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20th January 2013, 12:21 PM  #31 
diyAudio Member
Join Date: Nov 2011
Location: UK

Old caps dry out so be careful.
I would reform them anyway and use that check to see if they are still serviceable. 
20th January 2013, 12:35 PM  #32  
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Quote:
when things are so 'tightly matched' you need to measure your actual voltages ..... with your trafo or else its all just theoretical, and more or less waste of time but if you want to design for '100% safe everywhere' .... then it would be different, and Andrew would be right 

20th January 2013, 01:39 PM  #33 
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Join Date: Jun 2011

It is good that this case has come to a resolution and better suited capacitors have been found.
Therefore the rest of my input can totally be ignored: We know he had 40V rated Epcos capacitors. We know he had measured 37V no load output voltage. We know he had measured 35V at 56W. We assume he has 0.3V diode voltage drop at no load. We therefore know he has 26,4VACrms at his transformer output at no load, which is really close to a what ever toroid transformer available, for example a TalemaNuvotem. If the transformer was rated at 230Vin, we know that he must have measured the output at quite precisely at that exact voltage. If the transformer was rated at 240, we know that the input voltage during his measurements must have been 240v... So we assume he measured the output at (the worst case) 230Vin. Now: We take the British over voltage situation of 254V input (at that worst case 230v transformer of ours). 254/230x35=38,65Vout! And that will be the worst case situation! But as I said, ignore this... 
20th January 2013, 01:49 PM  #34 
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Join Date: Apr 2011
Location: Not where whatever I am looking for is at.

Derating voltage is advised to increase lifetime.
http://electrochem.cwru.edu/encycl/m...4appguide.pdf At this location, elco's are bound to be operating at >40 degrees and at high ripple currents. So, I would say it is really borderline and even slightly across it.
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20th January 2013, 06:19 PM  #35 
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20th January 2013, 07:54 PM  #36 
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Join Date: Apr 2011
Location: Not where whatever I am looking for is at.

The major ripple is from the bridge diodes transducting only briefly each cycle. In that brief period, all the Coulombs needed to maintain the current driving the amplifyer have to enter the capacitor. So, the more current you draw, the higher the ripple current in the caps. ClassA for that reason creates per output Watt larger ripple currents in the PS caps than classAB.
45 degrees will easily be seen inside a power amp, it's just over 20 degrees over normal living room ambient temperature. The caps will also heat up by themselves. Anyways, it will work without derating, but it is not good for longevity.
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20th January 2013, 08:05 PM  #37 
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high current would be something like welding
and those caps may be rated with 20A ripple current...each if they hear we call our amps high current, they will probably laugh at it 
20th January 2013, 09:25 PM  #38 
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Join Date: Apr 2011
Location: Not where whatever I am looking for is at.

I am not pulling it out of my hat. Please read Elliott Sound Products  Linear Power Supply Design under 'capacitor ripple current'.
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21st January 2013, 12:56 AM  #39  
just another
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Quote:
Without knowing the actual mains voltage at the time of the test then all anyone is doing is making assumptions. Yes it is likely that the 40V caps will be ok under the circumstances when load + diode drops are taken into account (as I said earlier). The maths is not hard, for anyone starting from scratch it doesn't hurt to do it and be safe knowing that no matter what the load is the caps should be fine. Personally I'm not one to cut corners (in anything). I've too many times seen situations where if a little more had been spent in the first place, the overall cost would have been a lot lower. Everyone though can make their own choices. The power in my street often goes to 252V, the suburb I used to live in was normally around 238V. If I'd used caps in a power supply that were marginal at 240V, then when I moved house I probably would have had problems. Tony. 

21st January 2013, 04:19 AM  #40 
diyAudio Member
Join Date: Jun 2011

I just calculated his input voltage, It was 230!
Error margins included 39.5Vout  still under 40V! And long life expectations to his capacitors... 2306% is totally irrelevant here. He has measurements! What kind of engineers are you? He has MEASUREMENTS! Use them! To make more accurate calculations we do not need to know the input voltage. We need to know his transformers make and model. And we need to know his diodes make and model. But we can ignore all this now. There has been an other solution to the case... 
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