Hi all, I have 8 new Epcos 47000microfarad 40V capacitors that I would like to use in a Power supply for a class A amplifier. 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.
My question is : is it safe to use 40V rated capacitors, or am I sailing too close to the wind.
Thanks for your help
Alan
My question is : is it safe to use 40V rated capacitors, or am I sailing too close to the wind.
Thanks for your help
Alan
Mooly, your wording their might be slightly confusing.
The 40V electrolytics will be FINE if the 37V measured is when the mains voltage is at its maximum. However, if it is measured when the mains voltage is at the low end of its tolerance - ie 216V - then the voltage will exceed 40V when the mains voltage rises.
The 40V electrolytics will be FINE if the 37V measured is when the mains voltage is at its maximum. However, if it is measured when the mains voltage is at the low end of its tolerance - ie 216V - then the voltage will exceed 40V when the mains voltage rises.
a 230:25+25Vac transformer is too high for 40Vdc capacitors when fed from the 240Vac UK supply.
I have posted the calculation many times before.The maximum off load voltage for a 7% regulation transformer will be about 41.3Vdc. Well over the 40V rating.
I have used up my quota of unhelpful posts, no more today.
I have posted the calculation many times before.The maximum off load voltage for a 7% regulation transformer will be about 41.3Vdc. Well over the 40V rating.
I have used up my quota of unhelpful posts, no more today.
I think I know where Andrew is coming from, though my calculations say that with 7% regulation (unless I'm calculating wrongly) I get around 39.9V
This is based on 25V being the nominal output at full load.
multiply 25V by 1.414. Multiply by 1.07 (for 7% regulation) and then by 1.054 (for the 13V extra that the mains may actually be)... that for me comes out at 39.9V (with 10% regulation I get almost 41V). perhaps I have forgotten how to take regulation into account.....
However as this is going to be a class A amp the current draw will be constant, so the regulation of the transformer can probably be taken out of the equation (assuming that the load is always on, and it is being driven close to it's VA rating). If you also take diode losses into account there should also be some loss there so the 1.414 multiplier is probably a bit generous.
Tony.
This is based on 25V being the nominal output at full load.
multiply 25V by 1.414. Multiply by 1.07 (for 7% regulation) and then by 1.054 (for the 13V extra that the mains may actually be)... that for me comes out at 39.9V (with 10% regulation I get almost 41V). perhaps I have forgotten how to take regulation into account.....
However as this is going to be a class A amp the current draw will be constant, so the regulation of the transformer can probably be taken out of the equation (assuming that the load is always on, and it is being driven close to it's VA rating). If you also take diode losses into account there should also be some loss there so the 1.414 multiplier is probably a bit generous.
Tony.
Manufacturers will sail that close to the wind to save money.
In the DIY World you would be better to err on the side of caution and go for the next voltage up which is normally 63V.
Depending on the cap manufacturer also depends on how conservative they were with their working voltage rating.
If you've got them already you will probably be OK. If you are buying new ones, go for the next voltage up.
In the DIY World you would be better to err on the side of caution and go for the next voltage up which is normally 63V.
Depending on the cap manufacturer also depends on how conservative they were with their working voltage rating.
If you've got them already you will probably be OK. If you are buying new ones, go for the next voltage up.
Hi all, thanks very much for your replies, I deduce from them that I will probably be OK, but if the voltage on the caps could be reduced a little it would be better. As I said this supply is for a class A amplifier with a bias of between 1.6 and 1.7Amps, with 4 47000microfarad capacitors on each channel and one 25-0-25 300Va transformer for each channel. Are any of these suggestions suitable for reducing the voltage slightly:
1) Place a CL60 in series with the primary windings.
2)Use two bridges per transformer ( one for each secondary winding )
3) Place a low value resistor ( 1ohm ? ) between the bridge and capacitor
4) Place a low value resistor (1 ohm ? ) between the two capacitors that are on each rail to give me a CRC.
Will any one of these, or a combination, give me a reduction in voltage, obviously I do not want to do anything that will affect the quality of the amplifier.
Thanks for your help
Alan
1) Place a CL60 in series with the primary windings.
2)Use two bridges per transformer ( one for each secondary winding )
3) Place a low value resistor ( 1ohm ? ) between the bridge and capacitor
4) Place a low value resistor (1 ohm ? ) between the two capacitors that are on each rail to give me a CRC.
Will any one of these, or a combination, give me a reduction in voltage, obviously I do not want to do anything that will affect the quality of the amplifier.
Thanks for your help
Alan
Unless you made your measurements when your mains voltage was at the extreme low of the tolerance bars, I think you'd better leave everything as it is.
Your class A amp will draw current, no matters what happens, and even if a fuse blows, you will still be within limits (and probably react rather swiftly if you have no sound).
Any additional impedance you add anywhere will adversely effect the quality and reduce the available power.
As long as you stay within limits (with some margins) with quality caps, you risk nothing.
As far as I know, Epcos is the inheritor of the Siemens brand, and if they keep up with the same quality standards, you should have no trouble at all (even if your mains happens to be 20% higher, but I shouldn't say that, I risk encouraging irresponsible behavior)
Epcos capacitors are widely used in the industrial field. If the 40V is working voltage the makers are well aware of mains and load variations then they will have taken that into account when establishing the rating. Everything in engineering has tolerances. That is why there are working and absolute maximum ratings.
You should be consulting the makers datasheet first, then asking how to interpret the ratings if necessary.
Another vote for leave it as is.
You should be consulting the makers datasheet first, then asking how to interpret the ratings if necessary.
Another vote for leave it as is.
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.
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.
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.
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
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
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.
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.
... 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
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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