What happens when you install electrolytic capacitors backwards?

I built an ignition coil driver with a ne555 timer I think I may have installed the electrolytic capacitor in the circuit backwards. The circuit oscillated then started to die out would this be the capacitor failing?  

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Jayefuu6 years ago
This happens.... (sometimes)

This one had 15V across it the wrong way because the schematic I was working from had it drawn backwards. Ooops!

It made a rather loud pop, a small cloud of dust, ejected its canister and covered the components around it in fluff.
SDC12289.JPG
He he. Now repeat with a 10,000uF 40V. That nearly took my ear off.
it works best with low voltage capacitors. i used a 16v 100uf. connected it in parallel to a charged 470uf 450v cap. that was spectacular. huge sound and a lot of dust(i think) in the air.
Hrmmmm yes, I can imagine. This was only a 25v 100uF.
The MadScientist (author)  Jayefuu6 years ago
But does the description of my circuit failing sound like the capacitor?
Post a picture of the circuit you're following and your current layout.

Does replacing the capacitor fix your problem? Make certain you know which way around it should go. Disconnect the later stages of your circuit before doing so, get this part working and oscillating as you want, THEN start plugging the next bits in. That way you can be sure your 555 timer circuit is working before you start adding complications.

The MadScientist (author)  Jayefuu6 years ago
I got it from this site http://uzzors2k.4hv.org/index.php?page=ignitioncoil I replaced the timer three times so I doubt it's that and I can't get another capacitor for about three days.
Which of the capacitors do you think you've broken?

Have you checked the output of your 7812? Do you have 12V there? Do you get 12V at pins 8 and 4? Do you have your 555 timer the right way around? Are you sure it worked/oscillated in the first place, or has it never worked?

Do you have an oscilloscope? Disconnect the output (pin 3) and check it works using your scope. Or if you don't have one, change the values of the astable to give an output frequency of a few Hz so you can see an LED blink, this will show you your 555 still works and you can put back in the old values to get your 400Hz signal.


The MadScientist (author)  Jayefuu6 years ago
The 10uF I've checked the regulator it works, I know the timer is in the right way and it did work for about 4 seconds then started failing and stopped. I did replace the 1.8k resistor with a 120k one to change the frequency might this put stress on the timer somehow?
That capacitor (and the 2 next to it) helps to stabilise the 7812 voltage regulator.

This post on allaboutcircuits explains when and when it isn't necessary What power supply are you using? Read the second and third posts by sgtwookie and ifixit.

This website has a calculator to help you find values of R1, R2 and C to lower the frequency of your astable to something visible so you can test it's working with an LED if you don't have an oscilloscope to look for the 400MHz signal your current R1, R2 and C values will give you.
iceng6 years ago

Electrolytic caps use a liquid like chemistry between the capacitor plates to increase the capacitance value beyond what the surface area alone could provide.
When you reverse polarize this chemical, it allows current to flow like a resistor.
If the current source is substantial, it will generate heat
( ever burn your finger on a resistor? )..
Lots of heat and steam with pressure will follow.
Depending on construction of the case you can get a fizzle or an eye altering BANG.

That's what happens quite fast.
seandogue6 years ago
Presuming a bypass cap for the supply, If you're really lucky, the circuit simply won't power up. If not, you'll need a new cap.
AndyGadget6 years ago
 
It won't go BANG!

It all depends on the type of capacitor and how badly you're treating it.
In your case, you are not being too cruel to it as the current through it is limited by the timing resistor, so what is happening is the capacitor's electrolytic plating is breaking down and the leakage current is increasing. It is becoming less of a capacitor and more of a resistor.

Because of this, the voltage on your 555 trigger pin is not getting to the trigger point so the astable stays in the OFF state. Try putting it in the right way and it may re-polarise and start working. (Or it may not.)

Where you've got a reversed capacitor directly across the power rail the current is not limited so they may well go bang.

