How to Check Aluminum Electrolytic Capacitors

7/30/2019 How to Check Aluminum Electrolytic Capacitors

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How to Check Aluminum Electrolytic Capacitors

One could write an entire book on this topic but I'm going to focus on a very limited situation, that of

servicing common consumer electronics, including audio amplifiers, receivers or video equipment.

The principles will be the same for all manner of electronics, but these devices tend to use similar

types of capacitor, all too often chosen for price over quality. Though I've no statistics, failedcapacitors seem to account for a large number of service calls.

In writing this I've realized that capacitors can be understood on many different levels, from the

practical to the purely mathematical. Some of the traditional analogies, like the "bucket of water"

analogy are misleading at best. Different datasheets and applications may use slightly different

terminology. Power people refer to power factor. Switching supply people talk about effective series

resistance (ESR). Traditional engineers may use loss tangent or phase angle. Test equipment makers

typically calibrate their dials in dissipation factor (D). OK, maybe you won't find that many dials these

days, but it's no surprise if people are confused by the different viewpoints and terminology.

One thing to remember is that whatever system of units is being used, it can be converted to any

other system of units. There will always be two numbers that describe capacitance and the

unavoidable internal losses. Series capacitance and dissipation factor are the most common, but

you'll also find reactance and phase angle or the somewhat obscure G & B. Loss in terms of effective

series resistance (ESR) has become a common buzzword in recent years, but it's just the plain old

series model resistance term, Rs, that's been familiar to engineers since the beginning of the 20th

century.

I have to admit to having some long held beliefs concerning the effect of various capacitor problems

on circuits. While writing this I built up some test circuits and installed various caps from mycollection of "defective" caps pulled from equipment over the years. The results were sometimes

surprising and I've changed my views a bit; some of my advice may now run contrary to conventional

wisdom.

Look at the Big Picture

Consider the function of the capacitor in the circuit. You need to know what's expected of the

capacitor to interpret your measurements and decide if the cap is sufficiently healthy or needs to be

replaced. Filter capacitors in mains operated power supplies, usually 50 or 60 Hz, will tend towards

large values, usually 1000 uF or more per ampere of output current. With a full wave bridge the

ripple seen by the capacitor will be twice the mains frequency, 100 or 120 Hz, so the capacitor's high

frequency losses aren't important. The cap does need to handle the ripple current; if the losses are

too high internal heating may occur, ageing the capacitor yet more rapidly, leading to premature

failure. Note that capacitors in consumer equipment, unlike industrial equipment, are usually chosen

to keep ripple to a minimum and do not have to support high current loads or carry high ripple

currents. In audio equipment heavy demands on the power supply are usually intermittent. The

worst threat is apt to be poor ventilation; watch out for ventilation slots blocked by dirt or

surrounding clutter. Another cause of early failure is proximity to a hot power resistor or thermal

connection to a hot power resistor via a heavy PCB trace, a subtle design error that happens more

often than one might expect.


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Note that the amount of ripple will be determined by the series capacitance (Cs), which will be

defined shortly. The losses will have no effect unless they're catastrophically high, nor will any other

capacitor parameter. If you want lower ripple from a conventional low frequency power supply, you

must increase the capacitance value. A cheap capacitor will perform exactly the same as an

expensive one, though the expensive one may last longer due to better seals and higher quality

construction.

Filters for switching supplies have more of an issue with ripple current and are specified mainly for

low ESR (Rs) to keep the internal power dissipation low. Internal power dissipation equals heat, and

heat is the enemy of capacitors. In switching supplies the capacitance value is often large and

somewhat irrelevant because the acceptable Rs and ripple current rating dictated the component

choice, not the capacitance value. When you replace a capacitor in a switching supply it's critical that

you know the original ESR specifications and insure that the replacement part is as good or better at

the frequency of operation. An ordinary low frequency filter capacitor installed in a switching supply

can immediately fail, sometimes violently if it overheats and the can vents or explodes. Always wear

safety glasses and don't lean over circuits under test!

Coupling capacitors have to pass audio frequencies up to 20 kHz or sometimes more depending on

application. They tend to be used in higher impedance circuits so losses aren't generally an issue.

What can be an issue is DC leakage, since the whole purpose of a coupling cap is DC isolation. It's

usually be necessary to measure leakage at the operating voltage; an ohmmeter check can prove the

cap bad, but it can't prove that the cap is good because it doesn't measure at a high enough voltage.

The non-polar electrolytics used in loudspeaker crossovers are a special case. Since they operate in a

low impedance circuit as a filter element, losses are important. If the designer voiced the speaker

with a specific capacitor, changing it to another type may very well alter the sound.

Bypass capacitors have to handle high frequencies so aluminum electrolytics are not the preferred

type. You may find high performance solid electrolyte (OSCON) or tantalum capacitors, but ceramic

and sometimes plastic film are the usual choices. These are all less subject to ageing and failure, but

they should be checked anyway as part of a complete service.

