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How much of a cushion for voltage ratings on devices?

Discussion in 'Solid State' started by saabracer23, Dec 5, 2018 at 7:15 PM.

  1. saabracer23

    saabracer23 AK Subscriber Subscriber

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    Im wanting to place a parts order for my next project, a Marantz 1120. Most of the electrolytics in the amp are either rated at 100v or 250v with the exception of the filter caps which are 50v. The polyester caps are rated at 250. Looking at the schematic in the power supply I see a voltage of 39.5 as the highest. On the amp board the highest voltage I see is 44.0v. So I don’t understand why they chose caps with such a high voltage rating. My guess is that since they’re axial they picked them for their physical size?

    Anyways, what’s the rule when it comes to choosing components? If the highest voltage in he amp is 44v, will 50v components be okay or should I step up to 63v?

    Dan
     

     

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  2. Blue Shadow

    Blue Shadow I gotta get me a new title

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    I'd go higher than 50 for a 44V system. I think 1.25x would be a minimum.
    Film caps are generally able to handle more voltage and you might not find low voltage films. They might start at 50 but 250 might be a bulk pricing that was cheap when the amp was built, no problem there.

    There were more steps in the past but you want to go up to the next size on small voltage units. 63 would be good for those on the 44V side.

    Back when this crap was made the axials were much more available and they might have picked them for their assembly or for characteristics or for fit. Who knows. Many times these axial lytics are replaced with radial because of cost. Most axials are a lot more expensive since they aren't everywhere anymore.
     
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  3. Retrovert

    Retrovert AK Subscriber Subscriber

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    Over time an electrolytic capacitor's dielectric (aluminum oxide) reforms to the applied voltage, thickening or thinning as depends upon conditions. (The oxide layer is fragile, and dissolves into the electrolyte when the capacitor is stored and not used. This is why reforming is recommended after two years in storage or disuse.) So using a higher-rated capacitor does not deliver greater tolerance for ripple voltage that one might expect. Such alteration of the oxide layer is not the case with film and PIO capacitors, which lack such layers, where the voltage rating depends upon the dielectric properties and does not change over time.

    Many times the ratings for components are chosen based upon (a) what components are already being ordered for other purposes, thus minimizing inventory and maximizing quantity discounts, and (b) what is the most common part available and thus had commodity pricing structure.
     
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  4. Powertech

    Powertech Well-Known Member

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    Just to re-iterate Retrovert would say don't worry too much about it. The main problem you are likely to face with fitting higher voltage large PSU caps is going to be their physical size and lead spacing that won't fit the circuit board. The second point is that, certainly with quality components, modern equivalents are of higher quality that their 20 - 30 year old counterparts. This is due to improved design and manufacturing techniques giving components which are going to have high reliability, tighter tolerances and longer life. The issue with the voltage is that the closer you get to the marked limit, the shorter the life of the capacitor is likely to be. Most these days are rated for around 10,000 hrs at rated voltage, and longer at lower voltages - 10,000hrs is a long time!
    The small film caps rarely fail and most don't really age much. The small electrolytic caps can dry out, but don't be too worried about the stated voltage. As long as it is above the service voltage, that should be O.K.
     
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  5. Retrovert

    Retrovert AK Subscriber Subscriber

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    Yuppers. Get the closest rating above the manufacturer's rating in the service manual and you're good to go.

    Good point. I looked up the release date for the Marantz 1120 and found 1974. The newer electrolytics are a bit denser than those from 44 years ago, so the new ones at a higher voltage may fit, nonetheless. But I would definitely measure the physical size of the existing units and check the specifications to make sure the replacements fit. Capacitance is a function of surface area, so there's two different dimensions to consider.

    Four hours a day is about 1,500 hours a year. So that's about 6 years. At two hours a day it's about twelve years. But that lifespan is at some given temperature, and the derating for higher temperature is considerable. The exponent in the formula varies by manufacturer, but it is along the lines that every ten degree rise in temperature halves the lifespan.

    The operating temperature, in turn, depends upon both (a) ambient temperature, so cooling fans can be beneficial in hot cabinets, and (b) the ripple current, which causes ohmic heating from ESR. The ESR for a modern electrolytic is quite low, but it does increase over time. Ripple current can be a big problem which shortens lifespan.

    Also, that lifespan rating does not, alas, include idle time. So if the capacitor is idle for more than two years the oxide layer dissolves into the electrolyte and reforming is needed. Check the manufacturer's specifications for that. It varies all over the place. Some of this is CYA, but the consensus is that after five years reforming is always needed, and some will say two others three is where the oxide layer thins enough that some sort of reforming is required.