My first job was as an electronics engineer in a printer factory where we would burn-in newly made dot matrix (that's how long ago it was) printers for 24 hours. to catch the 'early life' failures. The reversed polarity capacitors showed three main modes of failure . . .
The low voltage tantalum beads went with a satisfying 'CRACK' and a puff of smoke.
The large screw terminal electrolytics (47,000uF, 40V) had a pressure relief plug in the top and the pressure would build up until the plug shot out of the hole, followed by a copious quantity of a fluid which looked like it contained fluorescein dye which would bounce off the inside of the case and spray the entire inside of the printer with fluorescent yellow goo.
The axial aluminium can electrolytics (10,000uF, 80V) would explode and and cover the inside with what looked like shredded fluffy cardboard.
Ooo Given the right circumstance I can assure you electrolytic capacitors do indeed go bang, with a little effort quite violently as well. Modern electrostatics have a burst disc but can still go bang (OK often they don't but they can)


 
Read on Rick, read on.
What the OP described was exactly what would happen if the timing capacitor in a 555 circuit was reversed.  The current would be limited to a few milliamps at most and nothing spectacular would happen. apart from electrolytic breakdown.
My answer continues with what happens to various types of capacitor when the current is NOT limited.  Fun and games ensue.

However, now he's posted a link to the circuit I can see the only electrolytic there is across the power supply so my assumption of the timing capacitor was incorrect.

It may be a big assumption that he built the circuit as the diagram!

MAd Scientist - A good quality picture of what you have might help
Great minds . . .
Simultaneous postings - See my latest #;¬)
AndyGadget6 years ago

OK - My answer below is true, but as you've now posted a circuit link I see it IS across the supply. 
Was the 7812 getting hot?  If so, it's the same thing.  Depolarisation means it's less capacitor and more resistor so is drawing increasing current, possibly to the extent that the 7812 is going into thermal shutdown, dropping the voltage and stopping the circuit oscillating.
Whether the capacitor goes POP depends on whether it can dissipate the heat and the pressure caused by the electrolyte boiling.
The MadScientist (author)  AndyGadget6 years ago
Sorry for the delay. I think the regulator was getting hot but the timer was also getting hot what does this mean for the timer?
 
Jayefuu is giving you very good advice, so here's some more.

First, go for a walk and forget all about the circuit.

When you return,  thoroughly check your circuit. By thoroughly, I mean go through each node in the circuit and make sure each component is connected to the correct other components.
Then, with your multimeter on resistance, make sure each node is not connected to any other node, especially the power rails.

It is so easy to miss a stupid mistake or miss a tiny whisker of solder and remember, Occam's razor is the sharpest tool on your workshop.

Occam's Razor :
One should not increase, beyond what is necessary, the number of entities required to explain anything.
i.e. The simplest solution to a problem is the one most likely to be correct.


lemonie6 years ago

Give them enough power and like rick says, they go pop / bang.

L
The MadScientist (author)  lemonie6 years ago
Does the description of my circuit failing sound like the capacitor?
Could be, but it ain't going to work right if the thing is the wrong way around anyway.

L
Jayefuu lemonie6 years ago
That capacitor (and the 2 next to it) helps to stabilise the 7812 voltage regulator.

This post on allaboutcircuits explains when and when it isn't necessary What power supply are you using? Read the second and third posts by sgtwookie and ifixit.

This website has a calculator to help you find values of R1, R2 and C to lower the frequency of your astable to something visible so you can test it's working with an LED if you don't have an oscilloscope to look for the 400MHz signal your current R1, R2 and C values will give you.
Jayefuu Jayefuu6 years ago
What I was TRYING to say, before Instructables just posted my previous comment a second time, was that that capacitor isn't critical for operation of the 555 timer, it smooths the output of the 7812. It should work without it.
Jayefuu Jayefuu6 years ago
Argghhhh that's not at all what I wrote. Stupid website. It copied a previous post.
rickharris6 years ago
BANG! no really! BANG!