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Fuses

Discussion in 'Question of the Day' started by Mayhem, Jun 27, 2004.

  1. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Let me pose this question to you:

    What is the current rating of common household aluminium foil?

    Don’t know the answer? Well, nor do I and more importantly, none of us should know because it should never be used as a substitute for a fuse!

    The reason I even pondered this question was because a friend of mine came to see me yesterday with a sprinkler timer he has been having problems with recently and had now stopped working. I asked what the problem was and he told me that it kept blowing fuses. So I opened the front access panel up and saw a 1A 3AG fuse wrapped in aluminium foil sitting in the fuse holder.

    I asked what the hell he had done and was informed that he had run out of the correct size fuses, so he had improvised. He was actually rather impressed with his efforts and informed me that it worked for a little while longer than it had been.

    I then proceeded to open up the unit and inspect the board. What I found was quite impressive. The fuse had blown due to a short and by effectively bypassing the fuse the current draw caused by the short had heated up several components to the point that they not only failed, but lifted the tracks off the board. One resistor had actually caught on fire. All in all, given the chips on the board were quite probably damaged as well, he is up for a whole new unit. All this could have been prevented.

    So, perhaps the question should be what are fuses and how do they work?
     
  2. tenor_singer

    tenor_singer Active Member

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    Ah yes... the tin-foil bypass... lol,

    I remember as a kid, my father would decorate our front yard's two HUGE (45') pine trees for Christmas. Each tree would hold thousands of 4-W lights... usually 100 to a strand. Our problem was that when you linked several of these strands together, the built in safety fuse at the male end of the strand would pop. His solution was to teach me how to cram aluminum foil into the fuse receptacle... I was 12 at the time.

    The truly funny thing was that he couldn't figure out why the male ends of the strands kept burning out and why we kept blowing our fuse pannel in the basement. His solution there was to put in a seperate pannel and hard wire it to the studs of the original pannel. We then ran a couple of outlets outside from the new box which had 50-A fuses in it.

    Come to think of it... I am happy to be alive and I am happy that exterior mini-lights became popular! :D
     
  3. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    LAMO - I can see this being the next Christmas ad for VISA, with either Homer J Simpson or Chevy Chase as the lead!!
     
  4. Inaki2

    Inaki2 Active Member

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    Now thaaaats scary...I'd love to post a pic of the system my house had when I moved in...I'll add one tomorrow in my profile pic thingie and pleeeaasseee check that out.
     
  5. autophage

    autophage Member

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    As to the actual question - a fuse is meant to blow. The idea is that if a part costing less than a dollar and that is easily replaceable blows instead of an electronic system that cost lots of money, it's a good deal. It's a protective measure.

    When it blows, it means there's either a current spike from outside the system or a short/somesuch inside the system that could have led to voltages high enough to destroy the system. If a fuse is blowing repeatedly (and you know it's not due to external power spikes) you should get it in for a look/repair by someone licensed to repair it - elsewise you risk destroying the system/fixture and quite possibly hurting yourself or someone else.
     
  6. propmonkey

    propmonkey Well-Known Member

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    our soundboard power supply had a fuse problem. they didnt blow instead other diodes blew. the fuses should blew before them. i remember in 7th using a gum wrapper for a strobe light fus(like in macgyver...).
     
  7. soundman

    soundman Well-Known Member

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    To answer part two of the question; A fuse works by using a thin piece of wire, if the current draw is two much the wire gets hots and melts/blows/explodes saving the machine from further damage. Fuses unlike breakers are a one time use iteam.
     
  8. zac850

    zac850 Well-Known Member

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    Out of curiosity, how does a circuit breaker work? I understand perfectly well how a fuse works, and I sort of understand the concept of how a circuit breaker works, but how exactly does it work?
     
  9. rgsw

    rgsw Member

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    to put it in lame mans term:

    you have a electro-magnet close to a disconnector wire (i think) and the current flows through the electro magnetic and should anything get too big the magnets power gets bigger and pulls the rocker towards it cutting its supply.

    its like a door bell apart from better.

    have a look on the web or i'm sure ship will know exactly
     
  10. zac850

    zac850 Well-Known Member

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    That makes sense. I knew it was some way of cutting the power circuit (what's the correct term?) temporarily.

    Cool, that makes sense.
     
  11. Inaki2

    Inaki2 Active Member

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    Except that when the circuit is cut, the whole mechanism lock into place untill you reset it.
     
