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Old 19th January 2013, 01:44 PM   #21
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This thread seems to be going round and round in circles.
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Old 19th January 2013, 11:00 PM   #22
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Quote:
Originally Posted by Mooly View Post
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.
Thanks for the explanation Mooly That definitely explains why my caclulation were lower.

If I adjust:

multiply 25V by 1.414. Multiply by 1.07 (for 7% regulation) and then by 1.1 (for the 23V extra that the mains may actually be)... that comes out at 41.6V which agrees with Andrews worst case calculations.

7% is actually a very good regulation figure so in a lot of cases the figures would be even higher.

Again in this case though being class A, if the transformer is running close to it's VA rating it should be fine. I would agree in general with Andrew though that 40V caps are too low for a general 25V secondary.

I'm with Rundmaus and would generally prefer to have a 20% margin on the caps WV rating over he nominal rectified voltage.

katieandad I don't think it is going round in circles. This is an important consideration and I think should be made completely clear as someone less experienced may come along and see the thread and figure that 40V caps will be fine for their situation which may not be the case.

I use exactly the same method as Andrew when determining cap size putting in the actual regulation of the transformer being used when determining absolute minimum values for the filter caps ratings. If this is done then there should be no circumstances (short of the mains being outside of specs) where the caps will be over voltage. I think that this is just good engineering practice. Obviously as stated if the load is known to be constant then the regulation of the transformer can probably be taken out of the equation but the above gives a very good indication of worst case scenarios

Tony.
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Last edited by wintermute; 19th January 2013 at 11:03 PM. Reason: correct handle
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Old 20th January 2013, 07:03 AM   #23
Mooly is offline Mooly  United Kingdom
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I get the same figures although maths being my worst subject it took me around 5 minutes to work it out. My eyes glaze over at numbers lol

25 * 7 = 175
175/100 =1.75
1.75 + 25 = 26.75 volts AC. The transformer volts off load for the 230 volt specified transformer.

230 volts in and 26.75 out.
230/26.75 gives the ratio of the tranny. Thats 8.598 : 1

So 230 volts mains at its 10% limit above is 253 volts

253 / 8.598 is 29.42 volts ac

29.42 * 1.414 gives 41.6 volts YAY same answer

It would be interesting to know what alibears mains voltage really is.
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Old 20th January 2013, 07:17 AM   #24
AJT is offline AJT  Philippines
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i aim for 80% of capacitor working volts rating, say a cap is rated at 100 volts, i try to use it at 80 to 85 volts.....

it is the high line voltage that we have to consider, say that in your locality 260 volts is the highest voltage, then we consider the secondary voltage at that high primary line and size the capacitor voltage accordingly...
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Old 20th January 2013, 10:43 AM   #25
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quote.....The transformer has a secondary of 25 -0- 25 and when rectified under no load is about 37V dropping to 35V when loaded with 1.6 amp bias of the amplifier.

can I please ask, are you talking about measured trafo voltages, or theoretical ?


depends on whether you talk about measured actual voltages, or theoretical voltages
classA can be tough on the caps if they are wrong
but I think the type of cap is more important than exact voltage rating
meaning, it really needs to have high current rating, or it runs too hot

a cap deals better with a small over voltage than too high current
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Old 20th January 2013, 10:55 AM   #26
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Quote:
Originally Posted by alibear View Post
Hi all, I have 8 new Epcos 47000microfarad 40V capacitors .......
if they are the big ones(screw mount/terminals), I wouldn't worry about running them at 41V
they are very tough industrial caps
do you think they would of any use if the brake down at such small over voltage

but maybe they will last only 20 years instead of 25 years

btw, someone once told me many caps with different voltage specs are actually more or less the same cap .... just specced differently
more voltage ... less capacity (same physical size)
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Old 20th January 2013, 11:04 AM   #27
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As 40V is very close to the 42V (ish) that everyone is so worried about, you could always try a CRC circuit with the first C being 63V and safe, R being fairly small say 0R47 and then the voltage across the second C should be safely below 40V.
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Old 20th January 2013, 11:20 AM   #28
alibear is offline alibear  United Kingdom
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Thanks all for your valuable and informative replies, much appreciated. I will go with the informed replies and up the voltage rating.
Andrew T sorry to be a frustration to you. It's not that I "refuse" to read your posts, on the contrary,over the last two years or so that I have been a member of DIY Audio I have realised that your technical knowledge appears to be immense and your posts are of great help to those with inferior knowledge or experience such as myself. ( no I am not creeping, just telling the truth ) But the Forum is huge and it is not possible for me to read every post.
I will now use some 100,000microfarad 50 V that I have instead.
I am sure that in one of Andrew T posts in the past he has stated that you should bypass filter caps with caps of 100th the size eg.
100,000 bypassed with 1,000, then bypassed with 10, then 0.1microfarad.
Is this correct? or am I dreaming
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Old 20th January 2013, 11:38 AM   #29
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50V is an outdated voltage, replaced by 63V in most cases.

How old are the caps? They might need reforming.
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Old 20th January 2013, 12:17 PM   #30
alibear is offline alibear  United Kingdom
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The capacitors are at my place of work at the moment, will have to wait until Monday to look at them closely. If need be I will post a photo of the details on them to try to assertain their age. From what I remember they appear never to have been used.
Thanks for help
Alan
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