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Old 10-26-2020, 09:31 PM   #1
EasyGoing1
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Default Why would a 25V cap blow across 19V?

I bought this capacitor kit and I used a 100F across an input voltage of 19V (using a laptop power supply). Within about a second or so after connecting power, the cap gave up the smoke.

I've never worked with these types of caps before, I usually use the traditional black electrolytic caps.

Is there something about these caps that make them more sensitive or is there something I should know about this type that might be different from the standard type I normally use?
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Old 10-26-2020, 10:08 PM   #2
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Default Re: Why would a 25V cap blow across 19V?

Possibilities.

1. They are cheap no name caps from a non reputable seller who has a store on amazon.

2. They are mis rated for voltage. That is, it's really a 16V cap rated for 25V.

3. You put the cap in backwards.

4. Your laptop power supply isn't really 19V unloaded. It could be much higher and only when there is a load, it's closer to 19V. So if you have cheap no name caps with dodgy ratings, that cap could blow

5. All of the above.

6. None of the above.

The above reminds me of a professor in University who had multiple choice tests like this and it was right - minus wrong. So if you got 5 right and 5 wrong, your mark was 0%. It's designed to stop people from guessing.
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Old 10-26-2020, 10:10 PM   #3
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Default Re: Why would a 25V cap blow across 19V?

Since you are in the USA, you have reputable distributors to choose from like digikey and mouser who sell name brand caps like Panasonic, Sanyo, Rubycon, etc.

Pay a bit more. Support the little guys, not Amazon and not some no name seller.
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Old 10-27-2020, 01:58 AM   #4
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Default Re: Why would a 25V cap blow across 19V?

Did you try more than one?
100% sure the polarity is correct when hooked up to the power supply?
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Old 10-27-2020, 02:41 AM   #5
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Default Re: Why would a 25V cap blow across 19V?

Ive found fakes tend to pop within hours of first power on. And yes reverse connects are a quick way to pop.

Anyway normally you can subtract equal points if wrong on multiple guess, but only if they were true/false questions. I think it would unfairly penalize multiple guess with more than two options - like the college board SAT and AP exams IIRC docks a 1/(number of options) point for each guess so in the long run it would average to zero points awarded for random guessing - but educated guessing would still be rewarded.

I wish the USA amateur radio license exams were penalized like this, lots of people guess their way to their licenses...

Quote:
Originally Posted by retiredcaps View Post
Support the little guys, not Amazon and not some no name seller.
A lot of noname sellers are little guys... :-(

Last edited by eccerr0r; 10-27-2020 at 02:43 AM..
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Old 10-27-2020, 08:52 AM   #6
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Default Re: Why would a 25V cap blow across 19V?

Quote:
Originally Posted by retiredcaps View Post
Since you are in the USA, you have reputable distributors to choose from like digikey and mouser who sell name brand caps like Panasonic, Sanyo, Rubycon, etc.

Pay a bit more. Support the little guys, not Amazon and not some no name seller.
Sound advice for sure ...
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Old 10-27-2020, 08:59 AM   #7
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Default Re: Why would a 25V cap blow across 19V?

Quote:
Originally Posted by budm View Post
Did you try more than one?
100% sure the polarity is correct when hooked up to the power supply?
No, I didn't try more than one, and I didn't know that you could blow a cap just by hooking it up backward. I've done that a lot with the traditional electrolytic and I've never had any issues.

I'll try another one and use a variable power supply and slowly crank it up and see what happens.

Quote:
Originally Posted by retiredcaps View Post
1. They are cheap no name caps from a non reputable seller who has a store on amazon.

2. They are mis rated for voltage. That is, it's really a 16V cap rated for 25V.
Definite possibilities...

Quote:
Originally Posted by retiredcaps View Post
3. You put the cap in backwards.
Maybe...
Quote:
Originally Posted by retiredcaps View Post
4. Your laptop power supply isn't really 19V unloaded. It could be much higher and only when there is a load, it's closer to 19V.
No, the power supply is solid. With nothing connected to it other than my Fluke meter, it kicks out 19V on the nose.
Quote:
Originally Posted by retiredcaps View Post
5. All of the above.

6. None of the above.

The above reminds me of a professor in University who had multiple choice tests like this and it was right - minus wrong. So if you got 5 right and 5 wrong, your mark was 0%. It's designed to stop people from guessing.
Sneaky ... I still don't see how it could prevent someone from guessing correctly...
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Old 10-27-2020, 11:04 AM   #8
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Default Re: Why would a 25V cap blow across 19V?

Quote:
Originally Posted by EasyGoing1 View Post
Sneaky ... I still don't see how it could prevent someone from guessing correctly...
It does because it discourages people from making guesses.

Let's say you have a 2 question, 5-multiple choice whose correct answer is 'B'. The first question you know the correct answer: B, so you get one point.

The second question you don't know the answer. Do you leave it blank and get 1 point out of 2 possible points?

No, normally when you have no penalty you should guess. You have a 20% of guessing it right. So you have a 20% chance of getting a score of 2 and ace the test.

