[morzh]

Well, somehow (maybe because I am mostly solid state type of guy) while dealing with old radios I never asked myself that question: should I look closer to the ripple current rating of the caps we put to filter the full-wave rectifier?

After all tube radios even at huge power consumption overall draw very little from their rectifiers: tens of milliamps typically, with excursion into a couple of hundreds of those sometimes and even less frequent to more than that.
This is a little fraction of what I typically deal with. Transistors and solid state being low voltage devices will draw proportionally higher currents given the same power: in a simple Hi-Fi transistor amp one deals with several amps of load current at least. And in the switchers that I design mere 40 Watts make you handle several amps of ripple.

Anyway, I decided to take a look at this.
Usually when looking at the photos of people rebuilding the old aluminum caps or reading the description, or looking at the "kits" sold at DIY sites / eBay I mostly see General Purpose capacitors. They are used as long as microfarads/volts fit those of the capacitor being replaced.

Back in the days of yore when radios were, well, radios, the aluminum caps were simply aluminum caps. One did not have today's choice of "GP"/"low ESR"/"High ripple current"/"low leakage/"High Frequency" electrolytic caps.
Then again the caps were large: a then 10uF 450V cap was tens of times the size of today's one. This amongst other things accounted for better cooling.
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Let's take a hypothetical rectifier built around a tube. For simplicity let's make the load current 100mA. Let's make the peak rectified voltage 400V.
The LC filter typically employed by most radios will smooth the current and the voltage ripples making the radio more or less constant voltage / constant current load. But the very output cap sustains little ripple compared to the cap across the rectifier, so the latter is the one of interest.

So, first let's see what the voltage ripple is.
This one is dictated by the expression
Vr=Il/(2F*C)
Where the Vr - ripple voltage, Il - load current, F is the line frequency and C is our capacitance.
Let's pick a typical capacitance of 2x10uF which usually replaces that of 2x8uF.

So, our ripple voltage is:
0.1A/(120*0.00002)=83V.
Roughly sketching this it becomes visible that the discharge time equals approximately 3 times charging time. Which means that the average charging current that has to replenish the spent charge is 3 times that of the load (discharging) current.
So we have total charge of 3*Iload + I charge, or 6*Iload, and the average is 6*Iload/4=1.5*Iload.

So, our ripple is 150mA at the 100mA load.

Now let's look at the caps:

First the General Purpose:
A 10uF 450V cap by Nichicon, UVZ2W100MHD, has 65mA ripple rating.
Two of those in parallel will provide for 130mA.
A Panasonic ECA-2WHG100 has 75mA (two of them respectively 150mA.
So while having met the voltage/capacitance we are not meeting the ripple rating. This will at some point result in the premature failure of those caps.

But say a Panasonic EEU-EE2W100 cap will have 172 mA ripple current at 120HZ (the rating of 490mA is deceiving - one has to realize that the 120Hz ripple carries with it 0.35 multiplier), so in quantity of two that will give us nice enough 344mA ripple capacity.

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A neat trick to go beyond these limitations is this: instead of 10uF caps let's use 22uF caps of the same or slightly lower voltage.
Why?
The same cap type usually shows sizeable increase in ripple current rating with increased capacitance.

So, the same type 22uF cap EEU-EE2W220S will have 331mA ripple rating at 120 Hz. At $0.96 a pop.
Better yet, EEU-EB2G220S has 410mA. It is at $1.05 a pop.
And best of all EEU-EB2W220(S) has 560mA. This is the one I bought. At $1.34 a pop.

Now let's serialize the two we will have 11uF cap with the same 410mA rating. Stuff them in one can.
Do the same for the second can.
Now once paralleled, we will have whopping 820mA ripple rating.
Same done to 560mA cap will give us 1120mA.

This will get us out of the danger zone with a very safe margin.

