Electronics > Beginners
15v circuit, 35v capacitor, but only have 25v
Mp3:
The caps that were in it were no name caps from the 80's, neither brand name on either cap i recognized or had any meaningful google presence. They were also shockingly light when removed from the power supply, but that isn't surprising since the 4700uf was putting out just under 1000uf capacitance. I used some high temperature caps to replace the old ones which had some of the lowest listed ripple current on Mouser when i bought them.
I think most of the 16v 4700uf caps showed something like 2-2.5A ripple, and the ones I'm using have 1.2A ripple. The service manual made no reference to ripple or anything like that, just listed what model # of capacitor was used, but then again i wouldn't be posting in the Beginners section if i presumed to really know what I was talking about :-+
Anyway, the device has been happily working for the last few hours straight since I put these new capacitors in and touched up a lot of old crusty looking solder.
Siwastaja:
If you have no idea of the actual ripple current present, one way is to install the candidate capacitors and monitor their case temperature. As some sort of rule-of-thumb, it shouldn't rise more than roughly about 5 deg C over the ambient right next to the capacitor.
fcb:
--- Quote from: tggzzz on February 10, 2020, 10:48:25 am ---
--- Quote from: fcb on February 10, 2020, 10:33:52 am ---
--- Quote from: rdl on February 10, 2020, 05:16:51 am ---It's actually not a good idea to use an aluminum electrolytic cap at voltages far below its WVDC rating.
--- End quote ---
Can you explain why? Or provide a citation please?
Like many others I overrate capacitors (16WV on a 5V rail, 25WV on a 12V rail, 50WV on a 30V rail etc)- it's something I've always done and it hasn't bitten me in nearly 30 years.
--- End quote ---
Hand waving explanation... The dielectric is formed by the passage of a DC current, sometimes called a leakage current. Too little voltage can mean too little current and imperfect dielectric formation.
--- End quote ---
OK. So what rule-of-thumb would you use then?
tggzzz:
--- Quote from: fcb on February 10, 2020, 12:17:18 pm ---
--- Quote from: tggzzz on February 10, 2020, 10:48:25 am ---
--- Quote from: fcb on February 10, 2020, 10:33:52 am ---
--- Quote from: rdl on February 10, 2020, 05:16:51 am ---It's actually not a good idea to use an aluminum electrolytic cap at voltages far below its WVDC rating.
--- End quote ---
Can you explain why? Or provide a citation please?
Like many others I overrate capacitors (16WV on a 5V rail, 25WV on a 12V rail, 50WV on a 30V rail etc)- it's something I've always done and it hasn't bitten me in nearly 30 years.
--- End quote ---
Hand waving explanation... The dielectric is formed by the passage of a DC current, sometimes called a leakage current. Too little voltage can mean too little current and imperfect dielectric formation.
--- End quote ---
OK. So what rule-of-thumb would you use then?
--- End quote ---
I would read the data sheet to glean whatever info I could. I would google for (informed) answers.
rdl:
Dielectric deterioration of aluminum electrolytic capacitors used at voltages far below the WVDC rating is something I read about probably 40 years ago and has stuck in my mind. I don't know how much of a problem it is because it's rarely mentioned anymore. Maybe modern dielectrics are better and less affected or maybe it's becoming lost knowledge. I've always tried to stay at around 20-50% just to be safe.
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