  12. DMXtools

    DMXtools Active Member

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    The most common circuit breaker is thermal: a switch in series with a heating element. All the current for the load passes through both. A spring pushes the switch to the OFF position, a latch holds it in the ON position. The latch is made of two metals with different coefficients of thermal expansion, bonded together. As a result, when it gets warm, it actually bends, releasing the switch, allowing the spring to turn it off. The size of the heating element determines how much current it takes to generate enough heat to release the latch. There's also a time element - the temperature rise isn't instantaneous. At 110% of the rated current, it may take several minutes to an hour or so for the latch to release. At 200% it may still take several seconds.

    A magnetic breaker is similar, but substitutes an electromagnet for the heating element. At slightly more than the rated current, the electromagnet becomes strong enough to release the latch The advantage is that the time lag is much smaller than that of the thermal type. In many applications that's also a disadvantage - when a tungsten filament first sees power, it can draw many times the current it will draw when it's hot. Motors, when they're first starting also draw extra current. The time-lag of a thermal breaker allows lamps to light and motors to start - before the heating element is hot enough to release the latch, the current has fallen back to the normal range.

    A ground-fault breaker combines both a thermal and a magnetic component. The thermal element operates as a normal thermal circuit breaker. However there is also a magnetic element. Rather than seeing the full load current, however, it includes an electronic circuit that continuously compares the current in the hot and neutral lines, looking for a difference of only a few milliamps. If more current is flowing through the hot than the neutral, it must be getting back to the power plant a different way... possibly through the body of a musician who touched a microphone and his electric guitar at the same time. Sensing that small difference, the electronic circuit energizes the electromagnet, tripping the breaker very quickly, hopefully before the musician fries onstage.

    John
     
  13. zac850

    zac850 Well-Known Member

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    Which do theater dimmers usually use?

    Thanks for that, it makes a lot of sense.
     
  14. DMXtools

    DMXtools Active Member

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    Typically they'll use the thermal type, so they can handle the starting surge of tungsten-filament lamps. There's no need for the added expense of ground-fault protection because theatrical lighting fixtures are typically not meant for hand-held operation and, if properly installed, are well-grounded in any case. An exception would be follow-spots. If dimmable at all, however, the dimmer is internal. Some high-priced units may have built-in ground-fault protection. If not, it's best if the follow-spot is plugged into a ground-fault protected outlet if possible.

    John
     
  15. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Recently I saw a dimmer that had melted most of the fuse holders that protect the outlet for each channel.

    The reason the fuses didn' blow, which resulted in the subsequent damage to the fuse holders and some of the internal wiring was because the wrong fuses were used.

    The correct fuse for this dimmer is a 240V 10A ceramic fuse and someone hade used 12V 10A "car" fuses.

    So - what is the difference and why would this happen?
     
  16. ship

    ship Senior Team Emeritus Premium Member

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    Blown Fuses: Over loaded Fuses will have a clean window as the wire has broken without much heat build up (it wears down.)
    Short circuited Fuses have a darkened window as the wire as heat vaporizes the wire in a short time.
     
  17. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Yes - but the question is what is the difference between a 12VDC 10A fuse and a 240VAC 10A fuse?

    The 12VDC 10A fuses did not blow and as a result, caused damage to the fuse holders and wiring.
     
  18. darkfield

    darkfield Member

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    That's easy. 228VDC.


    Uh, and for say 500w, something about 41+ amps. I wonder why the fuse went out so spectacularly.
     
  19. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Not sure that I can follow your answer.

    The comparison is between two 10A rated fuses:

    The first fuse is a DC rated fuse, 12VDC 10A

    The second fuse is an AC rated fuse, 240VAC 10A

    The dimmer is a 12 channel 2.4K (per channel) 3 phase dimmer that uses 240VAC 10A fuses for the individual channel protection.

    The question is why is it that when (incorrectly) fitted with the DC rated fuse, the fuse did not blow, where the correct AC rated fuse would. Or simply put, what is the difference between AC and DC fuses (given the same amp rating)?
     
  20. darkfield

    darkfield Member

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    Sorry, I was just trying to add some humor. (Then as an afterthought threw in quick abuse of Watt's law, in my hurry skipping over AC/DC factors, and the technical issues you are interested in.)

    I'll stop bothering you now.
     

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