Reverse polarity Electrolytic

Thread Starter

Art

Joined Sep 10, 2007
806
Hi Guys,
I am wondering if a polarised electrolytic capacitor was connected across a power supply for a decade and did not blow up,
could it ever become a working electrolytic capacitor of polarity reversed from the marking on the package?

If not, assuming a polarised electrolytic capacitor is totally discharged by being shorted,
why does it have a positive and negative terminal?
Thanks :)
 

dl324

Joined Mar 30, 2015
10,938
I am wondering if a polarised electrolytic capacitor was connected across a power supply for a decade and did not blow up,
could it ever become a working electrolytic capacitor of polarity reversed from the marking on the package?
Electrolytic capacitors will tolerate small reverse voltages, on the order of 1.5V.

Reverse biasing them can cause dielectric breakdown, any that were abused should not be relied upon for normal usage.
 

Thread Starter

Art

Joined Sep 10, 2007
806
I know they should be replaced,
But have real examples that have not shorted or blown up (but don't expect they are nessecarily being capacitors) after more than a decade.
 

crutschow

Joined Mar 14, 2008
25,114
.................
If not, assuming a polarised electrolytic capacitor is totally discharged by being shorted,
why does it have a positive and negative terminal?
The terminal polarity tells you where you should apply the positive and negative voltage when you charge it.
If you apply a reverse polarity voltage greater than a volt or so it will start conducting current (somewhat like a diode) and can be damaged.
 

dl324

Joined Mar 30, 2015
10,938
But have real examples that have not shorted or blown up (but don't expect they are nessecarily being capacitors) after more than a decade.
Yes, it's possible for reverse biased electrolytic caps to not blow up. If the reverse voltage is small, it won't even harm them.
 

alfacliff

Joined Dec 13, 2013
2,458
it depends on the type, a cap that has set around discharged for a long time can be re"formed" with reverse voltage sometimes. when caps are made, they arent polarised, they are formed by putting a lower voltage across them and ramping the voltage up tol the rated voltage over time. note,---- tantalum and other high tech caps arent done this way, they can and will explode.
the plates of the caps are identical aluminum untill they have been formed.
 

ian field

Joined Oct 27, 2012
6,539
Hi Guys,
I am wondering if a polarised electrolytic capacitor was connected across a power supply for a decade and did not blow up,
could it ever become a working electrolytic capacitor of polarity reversed from the marking on the package?

If not, assuming a polarised electrolytic capacitor is totally discharged by being shorted,
why does it have a positive and negative terminal?
Thanks :)
Theoretically: you can re-polarise a capacitor. In aluminium electrolytics, the anode foil is "formed" during manufacture to grow an oxide layer (the actual dielectric) - but they usually do all that before rolling the 2 foils and separator and sealing them in the can.

In most cases; reverse voltage will break down the oxide layer and the fault current boils the electrolyte - with predictable results!

AFAIK: Solid tantalum capacitors are the least tolerant of reverse voltage - at the very least, they go leaky - I've seen bead tantalums go off like a match head.
 
If you apply more than a few volts they will explode.
When I was learning electronics the class all built up a pcb each.
One student put in his electrolytic in the wrong way around.
When he applied power it exploded and hit the ceiling.
 

ian field

Joined Oct 27, 2012
6,539
If you apply more than a few volts they will explode.
When I was learning electronics the class all built up a pcb each.
One student put in his electrolytic in the wrong way around.
When he applied power it exploded and hit the ceiling.
If you limit the current low enough, it wont boil the electrolyte and build up a head of steam.

Electrolytics stored for a long time eventually lose the oxide layer on the anode foil - as it gets thinner; the capacitance increases, but the breakdown voltage becomes less. Its the leakage current from the application of within normal voltage that maintains the oxide layer.

Reverse voltage will speed up the dissolving of that oxide layer, and cause a new one to form on the other foil - but the current has to be limited to avoid destruction of the capacitor.
 

Thread Starter

Art

Joined Sep 10, 2007
806
That’s where I was going, but I didn’t want to lead the thread there myself, and influence the answer.
I have 2 retro computer Commodore Amiga CD32, manufactured 1993-94.
They have an external linear PSU, and for at least one of two board revisions,
two SMD decoupling electrolytics are installed reversed.
The silkscreen is correct, but they are mounted opposite.

I have two units and there are two incorrect caps in each, so that’s four caps, no leakage,
and both units still chug along nicely. I used them a good six months before ever finding out about the error.
Something I can test with the old ones when the new caps arrive.

Ian, thery wouldn’t have been current limited in any way,
they all see the power supply and ground plane of the main PCB :D
Cheers, Art.
 

SLK001

Joined Nov 29, 2011
1,548
Are you sure that it isn't the SILKSCREEN that is in error? If the error was discovered after the PCBs were fabricated, the manufacturer might have just placed the caps properly and made the correction down the line.

It is unlikely that a electrolytic capacitor was mounted backwards without catastrophic results.

Also, what do you mean by "decoupling"? Electrolytic capacitors are generally used as filtering capacitors (mounted VCC to gnd) as opposed to signal decoupling.
 

Thread Starter

Art

Joined Sep 10, 2007
806
It's very well known in the community that collects these computers about the error.
I have also checked, and the silkscreen is correct, but components wrong.
I'll follow up here when the new replacement caps arrive.
There are plenty of other caps on the board mounted correctly,
But the two in question have positive terminal connected to ground plane.

By decoupling I mean the PSU is a brick seperated from the unit with 3 meter cable. The caps are local on the computer PCB across the power supply input.
 
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