Big capacitor's voltage rating in DIY amps

Hello

I just wonder, how much voltage one can put into an electrolytic capacitor rated for 100V DC.

I have used 100V caps in the power supplies of my first Leach Amps which have symmetrical 56V operating voltage. The newest has two pairs 100V and one pair 63V caps.

QSC USA1310 has 93V operating voltage and 100V caps. 300W BK module has 70Vcc and 80V capacitors. Professor Leach is ready to put 85 or 93V into 100V DC capacitors in his SuperAmp, as far as I understood.

As I wrote about this to sfnet.harrastus.elektroniikka, a Finnish newsgroup on electronics, the most conservative estimate I got was that one should have 30 (thirty) percent safety factor with capacitors, that is only 70V (or 77V) into 100V caps.

So, is it alarming or serious to have 56V in a 63V cap, or 93V in a 100V cap? Should I use only 100V caps for 56V (there are no intermediate ratings available) and use four 63V caps to make one 126V of equal capacitance for 93V (in the SuperAmp I hope I will soon build)?

This will of course cost me more than double compared to just 100V rating. 63V 10 000uF is 28FIM and 100V 10 000uF is 49FIM. So four times 63V is one 126V, also 112FIM. But I hate compromises with safety (that is, don't fancy killing anyone, burning anything or blowing up anything).

-Kimmo Sundqvist
 
We covered this pretty thoroughly during the summer. Ask fifty people and you'll get at least thirty-five answers. I run my caps at 80-95% of their rated voltage. Others run less voltage. To an extent, it will depend on how stable the AC is where you live. Even with stable AC, some are afraid to approach the rated voltage for their caps. Go track down that thread and see what everybody had to say, then flip a coin and decide what you feel most comfortable doing.

Grey
 

djk

R.I.P
2001-02-04 4:23 am
USA
De-rating the voltage in electrolytic caps extends the life in direct proportion to the de-rating, ie: 50V on a 100V cap doubles the life. More important is the temperature rise from ripple current. A 10*C change will double/half the life expectancy. Bigger caps run cooler than smaller caps because they have more surface area to get rid of the heat from ripple current. A 100V cap is much larger than a 50V cap of the same µF rating and will last much longer than just the difference due to voltage de-rating. 'Super' caps with really high capacitance to size ratios have very poor life expectancy in audio amps for this very reason. Avoid those little thumb sized 'snap-in' caps and go for the beer can sized caps. Frequently low voltage caps may be found on the surplus market and used in series.
 
When it comes to caps as long as you use at least what is called for the voltage rating won't affect the performance. You can use a cap rated for 450 volts instead of 100 volts and it will do the same job. You just can't do it the other way around. I like the old oil caps even in crossovers and the crossover doesn't care if the cap is rated for even 600 volts.
 
when a cap is rated for, say 100V DC then this means that the working DC voltage shouldn't eceed 100V.

However, this figure has a tolerance on it (i think it's quite tight) so really you can use it at exactly the voltage it says. And of couse you can go a little over say 2 or 3 volts. I have had amps with +-50V rails running with 50V caps fine for years.

Personally i think if you are within 98% of quoted spec you have absolutly nothing to worry about. You do not get any benefit from a cap that is massivly over rated (max voltagewise) for the job.
 

djk

R.I.P
2001-02-04 4:23 am
USA
"but I've seen manufacturers
put 75 volt caps into 85 volt rails without getting into
trouble." Yes, the Altec 771 did this. That amp blew the outputs so frequently that no one noticed that the main filter cap needed to be replaced every now and then too."And of couse you can go a little over say 2 or 3 volts." Bryston did this on the early 3B amplifiers, ran 52V on a 50V rated cap. If you have the original receipt and the amp is less than 20 years old they will even replace them for free when they fail. It was cheaper for me to buy my own 63V caps and be forever done with the problem than to pay the freight back to Bryston for a 'free' repair and risk UPS destroying the amp. I used to do flat rate labor repairs on Muzak amplifiers. It seemed like I charged more to do them than they could fix them for themselves. Upon close examination of the rental records they found that amplifiers that my shop serviced ran much longer between service calls than their own repairs did. Improving the mean time before failure doesn't cost, it pays. Get a parts catalog from a capacitor manufacturer, they usually have a tutorial on how to de-rate parts and determine lifespan. If you belong to the amp-of-the-month-club you may not care about this. I would like to think that somehing I build will last.
 
Can this be right?

Hi, I am a first time DIY-er and I am looking at upgrading my HK Citation 19 main power capacitors. The circuit diagram calls for 10,000uf 85V, but currently there are 75V rated capacitors in place. The circuit voltage rails on the diagram shows a +/- 55V going out. Is this the same situation as the Altec 771? I am confused as to why 75V capacitors are working here. Its difficult to find the right caps in the right size and I found some Nichicons that I want to use that are the perfect physical size at 80V. I am assuming that this should be o.k.? Would it help to bypass them with a higher rated film cap as well to add Voltage protection? Sorry to ask such a basic question. My knowledge is limited and I was hoping just to do some simple cap switching.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
If in any doubt measure the voltage across the caps with the amp on to see what you are dealing with. Circuit diagrams are not always correct particularly when regional/country variations are taken into account.

If the caps really are 75 volt and original then there is absolutely no problem fitting 80 volt devices.

Bypassing with a film does nothing with regard to voltage ratings. It's only of use in reducing the impedance of the supply at high frequencies... which probably isn't needed as there will be more local decoupling in the amp.
 
If you use the maximum line voltage you ever expect to see for moderate durations (ignore spikes), and under unloaded conditions for the PS, that should give a reasonable minimum for the cap rating. Most of the time it will be operating with a generous factor of safety and will even be OK if the circuit is disconnected. Most caps also have a higher surge rating, so the FOS is actually even better. The manufacturer has already put in some margin because building caps isn't as precise or consistent a business as one might hope.
 
Conrad is correct.
One must look for worst case operating conditions and design for those eventualities. The design decision could be to accept that some conditions are acceptably rare that the designer accepts responsibility for future failure when the foreseen risk is achieved. That's part of what a Manufacturer's warranty is about.

Guessing at a Factor of Safety (FoS) does no good. Determining a FoS based on testing and experience and actual variations in product specification is quite different.
There can never be an assurance that a guessed at FoS of 1.3 (that's 30% over the rating) will prevent failure. Particularly so when the worst case conditions may already exceed the "normal" operating conditions by more than that 30%.