Some Basic Capacitor Relationships

Apologies in advance for subjecting you to some theory and math, but understanding these

relationships will put you way ahead of those that don't.

There are two types of passive "component" that you can use to build a circuit, resistance and

reactance. The reactance can be either capacitive or inductive. An interesting thing about reactance

is that it can't dissipate power. Thus, pure capacitors and pure inductors by definition have no losses.

Unfortunately they don't exist except on the pages of textbooks. The only thing that can dissipate

power is a resistance, and every real capacitor and inductor will have some small resistive

component. At least we hope it's small. Here we arrive at the fundamental concept behind this entire

article: The ratio of resistance to reactance is a strong indicator as to the condition of an aluminum

electrolytic capacitor.

Most of the time we ignore the imperfections of real-world capacitors and treat them as purereactances. Not so when testing them, since it's the imperfections that make the difference between


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a good and bad capacitor. Those imperfections show up as resistive losses, leading to two different

ways to describe them. One way, called the series model, places a resistance in series with the

capacitor. The other way is the parallel model, placing the resistance in parallel with the capacitor.

Both models are used for AC analysis, so try to ignore the fact that DC can pass through the parallel

model. These models are just a handy tool; they do not reflect the actual "mechanics" inside of a real

capacitor. In particular, the models are valid for only one frequency; change the frequency and you

need to adjust the model. More complex models are used if dielectric absorption and/or self

resonance is taken into account.

Now let's consider capacitance value. Aluminum electrolytics typically have wide tolerances, +80%

and -20% is a common spec. The better caps might be as tight as 20%. That's still a wide range and it

means you may not learn much from a simple capacitance reading because you have no idea if the

capacitor is as good as the day it was made, or if it has lost a large amount of capacitance yet remains

just inside the specification, only to fail completely next week. It may also have high losses that aren't

evident in the simple capacitance measurement. We need to measure the resistive losses to get a

better idea of the capacitors health.

If you read the 2nd paragraph of this section carefully you noticed that we're really interested in the

ratio between the resistance and reactance, not so much the resistance itself. That number is the

dissipation factor.

ESR meters have become quite popular because they offer a quick and easy high frequency in-circuit

test. Hand held capacitance-only meters and DVMs with a capacitance function have also become

popular for the obvious reasons of low cost and convenience. The problem is both pieces of test gear

only give you half the information you need. A proper capacitance bridge or meter will give you both

the capacitance and the loss. Modern meters, unlike traditional bridges, can often express thecapacitance and losses in a variety of units, since it's just a processor calculation, but the most

common (and useful) are series capacitance & dissipation factor or parallel capacitance & dissipation

factor. In general you'll use the series model for low loss capacitors.

From those two numbers you can derive the series or parallel resistances and a variety of other

things. The beauty of those two numbers is that you rarely have to. With a bit of experience, knowing

Cs & D will tell you instantly if a problem exists or not. Still, here are some formulas for converting

between the two models and for deriving ESR. Notice that dissipation factor never changes between

the two models. In the formulas below, C will be in Farads, R, X and Z in ohms, D, the dissipation

factor, is dimensionless and omega equals 2*PI*F.

Capacitor Catalogs & Data Sheets

The manufacturers of aluminum electrolytics offer a myriad of different types, most identified by a 2

or 3 letter code. This is usually printed on the side of the capacitor body, along with the logo of the

manufacturer. As an example, I've pulled the capacitor below from my "stock" to identify and look

up.

You can see the small rectangle, but it isn't really just a rectangle. It's the stylized shield used by

United Chemi-Con, though admittedly you'd only know that if you were familiar with the various

capacitor company logos. You can also see the cap is clearly printed with "SXE", the series


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identification. The value and voltage are obvious, 330 uF at 35 VDC, and on the rear of the cap is the

maximum temperature spec of (M)105C. We also take note of the case size, 10 x 20 mm, since many

caps come in a variety of different sizes or aspect ratios, all with the same value, but each size with

different specifications.

Armed with that information we can locate the series in the United Chemi-Con catalog and see whatelse can be learned. We discover that this is a miniature solvent proof low impedance capacitor

suited to high frequency switching supply use. Naturally it can be used for any low frequency

application as well. Sorting through the various tables we also discover the following:

Voltage: 35 VDC (we knew that) with 44 volt surge capability (surprise!)

Temperature range: -55 to 105C

Tolerance: 20% (that's the "M" on the back of the cap that prefixes the temperature rating)

Leakage current: I=0.01CV after 2 minutes (20C) where I is uA, C is uF and V is rated volt. (115.5 uA)

Dissipation factor: 0.12 at 120 Hz and 20C

Maximum impedance: 0.13 at 100 kHz and 20C

Maximum cold impedance: 0.34 at 100 kHz and -10C

Maximum ripple current: 860 mA RMS at 105C, 100 kHz

Load life: 2000 hours, rated voltage at 105C with up to 200% of specified dissipation factor

There is more info available for the circuit designer but what we have is more than sufficient for our

purposes. We should also take note of some general trends in the data. The dissipation factor chart is

by voltage rating. The higher the voltage rating, the lower the dissipation factor. That explains the

generally poor performance of very low voltage capacitors. There is also an adder that states, "When

nominal capacitance exceeds 1000 uF, add 0.02 to the values above for each 1000 uF increase." Thus,

as capacitance goes up, so does dissipation factor. These trends are typical of all aluminum

electrolytics. The company seems to define the end-of-life as the point where the dissipation factor is

double the specification, so consider that in the testing of older equipment.

