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Old 19th January 2013, 06:50 AM   #11
Mooly is offline Mooly  United Kingdom
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I agree with Elvee. You've measured the OFF LOAD voltage and its fine. Unless your mains happened to be at 220v or less when you made the test and today it might be 250 or more then there is no problem.

Do the off load test again. Measure the DC voltage. Measure the AC mains. Calculate what the new DC would be if the mains were at 256Vac worst case.
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Old 19th January 2013, 09:54 AM   #12
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Mooly View Post
.................... Measure the DC voltage. Measure the AC mains. Calculate what the new DC would be if the mains were at 256Vac worst case.
There's the important part.
Check that the actual worst case voltage matches the calculated worst case.
A 230:25+25Vac transformer is too high for 40Vdc capacitors when fed from the 240Vac UK supply when worst case UK voltage goes to 254Vac.
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Old 19th January 2013, 10:12 AM   #13
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Epcos capacitor surge voltage =1.15x rated voltage = in this case 46V...
Just if someone would be interested in that figure...
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Old 19th January 2013, 10:36 AM   #14
AndrewT is offline AndrewT  Scotland
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Surge rating is NOT for a short term but continuous operating voltage.
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Old 19th January 2013, 11:12 AM   #15
lgabco is offline lgabco  Slovakia
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This issue with the applied/rated voltage vs. reliability is sufficiently covered in the US standard - MILL HDBK 217. Some years ago I was dealing with the reliability of electronic calculations. I remember that electrolytic capacitor are without any problem even at 1-3 % above rated voltage, so sleep well.
However it could be problem if your temperature inside the device is above rated temperature of the capacitor. The same problem is with humidity or vibrations.
My estimation for your case is that MTBF (Mean time between failure) is more than 100 000 hours - more than 12 years permanent work.

I will look into my files to give you additional info if necessary.

Ladislav
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Old 19th January 2013, 11:20 AM   #16
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Surge rating is NOT for a short term but continuous operating voltage.
Typo ?

Surge rating is for short term (where the duration will be specified) and not continuous.

You measure the voltage... you determine if the continuous rating is or can be exceeded by the normal mains tolerance... and you build a few percent safety margin into the figure.

37 volts across a 40 volt cap is fine and that figure will drop considerably when the Class A amp is connected and pulling current.
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Old 19th January 2013, 12:02 PM   #17
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Ah now I see where our calculations are different Andrew! You are saying a 230V primary..... Are transformers available in the UK 230V as opposed to 240V? Here in Aus the locally made ones have 240V primaries. I didn't seen any mention of the actual transformer in use.

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Old 19th January 2013, 12:07 PM   #18
Mooly is offline Mooly  United Kingdom
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Quite a few years ago our mains voltage was "harmonised" to bring it in line with other countries that used 230 vac. In practice nothing here changed distribution wise. All that happened was that the tolerance changed from the old 240 -/+ 6% to 230 +10/-6 % which gave pretty much the same limits. Appliances (and I guess new transformers too) are "designed" now for 230 volts. So an old 240 volt 30 v secondary and a "new" 230 v 30 volt secondary are actually different and will give different voltages when fed from the same supply.
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Old 19th January 2013, 12:34 PM   #19
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[QUOTE=Mooly;3332818]Typo ? [QUOTE]

I dont think so...
It does take the 46v pulses continuously, as long as its 'rms' voltage stays below 40v...
Although it does also mean the capacitor WILL NOT blow in 46V.

It may however shorten its life...
Now - for how long periods are we going to feed it with the max peaks in UK voltages?
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Old 19th January 2013, 12:49 PM   #20
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... I wouldn't climb a high building with a rope that was approved to carry exactly my own body weight.

Exploding electrolytics are loud and inconvenient at best, but a real risk if they happen to fly towards your face while the amp cover is removed for testing purposes.

Most people here seem to disagree, but I don't use components up to the maximum specs - usually I try to stay at 60-80% of maximum. Less risk of instant failure and longer component life.

Greetings,
Andreas
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