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Thread: Capasitor problems

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    Capasitor problems

    I am in the process of replacing the capasitors on an old board. All was going fine as most capasitors have a negative and a positive leg. Then I pulled one that doesn't. I have replacment caps but they are +/- does it matter which leg goes where if the cap that came out had no indications of + or - ?

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    The King of Vague Steve B's Avatar
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    Re: Capasitor problems

    Is the capacitor you removed a cylindrical electrolytic capacitor? i think you should really replace like for like.

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    DDY
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    Re: Capasitor problems

    The polarity (the +ve and -ve order) of a capacitor is very important, get it the wrong way round and you'll blow it up.

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    Re: Capasitor problems

    With the polarity you can see by the artwrk on thee board.

    On most electrolytics there is a dark band on the top of them. Look for the artwork and that may well also be shaded also.

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    The King of Vague Steve B's Avatar
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    Re: Capasitor problems

    Quote Originally Posted by DDY View Post
    The polarity (the +ve and -ve order) of a capacitor is very important, get it the wrong way round and you'll blow it up.
    Positive and negative with respect to what? Anode and cathode
    Thats only if its an electrolytic and if you reverse bias it with a loooot of DC, more than a PSU is likely to give you.

    Quote Originally Posted by Blitzen View Post
    With the polarity you can see by the artwrk on thee board.

    On most electrolytics there is a dark band on the top of them. Look for the artwork and that may well also be shaded also.
    Most electrolytics come in a dark blue container. Theres a light blue stripe going from top to bottom beside one of the pins, thats the cathode. That goes by the shaded line on the board.

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    Re: Capasitor problems

    Quote Originally Posted by Steve B View Post
    Positive and negative with respect to what? Anode and cathode
    Thats only if its an electrolytic and if you reverse bias it with a loooot of DC, more than a PSU is likely to give you.
    Most electrolytic capacitors will fail significantly below their forward bias voltage rating when reverse biased. For instance, I've managed to blow up 35V capacitors with only a 12V lab supply. It's never fun when they blow...

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    Re: Capasitor problems

    Quote Originally Posted by timothius View Post
    It's never fun when they blow...
    lies its the greatest fun on earth!

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    Re: Capasitor problems

    what actually happens when a capacitor blows?

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    The King of Vague Steve B's Avatar
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    Re: Capasitor problems

    its only electrolytics that blow. Electrolytics are like a piece of metal toast with a jelly spread. The toast is rolled up, like a swiss roll and put in the wee container. When you reverse bias it, they can blow up. Shards of metal (if its a big cap) go everywhere, and the jelly stuff turns to fluff and covers everything.

    Exploding an electrolytic capacitor by Synergic -- Revver Online Video Sharing Network

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    Re: Capasitor problems

    Theres a light blue stripe going from top to bottom beside one of the pins, thats the cathode. That goes by the shaded line on the board
    You misses what i said i think. You dont need to work out pos or neg if the artwork on the PCB is correctly done as it will follow the shading on the cap. (Well, the millions i have seen going through our SMT lines have anyway).

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    The King of Vague Steve B's Avatar
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    Re: Capasitor problems

    Ahh ok, i'm forgetting that solid state caps are now used instead of electrolytics.
    Like this you mean?

    Gotcha

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    Re: Capasitor problems

    Quote Originally Posted by Steve B View Post
    lies its the greatest fun on earth!
    Yeah, we had it demonstrated last year when we were building power supplies at Uni, was a tiny capacitor but it still made a decent bang!
    "If you're not on the edge, you're taking up too much room!"
    - me, 2005

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