What Is a Bad Capacitor?





Introduction: What Is a Bad Capacitor?

About: I was pfred1 but moved, changed my email address, and lost my password. I suppose worse things could happen.

A troubling trend in our technological society today is when good capacitors (caps) go bad. Perhaps this is a phenomena that you have heard of somewhere in passing, but do not know as much about as you'd like to?

In which case you have come to the right place! Because in this article I will endeavour to discuss when good caps go bad, and how you might spot the bad caps for yourself too.

Step 1: What Are Bad Caps, and Why Should I Care?

A bad capacitor is an electronics component that over the course of its life has turned to the dark side. It is evil now and is no longer serving its intended purpose in life. It is a hazard to all other electronic components that are relying on it functioning properly now too. In short, it is broken. We will soon learn it is a short. Here is one that my tester thinks is a diode. Good caps will never read as diodes.

Step 2: Let's Try to Read That Cap Again, Shall We?

In this image I have flipped the cap around in the test clips and read it again. Now it is coming up as a capacitor, and we can see readings related to capacitance too. Let us take a moment to decrypt the readings the meter is displaying now.

Reading across we see; 1-||-3 Vloss=34%

On the next line is shown; 943μF ESR=2.2Ω

1-||-3 are the test points the meter has found a component on

Vloss is the amount of voltage the meter has measured that the capacitor has lost.

943μF is the capacitance the meter has measured of the capacitor. 943 micro Farads.

ESR=2.2Ω is the measured Equivalent Series Resistance (ESR) of the device under test.

Well, you may ask, Why is any of this so bad? To begin with it is a 2,200μF capacitor so 943μF is not even close to its marked capacity. But that is not the worst of it. A voltage loss of 34% is terrible! This part is leaking electricity like a sieve. Which is not what capacitors are supposed to do at all. Quite the opposite in fact. Capacitors are supposed to be able to store electricity. They are supposed to have the capacity to store an electrical charge. Hence their name. The ESR measurement is pretty bad too, but not nearly as out of spec as the Vloss.

There is a lot of talk about ESR, so if you are interested in that I suggest you browse the web for additional information on that topic.

Step 3: How Can I Spot a Bad Cap?

You may not have one of these fancy meters at your disposal. But the good news is bad caps often appear bad visually! So you can just see that they're bad. OK so what are the telltale signs of a bad cap? Well, bad caps typically have a domed, or swollen top. Sometimes really bad caps can leak their electrolyte out of themselves too. Then you may see this brown crust around the capacitor, or perhaps on it. It often looks somewhat like a dried coffee stain.

In this image I have tried to photograph the slight bulge on the top of this bad capacitor. It is easier to see in person than in my picture. A good cap will have a very flat top. Any bulge is a bad sign.

You can also use some deductive reasoning to find the more likely places in circuits where caps may go bad. Capacitors that are placed under heavy stress are more likely to go bad than caps worked less hard. So power supply filter caps go bad often. The power supply is often where the power comes into equipment. You may see buck boost coils there, a diode bridge, heavy switching MOSFETs, and things like that. All of those items are a bad influence on impressionable caps, and can make them go bad.

Step 4: Why Should I Care About Bad Caps?

A fair question. Realize that a few inexpensive capacitors can be the difference between your expensive electronics working, or not. I had several caps go bad in my flat screen TV, and when they did it would no longer come on! $4 of parts replaced brought a $1,400 TV back to life. I bought my TV when they were more expensive than they are today.

Yes, I was extremely ticked when my TV seemed to die at such an early age too. But when I resurrected it I was happy then!

So you too could suffer from early infant mortality of your expensive electronic devices, just like I did. Bad caps cause a lot of electronics to fail long before they should today. This is a huge issue. Any electronics that you have that plugs into a wall outlet has filter capacitors in it. When they go so does your whole piece of equipment then too.