Note how the losses go up with decreasing temperature. If equipment has to run in the cold, make

sure the performance of the caps is up to the task. Older caps may work fine warm but because

losses have increased over the years, the device may fail when cold. This is another reason not to

power up equipment right off the truck in the winter. The other is condensation. Let things warm up

to room temperature before unwrapping or powering!

The load life seems very short. Operating full time, 2000 hours is only 83 days! This should be a hint

that capacitors should not be operated under conditions that cause high internal temperatures.

Operated at normal ambient temperatures, with low ripple current to prevent heating, this same

part can be expected to last decades with little degradation.

Measurement Caveats


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We want to measure capacitors in-circuit whenever possible. Even though this may affect the results

slightly, we're not generally looking for extreme accuracy, in fact there's nothing extremely accurate

about aluminum electrolytics to begin with. The big problem is any circuit component that would

shunt the capacitor and make it look worse than it really is. We can avoid errors from

semiconductors by simply keeping the test voltage lower than the diode turn-on voltage. For silicon

parts this is under about 0.7 volts peak, but to be safe let's say 0.5 or 1 volt peak to peak. If you're

working on very old equipment with germanium devices your life will be harder because low turn-on

voltages and typical leakage will make all in-circuit measurements untrustworthy. You may have to

remove caps or other components to get a valid measurement.

What about power supply caps? The problem with power supply caps is that the whole rest of the

circuit is usually connected across them. There's bound to be a resistive load of some sort.

Fortunately significant losses are usually tolerable. If a low frequency measurement show the

capacitance to be about right, and the dissipation factor (DF) less than 1 at 120 Hz, the problems are

likely elsewhere.

The Good, the Bad & the Ugly; Let's Make Some Measurements!

We'll start by measuring a perfectly good Panasonic FC series capacitor on the venerable General

Radio Corp. 1657 digital LCR bridge, the first modern digital bridge. Most of the capacitors used here

will be 47 uF so we can compare the information obtained using different measurement parameters.

The first measurement will be at 120 Hz using the series model (Cs) because the datasheet specifies

the capacitance tolerance at 120 Hz. Note that the test parameters are indicated by the LEDs under

the digits.

We see a capacitance of 43.8 uF and a dissipation factor (D) of 0.0671. The capacitance is a little lowbut it's only -6.8%, well within the published spec of 20%. The dissipation factor is low, which is

always desirable, but since these caps are touted for their high frequency performance we need to

look at that as well. The datasheet only gives us the total impedance at 100 kHz, ignoring low

frequency performance all together.

Most bridges and meters won't go that high, though some ESR meters will. Since we can do a 1 kHz

measurement on this bridge, let's see what that looks like.

If we calculate Rs, which is equal to ESR, from the above numbers, we get 0.872 ohms. Now, this

number is not constant with frequency, but the datasheet gives a value of 0.8 ohms at 100 kHz, so

we know we're well in the ballpark. I typically move through the capacitors on a board, making sure

the capacitance is approximately the marked value, but paying particular attention to the dissipation

factor at 1 kHz. Any DF greater than about 0.4 deserves closer examination. If the cap is used as a low

frequency filter I expect a low frequency (120 Hz) DF measurement to be less than about 0.25. Don't

get too hung up on the losses. Most circuits will work just fine with higher losses.

Here's a graph of that same capacitors actual measured performance from 20 to 20,000 Hz. Both

dissipation factor and ESR are shown. The scale on the left is both ohms for ESR and dimensionless

units for dissipation factor. Notice that when one gets to about 1 kHz, the ESR curve has flattened

out and will then slowly decrease as the frequency rises. At some frequency inductance will become

an issue and the total impedance of the capacitor will rise. The ESR will generally remain low, but the


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capacitor will become less effective because the inductive reactance is cancelling out the capacitive

reactance. At resonance XL = XC, so they subtract to zero, leaving only the ESR. The phase shift will

be zero degrees and you have a resistor! (the graph should have 4 decades but the numbers are

correct)

Now we'll move on to a more questionable part. This is a common 47 uF cap that you'd find in allsorts of consumer goods. It's only rated at 10 VDC and my experience is that caps rated for less than

16 VDC show poor performance and have short lifetimes. Here's the 120 Hz Cs test.

On the surface those numbers don't look too bad. If this cap was a low frequency filter cap it would

certainly do fine. Unfortunately these little caps are rarely used in power supplies, but it would be

common to find them used as coupling capacitors. Let's do a measurement at 1 kHz.