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What kills aluminum caps? Pretty much (with the obvious exception of the voltage breakdown) one thing: temperature. The temperature in turn is usually the result of the ambient (which in tube radios is quite high) plus the heat due to the ripple current against the ESR.
At 20 degrees C (room temp) most caps will last from 2000 to 10000 hrs at full ripple current. Which is not that much.

Choosing 105C rated cap with a good margin of the ripple current will ensure long life.

[codefox1]

I use recently manufactured 400 or 450 VDC caps and use them pretty much for everythng; any doubt I do a series job, and always employ bleeder resistors. Common sense, if not in the original design, really helps in the heat department. Seen enough just plain brain dead sets as we all have.

[Mondial]

Thanks for bringing up this topic. It always drives me crazy to see people replacing large can electrolytics with tiny modern replacements, with no consideration of the ripple ratings. Sometimes I wonder if we are replacing some of these electrolytics unnecessarily.

I recently bought a very inexpensive ESR tester and did a bit of checking.

http://www.ebay.com/itm/300899052511?ssP...1497.l2649

Measuring some older used twist prong can caps from the 1950's and 1960's, I found that the original used caps had measured ESR's less than some of the modern replacements. It is not unusual to see some of these good quality caps still meeting original specs 50 years later.

At this point I am reconsidering routine replacement of electrolytics in equipment from the 1950's and newer if the capacitance, ESR and leakage measurements are still within original specs.

In any case, I would recommend anyone interested in checking their electrolytics to get one of these testers, as it is well worth the small investment.

[morzh]

ESR is important and is linked to the ripple current rating, and usually high ripple current caps are also low ESR, but not necessaily otherwise.

ESR contributes to the ripple rating by dissipating heat while the current travels through the cap; this is usual heat dissipation, I-square-R, but the ripple rating takes into account overall temperature regime to which many factors contribute: size, ESR, loss tangent, mass.

What ESR itself is reponsible for is producing the ripple voltage while the ripple current travels through it. This parameter is more important when a cap is used say as the output cap in the rectifier stage of a flyback converter, or at the output of a buck converter. This is where the ripple voltage is less affected by charge-discharge or capacitance than it is by ESR.

However in the full wave rectifier working from Mains due to long discharge the ripple voltage is practically not affected by the ESR but is affected by the discharge current and the capacitance. And since the ESR only indirectly points to the Ripple current rating it is better to look at the latter directly.

[codefox1]

Just set old sets off if not present.

[Mondial]

Yes, while the ESR only indirectly relates to the ripple current rating, it does give an easily measured indication of the quality and condition of the cap.

Many restorers buy their replacement electrolytics online from sources and brands of unknown quality, so the ripple current rating is an unknown. Unless you are buying name brand caps from a reputable distributor, there are usually no published specs for the part. So at least, by measuring ESR, you can get some idea of the suitability of the replacement cap, especially if you have a cap with known specs to compare it to.

My other point is that ESR measurement can give an indication of the condition of the original or replacement cap. Typically electrolytics fail by loss of electrolyte by evaporation, resulting in a gradual increase of ESR. So a quick check of capacitance and ESR can give an indication of imminent failure. I recently checked some NOS twist prong caps, and while most were reading an ESR of less than an ohm, one read 140 ohms even though its capacitance was well within tolerance. Of course, this particular cap was obviously bad and would have failed completely upon use.

Ripple considerations are also very important when replacing small value paper filter caps with electrolytics. I recall a posting recently where someone replaced the 1 and 2 uF filters of a model 20 with tiny electrolytics more intended as bypasses. Needless to say, these replacements will not last long, and mylar film caps would have been a much more suitable replacement.

[morzh]

Actually the reason electrolytics should not be used for small caps replacement is not strictly speaking the ripple as the currents may be very small, but the frequency, leakage and such parameters which in them ar simply are dismal. Electrolytics were never meant for that; they were meant as a bulk storage with high per volume energy capacity. Using them otherwise is not expedient and often won't work.

Also when doing that people often forget about the fact that polarity should be observed. Which will kill it fast.