Electronics > Beginners
Using "new" old electrolytic caps in new circuits - Yay or nay?
WyverntekGameRepairs:
--- Quote from: Nusa on December 23, 2019, 03:14:26 am ---A bridge rectifiers is four diodes, and generally only one diode will fail at a time. They don't fail often, but when they do they generally fail open. What do you think happens to the rectifier output in that event? But it's possible for them to fail closed as well, so also think about that rectifier output case as well. Now think about the less likely case of multiple diode failures. How do they have to fail to actually achieve a scenario that passes direct AC? Not so easy, is it? And in any case, you're wrong....DC smoothing capacitors aren't going to go boom from a little 50/60 Hz AC in the short term...remember half the cycles are in the correct direction.
--- End quote ---
Hm. You know, you're right. I guess I was thinking of a different scenario? Or maybe that was just a bad example. My point was that generally capacitors tend to not accept AC voltage all that well (though now that I think about it, it might be dependant on the period that the voltage goes negative?)
Though that being said... If I take a 450V 150uF cap from a power supply and put it directly on mains voltage, will it explode? What about a 250V cap? I'm genuinely quite curious. I'm afraid to try it out myself in case it explodes with unimagineable force or excessive fire.
David Hess:
Aluminum electrolytic capacitors can fail with low capacitance and high ESR after being stored for decades. Even if they test out as acceptable, a large part of their remaining operating life may have expired.
This also applies to aluminum electrolytic capacitors which have been in service with essentially zero ripple current for decades. I have refurbished old test gear where all of the small output bulk decoupling capacitors were only at 10 to 50 percent of their marked value with the larger ones doing better indicating that the failure was gradual evaporation of the electrolyte over time.
helius:
The standard failure mode for old (or non-operated) electrolytic capacitors is "de-forming", thinning of the oxide dielectric layer through etching by the acid electrolyte. In this scenario, the measured capacitance increases (because the dielectric separation is thinner), but the charge leakage is very high. Capacitors in this condition can be restored to near-factory condition by re-forming.
Sometimes the electrolyte has evaporated (dried-out capacitors). This would lead to low measured capacitance (because less than the full foil area is acting as a capacitor). This condition cannot be remediated and the caps should be thrown away.
james_s:
--- Quote from: WyverntekGameRepairs on December 23, 2019, 04:01:44 am ---Hm. You know, you're right. I guess I was thinking of a different scenario? Or maybe that was just a bad example. My point was that generally capacitors tend to not accept AC voltage all that well (though now that I think about it, it might be dependant on the period that the voltage goes negative?)
Though that being said... If I take a 450V 150uF cap from a power supply and put it directly on mains voltage, will it explode? What about a 250V cap? I'm genuinely quite curious. I'm afraid to try it out myself in case it explodes with unimagineable force or excessive fire.
--- End quote ---
150uF at 60Hz is going to present an impedance of 17.7 Ohms which placed across 120VAC will draw 6.7A of reactive power. This is quite a lot of current and will result in heating the capacitor, whether it can tolerate this or not depends on the particular capacitor and how much ripple current it can tolerate.
If you are using a polarized electrolytic capacitor though it will be connected backwards 50% of the time and will not last long. I suspect it would blow the top but probably not immediately.
WyverntekGameRepairs:
--- Quote from: james_s on December 23, 2019, 07:02:57 am ---
--- Quote from: WyverntekGameRepairs on December 23, 2019, 04:01:44 am ---Hm. You know, you're right. I guess I was thinking of a different scenario? Or maybe that was just a bad example. My point was that generally capacitors tend to not accept AC voltage all that well (though now that I think about it, it might be dependant on the period that the voltage goes negative?)
Though that being said... If I take a 450V 150uF cap from a power supply and put it directly on mains voltage, will it explode? What about a 250V cap? I'm genuinely quite curious. I'm afraid to try it out myself in case it explodes with unimagineable force or excessive fire.
--- End quote ---
150uF at 60Hz is going to present an impedance of 17.7 Ohms which placed across 120VAC will draw 6.7A of reactive power. This is quite a lot of current and will result in heating the capacitor, whether it can tolerate this or not depends on the particular capacitor and how much ripple current it can tolerate.
If you are using a polarized electrolytic capacitor though it will be connected backwards 50% of the time and will not last long. I suspect it would blow the top but probably not immediately.
--- End quote ---
Actually, that clears up a lot! Thanks for that info, very helpful. I never thought about it that way. I guess I taught myself to not put capacitors in any AC situations because it could shorten their lifespan (as they do see negative voltage, basically incorrect polarity) or even cause them to explode depending on how badly they were constructed. But hey, guess I learnt something new today :D
Basically what you are saying is that it really depends on the construction quality and ESR ratings of the capacitor. It also is affected by the frequency and voltage, of course, and there are a lot more factors than I thought. Pretty cool info, even if it is off-track from what I posted (granted, I did ask in the first place, so it's my fault really).
--- Quote from: helius on December 23, 2019, 04:45:43 am ---The standard failure mode for old (or non-operated) electrolytic capacitors is "de-forming", thinning of the oxide dielectric layer through etching by the acid electrolyte. In this scenario, the measured capacitance increases (because the dielectric separation is thinner), but the charge leakage is very high. Capacitors in this condition can be restored to near-factory condition by re-forming.
Sometimes the electrolyte has evaporated (dried-out capacitors). This would lead to low measured capacitance (because less than the full foil area is acting as a capacitor). This condition cannot be remediated and the caps should be thrown away.
--- End quote ---
I saw BigCliveDotCom "activate" a fake electrolytic capacitor by adding electrolyte to it (it was a dry capacitor, like the kind you see in scam fake "oem" caps sold by shady bootleg vendors). In theory, would adding new electrolyte to a cap after it has dried out "reactivate" it in a sense?
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