Now things aren't looking so good. That 0.7 dissipation factor is very high. If we convert it to series

resistance we get 2.85 ohms. The parallel model is 26.87 uf in parallel with 7.82 ohms, a dismal

showing compared to a good cap, and likely to impact circuit performance in some applications. Agood capacitor will have a phase shift between current and voltage that approaches 90 degrees, at

least at low frequencies. This one is about 52 degrees. As the frequency goes up this cap looks more

and more like a resistor. That's not always a bad thing, but it shouldn't be happening at such a low

frequency. Now, that's just my opinion on the matter; I don't consider this a high quality capacitor.

Still, if the cap is used as a coupling cap, and if the value is good, and if the leakage is low, it will work

fine and isn't the cause of a problem. If I found this capacitor in a piece of equipment that I was

servicing, would I replace it? In a heartbeat! Modern parts are just way better than that.

Knowing only the series capacitance value, the thing that most inexpensive meters measure, leaves

you lost in the dark. That value of 42.28 uF looked perfectly fine, well within specification, yet thecapacitor was of poor quality due to high losses. Knowing only the losses, you can spot some bad

capacitors, but not all. An ESR meter is fast, but you should understand why it tells you what it does.

In the case of paralleled capacitors one could be missing entirely yet the ESR meter would report a

good number. It can also report high ESR for a capacitor that's perfectly acceptable for the frequency

it's operating at. In my opinion the ESR meter is still much more valuable than a C-only meter, but

you really need both numbers to fully understand and properly troubleshoot capacitor problems.

This is Confusing! How Do We Draw a Line in the Sand?

The $64,000 question is what value to use as a cutoff. If you have a datasheet for the part, it should

give some limits. If you can get a datasheet for a similar class of part it should serve as a useful

estimate. Hopefully it will specify a maximum dissipation factor, usually at 120 Hz. Here's the chart

for a general purpose Rubycon YK series general purpose radial that's typical of most general

purpose caps:

There is a note at the bottom of the table: "When nominal capacitance is over 1000 uF, tan shall be

added 0.02 to the listed value with increase of every 1000 uF."

So let's say you have a 4700 uF 50 volt cap. The base dissipation factor is 0.12 and because it's largerthan 1000 uF there's an adder of 0.08, giving you 0.20 (I rounded the value to 5000 uF). Now, the


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end-of-life dissipation factor is 2X, so the cap can be considered bad if the dissipation factor

measures over 0.40 @ 120 Hz.

Big Power Supply Caps

This is getting a bit long but I'd be remiss not to show a large value power supply cap. Here's aSprague "Powerlytic" 47,000 uF 50 VDC cap. Because the value is 47,000 uF, many traditional bridges

won't read it at all. Meters like the Digibridge will do it at lower frequencies like 120 Hz, but the

impedance is so low that they can't manage it at 1 kHz.

The dissipation factor of these big power supply filters can vary over a wide range, often much higher

than the smaller caps. Measured at 120 Hz you can use the same guide as above, but multiplied by

3X. There will not be an adder. You'll need good low resistance leads and possibly a 4-terminal

connection to get accurate measurements on the better quality filter caps. Even the arrangement

shown, with short heavy leads to a 4-terminal connection, is probably not adequate. Large caps need

a formal 4-terminal connection right to the lugs.

DC Leakage Current

DC leakage is a separate phenomena and if it's a concern you need to measure it separately. The

leakage resistance of the cap often doesn't offer enough loss to change the C & D readings, but offers

plenty of current to disturb circuit operation. Most caps that measure OK for value and loss will have

acceptable leakage, but some applications are more sensitive. A cap isolating the grid of a tube is a

good example. An old Black Beauty paper cap may measure perfectly in every respect, but have so

much DC leakage that it shifts the bias of the tube, resulting in a seriously distorted waveform.

Fortunately good designers don't use aluminum electrolytics in sensitive locations. You'll almost

invariably have to remove capacitors from the circuit to test for leakage.

To measure DC leakage you'll need a power supply that can reach the maximum voltage rating of the

capacitor. Connect the capacitor to the supply through a current limiting/sensing resistor and

measure the voltage across the resistor. Calculate the current, and the resistance of the capacitor (if

desired) using ohms law. Be sure to take all necessary safety precautions with both high and low

voltage caps since they can store substantial energy. The supply should be current limited in case the

cap shorts. I use a disposable 1/4 W sense resistor and DVM as described below, rather than a

current meter, in case of failure.

As an example we'll use the United Chemi-con cap above. Since the spec is 115 uA, it would beconvenient to choose the resistor such that 100 uA gives a 1 VDC voltage drop. 10 kohms (1 / 100E-6)

fills the bill. Since the typical DVM has a 10 megohm input impedance, we don't need to correct for

that. The cap and resistor are connected in series and 35 VDC applied. The voltage across the resistor

starts out at 35 VDC and falls as the capacitor charges. The official measurement doesn't start until

the cap is fully charged, but even after 19 seconds the voltage across the resistor has fallen to 1 VDC,

so the capacitor is well within the leakage tolerance. After a few minutes it fell to 10 mV, or 1 uA, and

was still dropping.

