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Old 22nd December 2012, 10:52 PM   #11
Pano is offline Pano  United States
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Old 22nd December 2012, 11:29 PM   #12
jcx is online now jcx  United States
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I see that DC = AC peak Vrating in the datasheets - not certain how strongly to believe - it turns out blocking high DC V is much harder on a real world dielectric over time vs AC - and these AC motor run caps really aren't expected to see appreciable DC for their entire working life

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Old 22nd December 2012, 11:38 PM   #13
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I'd just passed on what I understood - that DC is 1.4 x AC. I'm no expert on these things, and do please make any corrections needed!
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Old 22nd December 2012, 11:39 PM   #14
Pano is offline Pano  United States
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yeah, i've always wondered about that.
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Old 23rd December 2012, 09:00 PM   #15
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I've used 660VAC rated *used* motor run caps at 1170VDC for many years. They even survived an incident of soaring to about 2KV for a short period. But I do keep 'em inside the chassis. Not really recommended, but a data point.

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Old 23rd December 2012, 09:22 PM   #16
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That a bit over the 924 dc rating. Sounds like the rating is on the modest side of a rating .
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Old 23rd December 2012, 11:16 PM   #17
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Motor run, especially for 660VAC caps, is probably fairly hard work, in series for motor current, start-stop spikiness, etc. I don't know about "audiophile credentials" but they're plenty rugged.

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Old 24th December 2012, 12:08 AM   #18
jcx is online now jcx  United States
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its continuous DC bias at higher V that stresses dielectric

if "toughness" is just surviving seconds of transients then its no indication

Higher DC fields drive different degradation/breakdown phenomena to penetrate a dielectric over time - opto isolator WVDC ratings are lots lower than the 1, 60 sec withstand test V because of this


I'm suspicious that maybe the datasheet writers are just doing the AC Vpk math - and may have never tested at 1000s of hrs with DC bias

Last edited by jcx; 24th December 2012 at 12:15 AM.
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Old 24th December 2012, 01:04 AM   #19
12B4A is offline 12B4A  United States
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What kinds of failures have been reported using motor run caps in DC operation?
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Old 24th December 2012, 01:06 AM   #20
JoshK is offline JoshK  Canada
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Quote:
Originally Posted by andyjevans View Post
A capacitor rated 440v AC should be OK for 1.4 x that figure in DC, in other words around 600v.
I've seen that rule of thumb as well and given their original intention it seems *fair* but maybe not always relied on.
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