Exploding Capacitor, need help solving.

Thread Starter

sciengart

Joined Mar 25, 2020
36
Okay, so I am designing a basic power supply to get familiar with using ac to dc.
I am using basically this schematic, with a 115- 11.5v transformer from the line going into J1.

Full wave Rectifier.png
Now, I ripped out a 6800 25v cap out of an altec lansing sub I was gutting, and it was fully functional before hand.

I popped it in this design instead of the 470uf, and within 1 minute, it popped like a balloon.

my theory is that it, being in that altec sub amp for likely 15 years, had some sort of memory? Like perhaps the foil was corroded in areas that weren't used by the supply?
The transformer I used also came from the altec.

I am just trying to figure out why it would pop. The led lit up, and I swapped it for a brand new 470uf 16v and it works fine.

Any wisdom would be great.
 

dendad

Joined Feb 20, 2016
3,595
Are you sure the transformer is only 11.5VAC?
Have you measured the AC volts out?

Also, did you install the capacitor the correct way around?

The DC volts will be 1.4 times the measured ACV.
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
I may have mounted it with the wrong polarity.
The last time the altec was working was yesterday before I decided I wanted a new subwoofer and took it apart.

The cap was the cap used in the power supply of the altec, it is in pretty much the exact same configuration.

This is my first time working with transformers, so I will do tests on the polarity.
the cap that blew was 6800uf 25v.

I am 100% sure its its 11.5 volts out, it says it. on the label.
1.4x the measured voltage makes sense, as the new cap is reading 16v, and its a 16v cap. Which means the new cap will likely pop if I leave it on there too.

I need some guidance I think.
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
Thanks for your help. After testing by placing a new cap in reverse, I popped the cap instantly.
Looks polarity was the issue after all.
It was my initial thought, but for some reason I dismissed it.

Thanks for your help guys.
You are a great community and compliment my education very well. I appreciate all your help.
 

MrAl

Joined Jun 17, 2014
7,757
Hi,

Yes reverse polarity will damage the cap.
Also higher than rated voltage will damage the cap.
Also higher than rated ripple current could heat them up and damage them.
One other thing about old electrolytic caps that have not been used in a long time is that they should be "reformed" before using. That requires applying a small current over a relatively long period of time before using in any application.

I have seen all of the first three happen in real life with explosive results. One large capacitor bank blew up with such force that it bend the thick copper buss bars that connected them together.
The last one i never had experience with all of the older caps i ever used worked ok anyway.
 

atferrari

Joined Jan 6, 2004
4,069
Thanks for your help. After testing by placing a new cap in reverse, I popped the cap instantly.
Looks polarity was the issue after all.
It was my initial thought, but for some reason I dismissed it.

Thanks for your help guys.
You are a great community and compliment my education very well. I appreciate all your help.
Costly way to confirm facts!! Asking here, qualified members could tell you with zero resulting damage.
 

jpanhalt

Joined Jan 18, 2008
10,087
Aluminum electrolytic capacitors have a shelf life. Digikey and other distributors will often report that number. During manufacturer, they are "formed". On sitting, that chemical change is lost. They can be reformed. Usually, one uses a resistor to restrict current and gradually increases the applied DC voltage.

Look up "reforming" electrolytic capacitors for more details.
 

MrAl

Joined Jun 17, 2014
7,757
Aluminum electrolytic capacitors have a shelf life. Digikey and other distributors will often report that number. During manufacturer, they are "formed". On sitting, that chemical change is lost. They can be reformed. Usually, one uses a resistor to restrict current and gradually increases the applied DC voltage.

Look up "reforming" electrolytic capacitors for more details.
Gee i wonder where i heard that before. Oh yeah, two posts before yours :)
 

MrAl

Joined Jun 17, 2014
7,757
Reforming capacitors. Never heard that before. Looked up a YouTube video and found several. Here's one now.

I happened to learn about that on the job back in the 1980's because we used very large electrolytic caps in almost everything we produced. The key i think is to rotate stock.
 

Tonyr1084

Joined Sep 24, 2015
4,910
I'm glad to have learned of this because I have a big 470µF cap in the garage I've been wondering about using. Now that I know it may need "reforming" I won't be so quick to just throw 12 volts across it. Might not hurt it - but it could hurt me.

Just goes to show that an old dog like me CAN learn something new.
 

Tonyr1084

Joined Sep 24, 2015
4,910
Back when I was a young teen I came across a car radio. Found a 12 volt transformer and one of those large finned diodes. Thought this would be perfect for my room - a 12 volt source to play a car radio. Boy did it hum. Someone said I needed a filter cap. So I scrapped one out of an old short wave radio (non-functioning) and put it across the AC output of the transformer. Nothing. The hum was just as bad as before. So I put the cap across the rectified output. It was hardly any better. So maybe it needed to go across the AC mains. Boy did I learn that was wrong. But it IS an interesting way of salvaging the aluminum can and removing the entire contents of the cap fast, and spectacularly. I don't remember the value of the cap but it was probably around 16 volts. It should have popped when I put it across the rectified DC but it didn't. All that was before I took electronics in high school.
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
Sounds like I hit the perfect cocktail of problems. This cap was regularly used, But I am guessing it probably never exceeded a certain voltage.

The new cap i used just fizzed and brown liquid came out.

The altec cap sounded like someone shot a gun, and a huge plume of smoke erupted as the top snapped wide open.

Was a spectacle.

I have deduced that it is a little of each.


What I would love to know now is where to find ripple ratings. I have caps from china. Cheapies.
No ripple rating on them and this is the first I ever heard of such a wonder
 
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