What damage can occur when a filter cap explodes?

[Crystaleyes]

Hi all.

Following on from my previous thread...

Just wondering which other problems (if any) can be created further down the pcb when a reversed polarity filter cap explodes?

Trying to get a better understanding for future repair diagnostics.

[Per Hansson]

The diodes can fail because of the extreme current draw of the capacitor.

[stj]

high ripple can blow chips downrange if your really unlucky

[Crystaleyes]

Thanks for those replies.

It did make me wonder, as there would not be any collapsing coil fields, but clearly such dramatic events would not be without consequence.

@Per
Are you talking about the bridge rectifiers? Or any diode nearby?


@STJ
Regarding blowing chips downrange through high ripple, would that be instantaneous, or only if left switched on and in circuit?

[stj]

damage from ripple would be over time

[Crystaleyes]

Originally Posted by stj

damage from ripple would be over time

Cool.
That is what I would have thought.

At least I appear to be thinking along the right lines.

[Uranium-235]

Originally Posted by stj

damage from ripple would be over time

But if it "explodes" explodes, I'm imagining you might keep the thing off till you replace it

[CapLeaker]

Last time my main filter cap shorted in the psu of my oscilloscope. All damage it did was the main filter cap shorted and a fuse blown. So it really depends on the build quality of the power supply.

[Crystaleyes]

Originally Posted by Uranium-235

But if it "explodes" explodes, I'm imagining you might keep the thing off till you replace it

Actually I bought the unit already like that. It was part of a cheap 'job lot' from a repair shop, and considering how I am still learning all this electrickery, it seemed like a good opportunity to learn a bit more.

This particular cap was on the PSU of one of two Kenwood CS4035 scopes, which had both been dead when received. I've been messing with this one today (with a new cap) as changing the exploded cap has revealed other issues further along which I'm struggling to identify right now. No direct shorts but something on the main board is pulling the + & -8v lines down to ~6v on both the scopes.

These scopes have half a dozen 'hybrid' IC's which seem to be non-existent in the world of datasheets, so hopefully they are not the problem.
After pulling and testing all the caps on the PSU, it might be the next step to see if a cap or two is failing on the main board.

This is the area I need to learn how to speed up on...

[Crystaleyes]

Originally Posted by CapLeaker

Last time my main filter cap shorted in the psu of my oscilloscope. All damage it did was the main filter cap shorted and a fuse blown. So it really depends on the build quality of the power supply.

Thank you. Good to know.

The PSU is now working apart from the reference voltage adjust pot. This may well be one of these hybrid IC's playing up (a KMA 01).
For now, the -8v reference is -8.6v so I'm not stressing, more important is what on the main board is pulling the voltages down 25%

[stj]

hybrids can often be replaced,
the schematic will give you an idea of what they are doing.
it could be as simple as a bunch of transistors that have exactly matching spec.

[Crystaleyes]

Originally Posted by stj

hybrids can often be replaced,
the schematic will give you an idea of what they are doing.
it could be as simple as a bunch of transistors that have exactly matching spec.

There is actually a .pdf online of a template and component layout for one of them. Fingers crossed that none have failed.

I'll start a new thread regarding the scopes project

[PeteS in CA]

If it’s an electrolytic, the liquid electrolyte is very conductive and could cause shorts.

[petehall347]

it could blow your eardrums and blind you if it hits you in the eye .

[Uranium-235]

I can tell you guys this...if a polymer explodes, it smells like popcorn

[PeteS in CA]

Originally Posted by Uranium-235

I can tell you guys this...if a polymer explodes, it smells like popcorn

And the cases tend to launch.

[PeteS in CA]

Originally Posted by petehall347

it could blow your eardrums and blind you if it hits you in the eye .

I haven't heard any lytics going off that dramatically, but my experience is, obviously, finite. OTOH, when a wet slug tantalum went of a couple of feet from me my ears were ringing for an hour or two. I have seen both lytics and polymers launch.

[stj]

i had a high voltage dump cap from a big xenon strobe lamp explode in front of me once,
that wasnt fun - electrolyte all over me - paper all over the room.
metal can probably bounced off several walls!

[PeteS in CA]

Actually, I do remember a lytic launch. I was working at Boschert, and a new engineering tech was working on a 48V input DC-DC converter prototype. The assembler had accidentally installed a 7.5V cap on the input instead of the correct cap (75V?). He applied power and after a couple of second the cap launched and hit him in the chest. He wasn't hurt, but I didn't see him in the test are for an hour or so.

Another time, at Tandem Computers (not Tandy) we were doing rehearsals of "abnormals" tests for UL. The method was to select a component to short or open and then apply power to see if the power supply failed safely. In one of tests we shorted one of the series-connected input electrolytics. When 208VAC was applied, that put over 300V across 200V caps. What was supposed to happen was that one of those caps would short and blow a fuse. Well, this time it started venting steam, like it was going to launch. We all scattered for a few seconds until someone had the courage to switch off power to the power supply we were testing. This was not a good outcome and was totally unexpected. The safety people tried multiple time to duplicate what happened, but always had what was expected happen.

About a year before I hired on at Boschert they had moved their manufacturing to Manteca, a California city a couple hours' drive away. Some employees moved there, a few stayed, and some moved on. The senior engineering tech had been the manufacturing test supervisor. One day he showed me the ceiling above what had been the manufacturing test area. There were quite a few circular holes in the ceiling tiles from reversed or incorrect caps that launched.

Also from the 1980s at Tandem, I was testing some prototype power supplies that had screw terminal output caps mounted on bus bars. When I powered on one I heard a faint pop, switched off power, and found that the output caps were reverse-polarity. The little vents had popped open, and some electrolyte dripped out. The power supply had shutdown-and-retry current limiting, so I'm not sure whether anything more dramatic than dripping electrolyte would have happened.

All these memories are from the 1980s and are a bit dusty and dim.