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-   -   Electrolytic voltage rating (http://www.diyaudio.com/forums/power-supplies/228091-electrolytic-voltage-rating.html)

alibear 18th January 2013 09:26 AM

Electrolytic voltage rating
 
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

Mooly 18th January 2013 10:15 AM

That's acceptable although you also need to allow for the mains being at the top end of its tolerance which is around 253 volts AC in the UK. If the 37 volts you measure is with the mains at the lower end (around 216) then they won't be suitable.

KatieandDad 18th January 2013 10:36 AM

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.

AndrewT 18th January 2013 10:42 AM

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.

Mooly 18th January 2013 11:09 AM

I think the 37 volts dc that alibear measured is already off load and so is already a "worst case figure" regulation wise. That voltage can't go any higher for the applied mains voltage. It is just the mains voltage that was applied at the time of the measurement that is the unknown :)

wintermute 18th January 2013 12:15 PM

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.

KatieandDad 18th January 2013 01:06 PM

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.

alibear 18th January 2013 07:57 PM

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

Elvee 18th January 2013 08:13 PM

Quote:

Originally Posted by alibear (Post 3332001)
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

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)

metalsculptor 19th January 2013 02:10 AM

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.


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