    Anyway, as Powertech noted, pick a reasonable voltage above the spec and all is good. The new capacitors are quite good, as long as you do NOT buy Chinese counterfeits. Oh, yes, it's a thing throughout the entire supply chain. Only purchase from reputable suppliers for this reason.
     
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  6. saabracer23

    saabracer23 AK Subscriber Subscriber

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    Thank you all, so at 44v voltage on the circuit I’d be okay with 50v, but the higher I go the better.

    I’m not really concerned about caps fitting due to physical size and won’t have to measure to make sure they’ll fit prior as suggested above. I don’t want to go with higher voltages, I want to go with lower. In the amp where they used a 250v electrolytic I want to place a 50v. Since it’s newer and rated for lower voltage I know it’ll be much smaller.

    I keep a large number of capacitors on hand, though not many rated at 250v.

    3F3FB657-1E20-4126-8BBB-7AD0A664075B.jpeg
    E9C1E82B-7E62-434D-A60E-BB5DBDBA0184.jpeg

    Here are some pics of the schematic, the amp board sees 44v and the power supply highest I saw was 39.5v.

    Dan
     

     

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  7. merlynski

    merlynski Curmudgeon Electronicist Subscriber

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    This is asking for trouble. Electrolytic caps are likely to fail catastrophically when subjected to over-voltage. They leak, explode, catch fire, short out and/or take other components with them. Modern electrolytics have higher voltage ratings and better lifespans in smaller packages than ever before. Choosing a higher voltage rating in an equal or smaller size package just makes good sense. An equal to original voltage is ok, but don't go lower.
     
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  8. Blue Shadow

    Blue Shadow I gotta get me a new title

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    Why do you want to go lower?

    I know the voltage issue was mentioned in the Abyss thread so I searched it a bit and found @ConradH's comment:
    "The ideal voltage to run a cap at is probably 60-75% of rating" What with the voltage in the US creeping up a bit due to lower transmission losses, many here see up to 125 at the wall. This means older gear designed for 117V might show numbers in the schematic that will be a bit low today.

    Additionally, Conrad mentions the caps below 25V are just not as good as the ones at that level or higher. Just the way it is with small caps.

    I'd use that info from Conrad to determine some limits on the caps you might choose.
     
    Last edited: Dec 7, 2018 at 4:36 PM
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  9. Retrovert

    Retrovert AK Subscriber Subscriber

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    +1 for what he wrote. Sometimes failures don't catch fie, but will get very, very hot. About forty years ago I saw a smoking tantalum power supply capacitor which had shorted while I was trying to figure out why the circuit was unstable. It had not caught fire, but was well on its way. That day the capacitor gods smiled upon me. I've also seen cans and PIOs explode, streaming foil like confetti throughout the cabinet. Your luck may considerably vary, as @merlynski sets forth.

    BUT, having said that, sometimes the voltage ratings on the part are far greater than what is required for the reasons I above elucidated. An amplifier with, say, 40 VDC rails can't go to 250 VDC. The value was chosen based upon price, stocking, and other considerations which may have been external to the circuit parameters. Tinkering with values like this, however, requires some understanding of the circuit and this is not a sport for amateurs.

    Most, but not all, of the difference in rating is in the dielectric, but sometimes the electrolyte and construction change depending upon the intended use.

    Capacitors intended for power supplies, for example, often deliberately have higher ESR to damp out ripple current and prevent standing waves. This is one of the issues in having high-frequency motherboards or phones with many decoupling capacitors. The parallel layout reduces ESR to a vanishingly small number, and standing waves can arise in the groundplane. Plus the issues of having tank circuits created by mismatches in L and C.

    My point is not that such effects occur in audio frequency devices, but they can if high-speed digital logic is present and using the same ground configuration, but that capacitors are different depending upon the intended use.
     
  10. Retrovert

    Retrovert AK Subscriber Subscriber

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    Yes. The quality vs. voltage also varies by manufacturer and even across manufacturer's product lines.

    Many lower-cost capacitors are counterfeits, too. A few years ago I spoke to ELNA's tech support about this and was told that not only is the capacitor a counterfeit, but so the vendors of the counterfeit parts often forge literature which has different parameters than the real devices.
     
  11. sKiZo

    sKiZo Hates received: 8642 Subscriber

    Mfgs used to run close to borderline in many cases depending on how much room was available as older caps tended to be much larger than what we see today. I never saw any issue with stepping up a couple ratings, as new caps are much smaller than old production unless you're going with NOS or Elna Silmac II ...

