Blowing up capacitors

[Commander Kinsey]

[Brian Gaff]

Used to happen lot of course in the old valve days. What a mess. Brian

[alan_m]

My first job was working for a company that made PA equipment. Part of the testing after build was a soak test for 6? hours where the valve amplifiers were placed on a rack and just left on. One batch had some very large electrolytic capacitors fitted the wrong way around. A couple of capacitor cans hit the very high ceiling of a warehouse building before the assembly fault was realised.

In another job I also witnessed some tantalum bead capacitors glowing cherry red when fitted the wrong way around. The equipment was immediately switched off and then about 15 seconds later a capacitor exploded with a very loud bang sending small shards of the ceramic case in all directions.

[Commander Kinsey]

Is that what caused Monty Python's "And now a warning about leaving your radio on all night. Leave your radio on all night."

In about 2001 I left my TV on to keep my parrots amused. I told a technician at work I'd done this, and he said "that's how fires start". I tried to explain my TV didn't have valves, but he was old and stuck in his ways.

A bit OCD too. A lot OCD. I once got a lift from him when my car was in the garage. He'd told me he'd pick me up at 10 past 8. I came out of my house at 11 past eight. I got a lecture all the way there about how all his jobs couldn't be completed on time today. I wonder what he does if there's roadworks?

[Commander Kinsey]

They should make them all bisexual. No. Bipolar. Why do they care about charge direction anyway?

[invalid unparseable]

Only capacitors with small capacity are bipolar. Those kind of capacitors are just two conductive plates separated by air or a dielectric separator.

Large capacitors are called electrolytic capacitors. I have opened a few of those in my youth to investigate what's inside. I found that an electrolytic capacitor is a cylinder with two metallic foils rolled up inside and soaked in some kind of oily liquid (apparently the oily liquid is the electrolyte, and thus the name). It can store electrical charge like a battery, with very low internal discharge,  but if you connect in reverse polarity it will conduct electricity and heat up the electrolyte inside, and then the electrolyte will boil and the cylinder will explode. I had experienced a few blown up capacity in the 1970s when I was into building my own power amplifiers from circuit diagrams.

[The Natural Philosopher]

Connecting the mains across some big valve amp smoothing caps was apparently a weekly sport at Marshall amplification in the 1970s according to a tech I knew who had worked there.

Another story he had was a very sniffy call from British Rail who said their parcels were at the station, would they please collect them. That was in the days when Red Star was a fast way to get big items shopped across British rails networks.

At the station were to Marshall cabinets stencilled 'Deep Purple' in for replacement drive units. They *stank*..

When they opened them up, the reason became clear,. The Deep Purple roadies had expressed their contempt of them by using them as toilets

[Max Demian]

Electrolytic capacitors consist of two sheets of aluminium foil separated by absorbent paper soaked with an electrolyte. During manufacture a current is passed through the electrolyte which deposits a thin layer of oxide on one of the sheets. The oxide, which is an insulator, is the dielectric (its thinness allows for high capacitance). If it is connected the wrong way round the dielectric will be destroyed. (Maybe if you left it connected an oxide coating would form on the other sheet - but I expect it would overheat first unless the current was restricted.)

(A bit like lead acid accumulators which have no polarity when they leave the factory and could be charged up the wrong way so the + terminal is -.)

[Commander Kinsey]

So presumably if you knew what you were doing you could move the oxide layer?

Not really sure you'd gain much from doing so.

[Commander Kinsey]

So it will tolerate say one chargefull the wrong way? Although I've heard of them being in series and only getting reverse charged from the other slightly different ones discharging for longer can explode them. Especially supercapacitors, hence the monumentally complicated evening out circuit which comes with a 6 pack.

[Commander Kinsey]

Would have loved to see the look on a roadie's face when a capacitor discharged through the urine flow into his penis.

[invalid unparseable]

That's also true for connecting many batteries in series. The weakest in the lot will eventually be killed by being reversely charged by the stronger ones.

However, nobody with a sound mind would connect electrolytic capacitors in series because connecting two capacitors of equal capacity will end up with a combined capacity of only 1/2 of a single capacitor.

[invalid unparseable]

Correcting "capacity" to "capacitance", for completeness.

[invalid unparseable]

Are you from the hippies era? It's called  SPONTANEOUS  COMBUSTION. It was common knowledge back then.

[The Natural Philosopher]

But twice the voltage rating. It's done often enough in certain applications

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[invalid unparseable]

For voltage multiplier applications, they use high frequency and small capacitance capacitors. Small capacitance capacitors can withstand high voltage and have no polarity.

[Clive Arthur]

<snip>

I have a PCB on my desk with 5 off 33uF 35V surface-mount Tants in series, and 4 off 47uF 50V SM Tants in series. There's a good reason. Often I see series parallel arrays of tens of non-polar SM ceramics, again with good reason.

You can't always get what you want, so you have to DIY it.

[Commander Kinsey]

[whisky-dave]

True, but it could be useful way of doing it if you don;t have a capacitor of say 10uF of 50V but do have 2 of 22uF at 25V

[Commander Kinsey]

Also the good batteries will be worn out by overcharging. Even worse, put three 12V lead acids in series, and one cell of one battery collapses and shorts, the charger doesn't know this, and the good batteries all get too high a voltage. Which is why I use equalisers.

Supercapacitors tend to come in about 2.7V. I often need a higher voltage.