However this does not show the test taker's true understanding of the material. This type of scoring would result in a score of less than 1 if the person guesses wrong. In life it's good to know what you know and good to know what you don't know. So on the College Board exams if I remember correctly, you get docked 0.25 points for a wrong answer (if you chose one of the A, C, D, or E answers). So if you have to guess, 80% of the time you get docked 0.25 points and 20% of the time you get a full point for a correct answer.

So the average, (-0.25 +1 -0.25 -0.25 -0.25) / 5 = 0 points gained.

Of course over a large number of questions the average will indeed be 0 as if you left the whole test blank. But if there are specific questions now you have a risk of losing quite a bit of ground if you truly don't know and you may be better off leaving a question blank.

BTW if someone gets a very negative score due to the docking, it does not mean the person doesn't know the material - in fact a very negative score means the person does indeed know some of the material, perhaps a lot - because someone cannot guess all wrong answers by chance. Does not show as much as all correct answers however.

Of course on the USA amateur radio written exam they don't dock points for incorrect answers and it is in your best benefit to pick one:
(0 +1 0 0 0)/5 and you get an average of 0.2 points per guess, which can add to your score.

Too much statistics here, hope I guessed this correctly
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Old 10-27-2020, 03:31 PM   #9
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Default Re: Why would a 25V cap blow across 19V?

Quote:
Originally Posted by eccerr0r View Post
you may be better off leaving a question blank
...
Too much statistics here, hope I guessed this correctly
I actually liked statistics even though it wasn't true deductive math, I liked what you can do with it ... confidence levels and what-not.

My advanced statistics instructor was a couple of years younger than me when I took his class, and he told us on the first day, that he doesn't do statistics because he likes it ... he does statistics so that he can afford to do what he likes, which was purchasing old Vespa's from Italy and restoring them, then selling them...

He was apparently a highly paid consultant for several media broadcasting companies. He would often make sure the data they were going to present to the public was accurate as well as work with various companies on their behalf to gather accurate information.

The man knew his stuff and was quite impressive.

Point docking makes sense as a deterrent for guessing on a test. As long as leaving the answer blank doesn't cost any points, then it makes a lot of sense and I can see how that would work.

I use to talk on the CB radio when I was a teenager. I had a Uniden base station with a sizeable antenna on the side of the house and a 2,000-watt foot warmer hand-built by the local guru that all the CB hobbyists went to when they needed something that sounded clean and loud.

After I got my CET, I was at his house for some reason, and when I told him that I finished the electronics courses at the local college, he told me he would teach me REAL electronics ... said he would teach me everything he knew and he knew a lot! However, I had just gotten married a couple of years before and my wife didn't like him. He was a biker type ... long beard, lived simple and distilled his own moonshine, and cussed like it was a second language. She forbid me from going over there probably fearing I would end up like him. So I never got to take him up on the offer. He died a couple of years later of a heart attack.

I always intended on getting my HAM license, but I've always hated rote memorization and when I started to study morse code, I just got frustrated with it so I never did take an exam. But in the mid-nineties, the Internet started and Instant Messaging came soon after and I guess that replaced my desire for HAM radios. I had a friend though who owned a few packet repeaters that operators could use for data transmission ... this of course was long before 56k modems, ISDN or DSL so I assume it was a handy technology for the time.
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Old 10-28-2020, 03:11 AM   #10
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Default Re: Why would a 25V cap blow across 19V?

I still can't identify the polarity of SMD capacitors and diodes reliably.

Got so many mishaps.
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Old 10-28-2020, 03:16 AM   #11
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Default Re: Why would a 25V cap blow across 19V?

Or sometimes even confused SMD components and caused havoc.
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Old 10-28-2020, 08:16 AM   #12
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Default Re: Why would a 25V cap blow across 19V?

Diodes Polarity is easy - just use a DMM on diode test mode. When it is conducting (0.2V shottky diode, 0.6V silicon rectifier) then you have the positive lead on the anode and the negative lead on the cathode

With capacitors to find the max working voltage (or polarity) - connect a variable bench PSU via a resistor something like 1K to the capacitor like this

+ve -----/\/\/\----!!---- -ve

measure the voltage across the capacitor and slowly increase the voltage on the PSU. When you reach the maximum working voltage of the capacitor, the voltage across the capacitor will stop increasing when you turn up the voltage on the PSU

For electrolytics, if you are not sure of the capacitor polarity, try with the capacitor both ways. Whichever way gives you the highest voltage across the cap is the correct polarity

The same technique works to identify the polarity and voltage of zener diodes too, bear in mind with a zener when you read the highest voltage you have the +ve to the cathode - opposite to a normal diode.

You can do the same trick with high voltage electrolytic capacitors, like the 100V ones often found in PA power supples using a variac, a bridge rectifier and a higher value power resistor like 2K7 or 4K7 or something similar. Do be careful not to electrocute yourself though, or use an isolation transformer!

If the capacitor stops increasing the charged voltage before you reach the rated voltage, then it is a fake (or faulty).

These above methods will not damage the capacitor as long as you stop increasing the supply shortly after the voltage across the capacitor stops increasing.

Last edited by dicky96; 10-28-2020 at 08:26 AM..
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