Leakage limits will commonly be specified by a factor of C*V. A common spec would be 0.03CV or 4

uA, whichever is greater. Since you're typically using uF and looking for uA, no conversions arenecessary. Just multiply the capacitance in uF times the rated voltage times the multiplier.


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Specifications don't typically allow increased leakage over the life of the cap, unlike dissipation

factor, which is allowed to double.

Forming & Reforming Aluminum Electrolytic Capacitors

When capacitors are made the manufacturer places a voltage across the terminals to form the oxidefilm on the plates, always a higher voltage than the cap is rated for. The oxide film is semi-

permanent, but if the cap sits unused for a long period of time, the oxide film can degrade. This

makes the capacitor vulnerable to shorting out the first time power is applied. Thus the advice to

slowly power up old equipment using a Variac. This builds up the oxide film until it can support the

full operating voltage. When a new or long-unused cap is installed in a circuit and first powered up, it

will have a significant leakage current. This current falls off for quite a long period until it reaches

near zero. The process can actually take days to weeks before the minimum current is observed.

Remember that significant leakage current equals heat being generated inside the capacitor. When

powering up old equipment, don't assume that all is well just because the caps support the workingvoltage briefly. Failure can occur as the cap heats up because the leakage current is still too high.

When bringing old equipment back to life, raise the voltage slowly and in several stages. Power down

frequently and allow the caps to rest and cool off internally. Then, after half an hour or more, power

up again to a slightly higher voltage. After the caps have accumulated some total power on time,

they'll have a better chance of survival. That said, if they still fail standard tests, replacement is the

only remedy.

Shaken Confidence

So many times I've measured a capacitor and immediately questioned its health because the value

was slightly low. Not out of spec, but just 5-10% low. Surely the manufacturer aims for the value

printed on the cap or do they? Though I have no proof, I'll suggest that they don't. With automated

equipment the manufacturer can probably hold the tolerances far closer than needed, and might

well aim for a value that's below nominal, yet always above the minimum. Why? Because saving a

few percent on the expensive etched aluminum foil, plus the separator paper, will save big money

over a long production run. It takes less surface area to produce a lower value cap and I'd be amazed

if some manufacturers weren't taking advantage of this on the highest volume parts.

You will sometimes see see capacitors that measure substantially higher than nominal. The tolerance

on many caps was as high as +80%, but it would be rare for them to be that high when new. What

happened was that chemical changes over time caused the value to increase. Unfortunately this is a

sign that the caps are near the end and need to be replaced. It's interesting to note that, for the

moment, those caps are probably doing a better job of filtering at 120 Hz than the new replacements

will do. Still, they're toast, so get 'em out of there. I tend to see this increase in value with caps that

are 30+ years old.

My Capacitor Leaked Brown Goo On My Circuit Board!

This complaint shows up frequently on Internet circuit forums and has probably caused the

unnecessary replacement of untold numbers of capacitors. The brown goo is just adhesive that anysensible manufacturer squirts on the board to hold the larger capacitors in place. If they didn't use it,


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vibration in shipping could easily cause the leads to fail or pull out, resulting in a DOA unit. A tall

capacitor with a small base creates a good lever arm on the leads and extra support is always a good

idea.

Since doubt always exists about the brown deposit, let me point out that aluminum electrolytic

capacitors are not filled with large quantities of fluid of any type. The internal paper will be moist,with possibly a few drops of condensation on the inside of the case, but there is never enough

electrolyte to come spilling out of the case to form a big puddle on the circuit board. That said, a

major failure of a big high voltage cap that causes it to explosively vent can put a thin film of

electrolyte on just about everything in the chassis.

Just What Is This Electrolyte Stuff

The manufacturers probably aren't going to tell you the details, but the traditional electrolyte used in

85C caps was a glycol/borate system, specifically a mixture of ethylene glycol (yes, antifreeze) and

ammonium pentaborate. Or, they used boric acid and bubbled ammonia through the mix. Theperformance of this mix leaves much to be desired at low temperatures, nor does it give low esr.

Adding more water will lower the esr, but reduce reliability. Makes you wonder about cheap low esr

capse used in computer power supplies that seem to fail so often. Higher performance caps use

more advanced electrolytes and additives to achieve wider temperature range operation and low esr

without a reliability penalty. All electrolytes are toxic so avoid contact with electrolyte deposits from

vented caps and wash thoroughly with soap and water if contact is suspected.

What Factors Affect Electrolytic Capacitor Life?