    ** Some say larger caps take longer to charge, but we're probably talking milliseconds here. Go get yourself a really really really small cuppa coffee if you don't feel like waiting that long ... ;-}
     

     

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  12. saabracer23

    saabracer23 AK Subscriber Subscriber

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    I don’t think anyone is getting what I’m saying. I don’t want to go lower than the voltages seen in the circuit, I want to go lower than the values Marantz used. I don’t see the need to use the 250v rated caps they used in a curcuit that sees only 44 volts. I’m saying I’d be fine buying 63v rated caps for a circuit that sees 44v.

    Therefore I’d be going lower than what Marantz was using. That’s what I’m saying. I don’t think using 63v rated caps in a circuit that sees 44v would fail catastrophically.

    Am I wrong?

    I initially ask why Marantz would use 250v rated electrolytics in this amp and I’m told it’s probably what they had on hand, they buy in bulk for many designs. I don’t see the need for buying 250v rated caps for this integrated when it sees nowhere near that. So why would I buy equal to the original as you say when they used 250v rated?

    Don’t worry, I’m smart enough not to use on rated lower than what’s actually in the circuit.

    Dan
     
  13. saabracer23

    saabracer23 AK Subscriber Subscriber

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    I understand that newer caps are smaller, I pretty much have every value of Elma Silmic II, Panasonic FC, and Nichicon FG on hand. I have thousands of caps on hand. The FC and Nichicons are definitely smaller than the Elna. Problem is that all the caps I have, 90% I only have the values up to either 63v or 100v.

    I’ve fully restored maybe 50-60 amplifiers, receivers, preamps, or integrateds. Not just recaps, but relays, bridge rectifier diodes, checking outputs and drivers on a curve tracer, replacing matched differential pairs, etc. Not only restored, but half of them needed repaired. They either wouldn’t power, blow a fuse, no output, etc. I had to diagnose (with help from the community on many, big thank you to all!) I had a pretty big collection that my wife wasn’t fond of lol. Anyways, that’s why I keep a huge stock of parts on hand. I have an entire room of the house dedicated to this, may not be a pro, but I’m learning. I just really really enjoy bringing these beauties back to life.

    This is the first time I’ve run into a unit that has devices that are grossly overrated for their circuit so it’s just a little confusing for me. If you think there is a reason they specifically went with 250v caps other than it’s because they bought them in bulk so they used them, than that’s what I’ll get.

    On the other hand, if they did use them because they had them in bulk and there would be no harm to me using my 63v rated caps then I’d rather use them because just as Marantz did, I have them sitting around.

    If I absolutely must buy 250v caps, what are some good quality ones that would match the Elna or Nichicon FG? Obviously they don’t sell that high of voltage.

    Dan
     
    Last edited: Dec 7, 2018 at 11:40 PM
  14. Retrovert

    Retrovert AK Subscriber Subscriber

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    Electrolytics in the power supply have no sound. If the power supply has a sound, then the designer (or rebuilder) has found a new way to fail.

    Electrolytics in the signal path cause distortion and wherever possible should be avoided.
     
  15. saabracer23

    saabracer23 AK Subscriber Subscriber

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    So let’s say there is a 1uf 250v cap in the audio signal path (there is). According to the schematic it says the board sees 44v (+/-). I replace it with a Wima film, which I also have about a 1000 of. Now I have 1 uF in 50v, 63v, and 100v. I’m assuming I’d be okay using one of those. Or should I order 250v films even though there is only 44v?

    I apologize for asking again, I’m just very confused as at first I was told that Marantz likely used the 250v because that’s what they had on hand. Then it seemed like I was told that I’d run into issues (failing capacitors) if I went lower than the 250v used.

    Dan
     
  16. Retrovert

    Retrovert AK Subscriber Subscriber

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    Good choice on removing electrolytics from the signal path.

    The question which must be answered: what is the peak voltage seen by that capacitor? That peak can occur in peculiar ways. Most capacitors have some over-voltage tolerance, but it isn't huge. The 50 VDC rating is very close to the expect voltage. I would therefore go up in rating to the next one you have. The 100 VDC rating gives you so much headroom you'll never have an issue. Even 63 VDC would likely be more than enough, being 50% over the maximum expected voltage. But the size difference in a film capacitor of that size is very small, so either 63 VDC or 100 VDC would be fine. I would not, however, use 50 VDC on general principles.

    Always far better to ask than to have a problem down the road, so no need to apologize.
     
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  17. saabracer23

    saabracer23 AK Subscriber Subscriber

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    Thank you very much!

    Dan
     
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  18. Blue Shadow

    Blue Shadow I gotta get me a new title

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    I would do exactly what some say Marantz did, use what you have on hand with a proper safety factor. Using 250V caps is not necessary.
     

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