If you can spot bad caps for yourself you can fix this equipment for yourself too. Saving you hundreds, if not thousands of dollars. Over what repair, or replacement costs may add up to.

Step 5: Enough About Bad Caps Are There Any Good Caps?

Yes there are! Not all caps are created equally. The Japanese make a superior capacitor than what is available from some other parts of the world. When you buy replacement caps it pays you to spend the few pennies extra and get these better capacitors for yourself too.

In my capacitor testing I have found that the absolute best capacitors I have run across are manufactured by Rubycon. Rubys are some fine capacitors. Although Nichicons are no slouches either. These are called Jap caps. No slur intended. Because they are the very best caps you will find today. They generally do not cost much more than the inferior ones do either.

But if you are a manufacturer buying tens of thousands lots of capacitors then pennies add up I suppose. Still what is going on is a grave injustice to all of humanity. Expensive equipment dying prematurely over such an insignificant cost difference is a travesty.

Step 6: For Folks Curious About My Meter

Someone is bound to ask, so I might as well say something about the meter I used in this post. I am using something called an "AVR TransistorTester by Karl-Heinz Kübbeler". There are a number of models available from the Far East. The first one I purchased was called an LCR-3 (now going under the name of Mega328 Transistor Tester). Sometimes it is referred to as the "graphical meter". Because it has a larger screen that it draws the parts out on graphically. Unfortunately the designers of it took some license with how they made the circuitry. This makes the meter less accurate as a result. (I have thought about trying to rectify that but I am not sure if I can due to a lack of firmware availability)

I learned all of this later as I researched the topic more myself. So then I bought a kit that I assembled my "AVR TransistorTester by Karl-Heinz Kübbeler" clone from myself. I might as well supply a link to that here now too, because people are likely going to ask for that as well.


I am in no way endorsing that company, I am merely providing information people may want. But honestly I think the whole deal is pretty sweet. I am extremely satisfied with my meter today too. Here is a link to the original open source project all of these meters were derived from


Which is in German, but you can translate it. Anyhow, the meters are pretty neat and anyone with even a passing interest in electronics would be wise to get one for themselves. They are an awesome deal for what they are. They will help you pinpoint bad caps even better too. Because not all bad caps look bad.



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    58 Discussions

    People should be aware that capacitors, especially the variety found in older appliances may have a brown or whitish gunge looking substance around their bases. This isn't always a sign of problems, it could be simply the goo manufacturers slap around caps to anchor them to the PCB. Also be very careful not to go prodding capacitors, especially the larger types. I've seen one decide to spurt a fountain of substance, something like spider web when ejected and if it lands in your eyes it could prove nasty. By the way, I have one of the BangGood test meters and it's excellent for all common components. If you buy one opt for the plastic case made for the unit, it's worth the extra few dollars.

    7 replies

    Yes the thicker goo is some kind of a sealer, or glue, that manufacturers use, for an unfathomable reason. Maybe the foreman's kid needed a job at the factory? I don't honestly know (perhaps to keep parts stable while the board is being wave soldered?). When a cap squirts we often see just the dried residue, and it looks a lot like someone spilled coffee on the circuit board, and it dried up. It always looks like a brown crust to me. The stain is quite thin. It can often be very subtle, the residue left over from a leaked capacitor. Like residue left from just a small drop of fluid.

    But folks looking closely for such things should be able to notice it if peering carefully enough. When troubleshooting any telltale sign is a valuable clue. Although every clue does need to be investigated thoroughly, to determine precisely what it means.

    When I strip boards for parts I hate that glue goo that is on them sometimes. I was just peeling some off a pair of capacitors yesterday here. It was some better translucent silicone based glop. The brown stuff is like cheap hot melt glue. Worst industrial quality.

    The fumes that come off the brown stuff when heated probably reduce lifespan. Some electronics are doped to smell terrible when they burn, Just so folks realize it is burning. Maybe they'll unplug it then too? One can always hope!

