Capacitor safety, preventing fires

Discussion in 'General Electronics Chat' started by MrSoftware, Apr 8, 2016.

  1. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    122
    I've been struggling with how to make an idea extremely safe for a while, and I think the answer was so obvious I don't believe it took this long to think of it. I'm looking for opinions one way or the other.

    The long story short is I want to use a capacitor to enable motorcycles to run without a battery, many of you have participated in the discussions. The electrical system is 12v, and a 10,000uF 25V capacitor works perfectly. However if you connect it backwards it fails in a bad way, so I've been nervous about giving them to the general public. But why not use a start/run cap from a small electric motor? They're not polar so there's no way to connect them backwards, and the voltage ratings are very high so over-voltage is not a concern. Plus they come potted so when they do fail it looks like the failure is contained well (see the image below). I've found a similar 10,000uF 250VAC potted capacitor for only a few bucks. Is there any down side from a technical point of view?

    I haven't tried this yet, but I 'm going to order one for this purpose and see if it works as expected.

    Here's a failed cap, there is some melted plastic bulging out but it looks like the potting basically contained the failure.
    [​IMG]
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    No technical downside I can see if the polar capacitor works satisfactorily.
    But wouldn't such a capacitor be huge - like 1000 times the volume of that popped 10uF one?
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,779
    1,103
    You are rightly concerned with safety, but wouldn't it be unsafe to have no battery on the bike? What happens if the engine fails on a dark night and you have to push the bike home? You would surely need battery-operated lights to be seen by other road users?
    Is 10,000uF enough for anything useful? A discharge current of only 3A would be sustainable for only 40mS.
     
    Last edited: Apr 8, 2016
  4. MrSoftware

    Thread Starter Member

    Oct 29, 2013
    497
    122
    Ha! You're right! Somehow I mis-read that as 10,000uF... but you're right, it's only 10uF... darn! well.... I'll shop and see what I can find, but I suspect you're correct..

    @Alec_t - is an emergency get-me-home device for off-road adventure bikes and motocross bikes. They have kick-start backup, but if the battery is damaged or completely dead, the bike won't run, either leaving you stranded, or in the least wrecking your ride day. Adding the capacitor filters the power enough that the bike starts and runs just fine.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    Just a general note:
    The Start/Run caps in AC motors are generally short term operation on AC as they are bi-polar and heat up on AC if used continuously, they relay on the start switch to turn off, these are usually the high value caps.
    Of course on DC they can be ran continuous.
    The continuous run type are usually a smaller value and are typically oil filled paper etc, and can be ran continuously on AC.
    Max.
     
  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    I have used two normal polar aluminium electrolytic capacitors back-to-back as a bipolar capacitor which works very well.
    I don't know how long they would last if subjected to DC for an extended period though.
     
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