Temperature

Operating voltage

Seal integrity

Capacitor formulation

Contamination

Manufacturing defects

All electrolytic caps will eventually fail due to internal reactions that destroy the dielectric. Theprogress of these reactions is determined by the factors listed above and can be remarkably slow or

disturbingly fast. Starting from the top, a general rule is that capacitor life will be reduced by 50% for

every 10C increase in operating temperature. 105C caps should last longer in most cases because the

safety margin is higher. Heat may be from outside sources or generated internally due to ripple

current. Usually both!

Older literature referred to a power law that said cap failure rate was inversely proportional to the

operating voltage raised to some power, N. The problem is that N varies over a huge range, maybe 2

to 10, depending on the capacitor "recipe". The information is still useful because it tells us that

operating near the voltage rating of the cap is worse than allowing some safety margin. An operatingvoltage of about 60% of rated voltage is a good place to start, if size and other factors allow. Also,


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avoid caps with ratings under 16 VDC as they have a higher failure rate. There is no downside to

running modern caps well below their maximum voltage rating.

There is a certain amount of paranoia concerning capacitor seals but they're usually a minor issue.

They are not tires and they are not generally exposed to mechanical stress, ozone and ultraviolet

light. Seal materials in any quality cap are chosen for extremely long life and compatibility with theelectrolyte. That said, if you lose the seal, you lose the capacitor, so buy quality.

There are many capacitor "recipes" and they break down at different rates. The only advice I can

offer here is to buy premium long life parts. Manufacturers catalogs all list products with lifetimes of

2-3X over their standard parts. You may pay a bit more but it's money well spent.

Contamination is mostly a manufacturing issue. An aluminum electrolytic capacitor with the smallest

amount of chloride (and certain other contaminants) will rapidly degrade and can fail within weeks of

manufacture. One fingerprint on the internal materials is all it takes. Buy from well known and

established suppliers. There used to be an issue using chlorinated solvents to clean circuit boards. Ifthe solvent managed to get past the seals, the cap life would be degraded. Most caps are now

solvent resistant, but check the datasheet. Try to keep cleaning solvents away from electrolytic caps,

especially the seal end.

Electrolytic caps, like most electronic components, suffer from a certain amount of infant mortality.

They display the usual "bathtub" curve where there's an initial failure rate followed by a long trouble-

free service life, after which the failure rate rises due to wear-out mechanisms. Those initial failures

at the beginning of service are the result of defects in the foils, paper or other details, so don't

assume that changing out capacitors that have a proven history of reliability, with brand new

unproven parts, will somehow guarantee zero failures. It won't. You can, however, improve yourodds by buying "hi-rel" parts that should have a lower initial failure rate. Realistically, hobbyists and

small shops have statistics on their side because the number of caps used is quite small. Most of us

will never get a defective cap from new production.

Please remember that everything stated above consists of generalities distilled from manufacturers

literature. It isn't close to absolute and your (and my) experience with a small sample of parts may

not follow the "rules".

Myths About Replacing Old Capacitors

Capacitors degrade as they age, both on the shelf and inside operating equipment. The capacitortested above was a NOS part only a few years old. The entire bag has high loss, though I have no idea

if the numbers are normal for this part. Many failures on older equipment are due to failing

capacitors. Beyond a certain age, it seems to make sense to do wholesale capacitor replacements

when equipment is in for service. Hold off, it could be a Bad Idea!

Like a doctor, the service person should "do no harm". Doing needless component replacement often

tears up circuit board pads and traces. It also contaminates the board unless you're careful to clean

it. It may make a classic piece of equipment even more non- original. Worst of all, the original

capacitors may still be of better quality than the ones you're installing. As counter-intuitive as this is,

there were many series of Sprague and other makers capacitors that were incredibly good 30 yearsago, and remain so to this day. As an example, here's a Sprague 30D cap that's well over 30 years old.


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It has lower losses than the fresh and well respected Panasonic FC up above. It's quite large and

would handle far more ripple current. It will probably outlast and outperform several replacement

caps unless you can find something of equal quality. Only a fool would replace it with a new cap.

Many of the old caps with epoxy end seals are even better. I have test equipment that's going on 50

years old and the caps show no sign of decreased performance. Now, you'll certainly find bad

capacitors and should replace them. You'll even find bad Sprague 30Ds, but replace parts because

they're bad, or because they have some physical problem, or because they have a history of failure,

not just because they're old.

One place where I do recommend wholesale replacement is where an instrument contains a large

number of similar caps, and more than a few have failed or show high dissipation. This seems to be

common in '70s audio receivers and some video equipment. In those cases you can easily predict the

future, and the future is bad; go ahead and stave off trouble by getting them all out of there.

You have to work to your own comfort level. No one can say with absolute certainty if a given

capacitor is going to fail in an hour or in a year, though it would be very rare for a cap that measuredclose to its nominal value, had low losses and low DC leakage to suddenly fail, regardless of age. Also

note that brand new electrolytic capacitors have a non-zero infant mortality rate due to fabrication

and contamination issues. If your experience includes a lot of hot high voltage tube equipment you'll

likely be more conservative than I am. If the consequences of a failure are particularly serious, you'll

also be more conservative. Service is a balancing act; do what's appropriate for the situation.