    I saw a youTube video that showed that the leaked dielectric is more visible in UV light (black light).

    it's a very professional touch and something I sure appreciate - when a manufacturer uses Silastic or other elastomeric compound to stabilize and dampen parts against the circuit board.

    A brown coffee-like stain could also be (burnt?) flux, used during the soldering process.

    Not on top of a capacitor can it can't be. Plus flux is shiny, and cap leak goo is not. When you see flux you should clean it off anyways. Flux left on circuit boards is just sloppy. But in consumer electronics that is common. Because cleaning rosin flux is pretty time consuming. Most mass produced electronics are assembled by machines and they use water soluble flux. But often some parts need to be hand soldered too. I haven't seen completely hand soldered circuit boards in decades. Like old electronics that came out of Japan in the 60s were hand soldered. No one is doing that kind of thing today though.

    Well, I'm sure someone is, but it is extremely rare.

    The glue is used for one of two reasons.

    1) It holds the cap in place during assembly, prior to soldering.

    2) It provides mechanical strength in case there is vibration or limited width or thickness PCB copper holding it down.

    The glue is also used to prevent the can from oscillation. Many times over the years I have had customer complaints about hums or squeals coming from consumer appliances and electronics, only to find that a cap is making the noise. Most manufactures are aware of it and so they put the glue in place to prevent the stray oscillations. In the mid-80's to about the mid 90's when the transition from transformer / diode pack with very large capacitors gave way to switching power supplies, it was common to carry a hot-melt glue gun in the tool box to "quite" noisy power supplies from the first-gen switching power supplies.

    My tried and tested method of putting capacitors on trial are as follows:

    Connect both leads to the L/N of a mains plug, turn it on and see what happens; if the capacitor explodes spectacularly, then it was a capacitor of pure heart, if it doesn't explode, then it is an evil capacitor.

    1 reply

    "If she drowns and dies she wasn't a witch. If she floats and lives she's she's a witch!"


    2 years ago

    Only buy Japanese capacitors! All Chinese capasitors are rubbish.

    Nice tut!
    Good to know about this things, but a question arises to me, how do I take out the cap, and how do I replace it back? Whats's your best method to do so?
    In advance thanks for your reply ;)

    1 reply

    Desoldering components is beyond the scope of this article. But generally heating one lead, and levering the part out works. Usually after going back, and forth a few times they come up. It helps to add solder to the joint too. The new lead free solder they use today is horrible. That is another conspiracy being perpetrated on the consuming public. Did you know the military, and aerospace are exempt? Strange how only we have to suffer, isn't it? I only use 60/40 solder myself. Don't bother with that lead free stuff, it is trash.

    To put a new cap in you need to clear the board holes the cap was sitting in. I use a solder puller for that. Electrolytic capacitors are usually polarized, so observe proper orientation when installing one!

    very lovely and useful clues to some not that excellent but have records in doing some good trouble shooting. My Samsung 37 inch TV has white rings shown on LCD screen with no problem in the input signal. In the power supply one 820 microfarad has a slight bulge on the top. 1500 microfarad 35 volts also slightly bulged. Do you think changing the caps will cure the problem.

    1 reply

    Rather late, but YES definitely change that cap and any which are close to it. Just make sure you take note of the voltage, going over is OK but not under. You can often get away with something close in value, eg a 1680uF instead of a 1500uF (uF = Microfarad.

    PhenomenON. not 'phenomena' (which is plural) - just my two cents. professionalism please.

    That looks like the same as my kit. I just ordered the graphical kit. Because I do like the graphical display for checking transistors. I have a prebuilt graphical without the precision voltage reference. So I just want it all in one meter.

    I saw one of these available on Amazon (not a kit) for about $20.00. Someone compared readings from it to a Fluke for various good capacitors and the cheap unit was not reliable. I also saw one for around $40.00 which had no complaints. Has anyone tried such an expeeriment with this unit?