Capacitor goes BOOM! What went wrong ?

[ThermallyFrigid]

So,
I built a circuit from a schematic that is a battery deSulfator (or claims to be)
I realize most likely it does nothing....but I built it anyway as an experiment.

So after connecting the battery to be desulfated to the output end, I connected 12.9v to the input end.

For about 12 seconds all seemed well.....them a startlingly loud pop as the circuit's 100uF 25v capacitor blew itself apart.
I'm wondering if someone can look at this schematic and tell me what is wrong?
Thanks


Here is the circuit Schematic

[jpanhalt]

What type of capacitor?  Was it installed with proper polarity?

[ThermallyFrigid]

Hello,
It was a 100uF 25v Electrolytic capacitor and I'm nearly certain it was installed correctly.

[rsjsouza]

The remnants (black side to the positive) seem to indicate the capacitor was installed in the correct polarity.
My suspicion is that the capacitor was already faulty - either it went through excessive stress during soldering or was already defective. 

[bdunham7]

That's exactly what would happen if the capacitor was installed in reverse polarity.  I can't see enough remnants to know either way.  Don't overlook the possibility that the silkscreen is wrong--it's happened before.  Verify that there is continuity between the negative side of the capacitor and GND.  Then try again with a new one.

[ThermallyFrigid]

With the capacitor removed, I verified continuity between GND and the Negative marked side on the silkscreen.   Maybe bad capacitor.

[rsjsouza]

That's exactly what would happen if the capacitor was installed in reverse polarity.  I can't see enough remnants to know either way.  Don't overlook the possibility that the silkscreen is wrong--it's happened before.  Verify that there is continuity between the negative side of the capacitor and GND.  Then try again with a new one.

It would happen with a defective capacitor as well. About three months ago I had the exact same thing happen in a new equipment of mine. It had three bypass 470uF/50V in parallel in a 32VDC input supply line fed by my ridiculously low noise Power Designs 5015A. Out of nowhere, smoke and noise came out of the equipment's internals - the capacitor (low quality ChongX) simply gave up.

But good point on the silkscreen - I have been bitten by that in the past.

[BrokenYugo]

I've heard of defective caps where the label was 180* out, but the leads were correct (negative is shorter).

[SteveyG]

Based on your other electrolytic capacitors on the PCB you probably had it the wrong polarity, the top one is definitely the wrong way around!

[ThermallyFrigid]

Based on your other electrolytic capacitors on the PCB you probably had it the wrong polarity, the top one is definitely the wrong way around!

oops.  You jumped to the wrong conclusion.
Optical illusion.  It definitely is not installed wrong.

I could take (and post) a series of photos that would show you this, but just take my word for it.

I can see in the photo where you get that, but it is absolutely positively installed correctly.
You are seeing print on the right side edge of the casing that looks like the negative bar but it is not.

The Negative bar is on the Left, just beyond the edge of the capacitor, and is indeed aligned with the white indicator on the PCB.

[SteveyG]

I'm not sure I understand that? Is that not the negative stripe on the capacitor on the right?

Do you have an oscilloscope? With that inductor there, I wonder if you're generating some very high voltage spikes when the MOSFET turns off.

[bdunham7]

Whoa!  You're telling us that this is NOT the negative side of the capacitor?  If an experienced tech or engineer that I trusted told me that, I'd still have to see for myself.  Coming from someone that is asking why their capacitor blew up, well....    Seriously, show us a better photo and maybe tell us the make/model/part number of the capacitor.  I have extreme doubts....

[josh132]

Whoa!  You're telling us that this is NOT the negative side of the capacitor?  If an experienced tech or engineer that I trusted told me that, I'd still have to see for myself.  Coming from someone that is asking why their capacitor blew up, well....    Seriously, show us a better photo and maybe tell us the make/model/part number of the capacitor.  I have extreme doubts....

you are right. if you look at the silk screen capacitor footprint the negative side of the cap is on the left according to the silkscreen footprint on the PCB. But the positive side of the cap is on the negative side of the silkscreen footprint.

[SiliconWizard]

So have you tried it with a new cap?

[ThermallyFrigid]

Whoa!  You're telling us that this is NOT the negative side of the capacitor?  If an experienced tech or engineer that I trusted told me that, I'd still have to see for myself.  Coming from someone that is asking why their capacitor blew up, well....    Seriously, show us a better photo and maybe tell us the make/model/part number of the capacitor.  I have extreme doubts....

Easy there Nelly....it's not the end of Silicone.....
It was the correct direction when I checked it today.   No bad intent there.
I guess at some point after I took the photo I must have caught it and turned it around.

I guess I'm still not perfect.  Dog gone.  Working on it.   

To be honest, I'm making a lot of mistakes.   I know that.   It's all a learning process.
It's likely I did something wrong to blow that capacitor.
When I replaced it it didn't blow again so maybe I had a few of them reversed.

Gotta be more careful

[ThermallyFrigid]

So have you tried it with a new cap?

Yes.  It didn't blow.  Perhaps I did install it backwards.
Probably a number of other mistakes to go along with it.