The Bottom Line

Test caps in the same frequency range they have to perform in.

Consider whether losses are important for the circuit in question.

You should have a schematic or at least know what part of the circuit the caps are in.

Reject caps with excessive losses for the application.

Reject caps with excessive DC leakage for the application.

Reject caps with low capacitance.

Reject caps with unusually high capacitance.

Reject caps with visible leakage, corrosion of the leads, deep dents or bulging.

Reject caps whose similar neighbors have failed.

Keep caps, regardless of age, that don't exhibit the above criteria.

Long-term stability of aluminum electrolytic capacitors

July 2007

Built to last


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Thanks to the manufacturing process, aluminum electrolytic capacitors for automotive electronics

from EPCOS offer long-term stability for both storage and operation. This is of particular benefit to

automotive customers with their need for high-quality components.

These capacitors are used in a variety of automotive applications. These include engine management

systems for fuel injection as well as control systems for fan and windshield-wiper motors, electronicsteering systems, airbags and multimedia equipment.

Storage affects leakage current behavior

A key parameter of aluminum electrolytic capacitors is the behavior of their leakage current when

they are operated immediately after storage. The leakage current is the current flowing through the

capacitor on DC voltage: it remains relatively high shortly after a DC voltage is first applied to it and

then drops after several days to a low current, which is known as operating leakage current. As a

rule, leakage current behavior is determined by the leakage current that continues to flow through

the capacitor after the DC voltage has been applied for five minutes.

FIGURE 1: LEAKAGE CURRENT OF ALUMINUM ELECTROLYTIC CAPACITORS

The forming requirement of an aluminum electrolytic capacitor from themanufacture of the anode foil up to final operation. The reforming requirement is

reflected by the leakage current value, marked here in red. When no voltage is

applied (green range), the mean conductivity of the oxide layer increases again.

When a voltage is subsequently applied, the increased conductivity occurs in the

form of an increased leakage current that rises in line with the duration of

preceding storage period.

The dielectric of an aluminum electrolytic capacitor consists of the aluminum oxide formed

electrochemically on an etched aluminum foil. The quality of the oxide, which changes during themanufacture and subsequent use of the capacitor, determines the insulating properties of the
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dielectric. The DC conductivity of the oxide increases as a result of processing the oxide-coated

anode foil during capacitor assembly and the conductivity is again lowered in the final forming

process. When it is subsequently stored at zero voltage, the insulation properties of the dielectric are

impaired. In order to minimize this perturbing conductivity, it is eventually necessary to repeat the

forming process under voltage in order to repair and build up the oxide layer again. If the oxide has

degenerated more pronouncedly, a high-forming current flows during the reforming process, thus

increasing the forming requirement (Fig. 1).

Storage of aluminum electrolytic capacitors

Two different phenomena can have a negative impact on the internal insulation of an aluminum

electrolytic capacitor during storage: oxide degeneration and post-impregnation effects. When a

voltage is subsequently reapplied, the regeneration leakage current may initially rise again.

a) Oxide degeneration

Depending on the electrolyte class and temperature, ionic parts of the electrolyte can diffuse into the

dielectric or oxide and alter the oxide crystal structure. Electrical defects and ionic charge carriers are

then produced in the oxide.

Although glycol-based electrolytes have the drawback of producing higher leakage currents, they

offer the advantage of repairing defects in the oxide very effectively when current is flowing. This

makes them especially well suited for high-voltage aluminum electrolytic capacitors.

In the low-voltage range, in which oxides are more homogeneous, electrolytes based on gamma

butyrolactone solvents are sufficient to produce a reliable and voltage-resistant dielectric. In this

case, it is advantageous when these electrolytes are almost completely unable to penetrate the oxideor break bonds, thus ensuring a well-insulating oxide even after potential-free storage lasting

decades. If these electrolytes nevertheless sporadically and temporarily lead to high leakage currents

after such storage, this is due to post-impregnation effects.

b) Post-impregnation effects

The oxide can be electrochemically formed in the component only where it is also coated with

electrolyte and is connected electrically to the cathode foil via the electrolyte, thus allowing the

required forming current to flow in these regions. In a new capacitor, this is the case on more than

99.9 percent of the oxide area to be formed.

In the radial capacitor shown in Fig. 2, the positive feedthrough also known as a paddle tab, was

formed only in the winding element area. No oxide was able to form in the vicinity of the rubber plug

where the electrolyte has not penetrated. This is not a disadvantage for the insulation because no

leakage current can flow in the absence of electrolyte. However, if some electrolyte subsequently

penetrates this region, a supplemental forming process must be conducted to create an isolating

oxide the next time a voltage is applied (Fig. 3). This means that increased leakage current flows until

the anodic aluminum surface newly wetted with electrolyte has been formed.

FIGURE 2: PARTIAL FORMING IN THE TERMINAL AREA


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Example of an anode

surface in thefeedthrough area of a

radial aluminum

electrolytic capacitor

not coated with

electrolyte.

