference. Internal resistance would add to external resistance and alter the RC time constant of whatever circuit is being used. DA would still be the same.
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