FIGURE 3: REFORMING IN THE TERMINAL AREA

Example of an anode

surface in a radial

aluminum electrolytic

capacitor with

supplemental

impregnation during

storage.

In low-voltage aluminum electrolytic capacitors with solvent electrolytes, all areas can be expected

to be wetted and as a result show a very low leakage current in the long term, i.e., after the storage

and transport periods and before their first operation in the application.

Supplemental reforming effects are caused by subsequent wetting and in principle also apply to high-

voltage electrolytes, although they are of minor importance due to the dominant effect of oxide

degeneration in high-voltage capacitors. However, test operation under voltage is also of benefit for

the long-term leakage current behavior of high voltage capacitors in this electrolyte class, because

each forming step makes the isolating properties of the oxide more stable.

In order to also keep the forming condition stable during storage of the equipment, larger

temperature fluctuations and prolonged shocks should be avoided. Aluminum electrolytic capacitors

with solvent electrolytes of the SIKOREL class can normally be stored over a period of more than 15

years without exceeding the leakage current limit specified for the new component.

In capacitors with polar electrolytes used in high-voltage capacitors made by EPCOS, the chemical

interaction between electrolyte and oxide dominates the blocking behavior of the dielectric. The

storage temperature for these capacitors should be as low as possible, and certainly below 25C. This

enables reaching storage periods longer than the specified two years. However, even after the

permissible storage time has been exceeded, no damage to the capacitor is to be expected; there is

merely an increase in the leakage current lasting several minutes.
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Irrespective of the electrolyte used, the operating leakage current at equilibrium is very low. The

leakage current adapts itself to the equilibrium state (voltage, temperature distribution, insertion

geometry, shocks). If the equilibrium changes after long constant operation due to a higher voltage

or temperature, charge carriers in the dielectric are reactivated so that a higher leakage current

flows again. Under certain circumstances, equilibrium changes that make the electrolyte flow may

also wet anode areas that were not effectively wetted under the old equilibrium. These anode areas

may also include tiny regions with sporadic weak points that were incompletely formed. Apart from

the classical production of the dielectric by formation of a non-conductive oxide, foreign inclusions

may produce a defect site that continuously generates a local leakage current. These defects (Fig. 4)

can apparently be corrected in equilibrium by the formation of a gas bubble. The gas generated by

the local leakage current expels the electrolyte at the defect site so that the local current flow also

stops.

FIGURE 4: DEFECT SITES LEAD TO GASING

Every change in equilibrium that

affects the gas and electrolyte quantity

and its gaseous solubility can cause the

leakage current to rise. This also

explains the paradoxical observation

that even when a capacitor cools, the

leakage current can initially rise

contrary to expectations.

The leakage current certainly also causes the capacitor to age by consuming some of the constituents

of the electrolyte for the oxide forming or regeneration. As a rule, however, this mechanism does not

determine the rate by which the capacitor ages. It should be noted that high perturbing leakage

currents occur only a short time after the voltage has been applied, i.e., they can essentially be

neglected during the entire operating period. In practice, a high leakage current leads to premature

capacitor death only when the current cannot drop fully due to an excessive voltage or incorrect

polarity, which is increased by high temperatures. The rapid gas generation then leads to bursting at

the predetermined breaking point.

In applications supplied by a standard or rechargeable battery, as is typically the case in motorvehicles, there is always the concern that the high leakage current of a component could discharge

the battery. This hazard is negligible with state-of-the-art electrolytic capacitors from EPCOS. This

also applies for self-extinguishing high-voltage electrolytes. Undamaged aluminum electrolytic

capacitors can produce high leakage currents only briefly, but never over long periods of time. Under

suitable conditions of use, they may be operated over decades.

BACKGROUND: PROCESSING OF ALUMINUM ELECTROLYTIC CAPACITORS

Long storage periods for aluminum electrolytic capacitors
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For the series of aluminum electrolytic capacitors listed below, which are suitable

for automotive applications, EPCOS specifies in regard to the leakage current a

storage period of up to 15 years at a temperature below 40C. After the storage

period, the expected leakage current is still in the order of magnitude of the

original limit.

Radial series:

B41853, B41858, B41888, B41866, B41896, B41868

Axial/solder-star series:

B41684/B41784, B41691/B41791, B41692/B41792, B41693/B41793,

B41694/B41794, B41695/B41795, B41696/B41796

Snap-in and large-size series:

B41505, B41605, B41607

Screw-terminal series

B41550, B41554, B41570

If the capacitors are stored for a prolonged period, the properties of their leakage

current may change and lead to errors at the final inspection. Users are

recommended to insert the capacitors at an early stage and then store the

completed equipment. The capacitors will then not only be optimally formed, butthe soldering on the circuit board will be performed while the solder points of the

component are still new. The completed equipment should be stored at a low

temperature and be exposed to minimum temperature fluctuations.

How to Check Aluminum Electrolytic Capacitors

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