fuse: i thought the bigger the better??

Discussion in 'General Electronics Chat' started by vadimo, Aug 14, 2008.

  1. vadimo

    Thread Starter New Member

    Jan 20, 2008
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    Hi Guys,

    I was helping out my mate to deal with some CCTV repair jobs, i came across faulty camera which i discovered had blown fuse in its PCB circuit, however the power source supplying 27AC was live. While power connected to the camera, I removed faulty 1.6amps glass cartridge type and replaced it by 4amps (the lowest i had) then next second some micro component on the PCB set itself on fire and blew up with smoke, it of course blew my 4amps fuse too - i was shocked by what just happened. Nothing to loose i disconnected power from the camera, installed another 4 amps fuse, connected power and cameras worked like new.

    My friend suggested that the blow was due to oversized amperage fuse used which probably upset that particular part of PCB.

    So my question, is that possible? If so, can you explain it to me? Please educate!

    :confused: :)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    Well, you now know for certain what was causing the fuse to blow in the first place :eek: except that's not the usual way to find it.

    You should never use a fuse of greater amperage than is specified by the manufacturer. The idea of having a fuse is that it is the "weakest link" in the electrical circuit. In case something goes wrong with the circuit or the power supply, the fuse melts rather than components in the device self-destructing, or the wiring to the device catching on fire.

    The device that went up in smoke may very well have been a voltage regulator that had gone bad or perhaps a shorted capacitor, and now the rest of the device will have a short life span due to the lack of that component.
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    The original fuse blowing was probably due component failure somewhere within the power-supply causing a short circuit. When you put the 4A fuse in the short-circuit was still there but rather than the fuse blowing this time the failed components burned - no more short-circuit.

    However, there is still a problem even if it does appear to work OK at the moment.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Fuses are safety and protection devices. If the circuit is working correctly it won't matter, but the blown fuse is an indictation there is probably something more serious wrong with the camera. Putting a bigger fuse usually allows the fault to progress to a greater level of damage. What sounds like happened here is a power supply capacitor had shorted, and the original fuse did its job. The second fuse allowed the capacitor to finish burning out. You were lucky, many times these devices catch on fire when this happens (which is why there was a fuse). Having burned itself out, the capacitor was effectively removed from the circuit, so the 3rd fuse allowed the circuit to power up normally. Of course, if something else ever goes wrong that fuse will allow the circuit to fry itself to oblivion, and the camera will be scrap. I guess the counter arguement would be most electronics is thrown away when it dies anyhow, so tain't no big deal.

    ***********

    Looks like 3 people jumped on this post at the same time. :D
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Yeah, we did :D Looks like we all agree on something in the camera's power supply failing, too - to get such a rapid consensus on the board from three experienced individuals separated by quite a distance (not sure where Blocco is) is somewhat unusual.
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    The fuse didnt blow because it had a larger value but because there was a fault in the circuit. Its not good to replace the fuses with higher value fuses because you may damage other parts of the circuit if something goes wrong.
     
  7. vadimo

    Thread Starter New Member

    Jan 20, 2008
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    thanks for that guys, i guess now i will be more considerate about fuses, especially when working with old equipment.

    Last question; if i would of disconnected power from camera before removing old fuse and placing new one in, would that might of prevented the blow? I dont think it would, but still gotta ask.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Probably not, but replacing a fuse in a live circuit can put you in a position of getting a lethal shock. That 27 VAC is probably not likely to do you in, but you are the only conduction path when fumbling with a fuse. Never touch live power supply points.
     
  9. vadimo

    Thread Starter New Member

    Jan 20, 2008
    3
    0

    yes i agree, even when its low voltage ac/dc i try to avoid touching it, if i had to i would of disconnected each wire one by one into and placed it into block connector, i have never been electrocuted and planning on to keep it that way.

    thanks for advice.
     
  10. Doktor Jon

    New Member

    Jan 22, 2008
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    Very good advice all round :)

    The only other thing that I'd quickly mention in passing, is that as well as using the correct value for the fuse (never larger, as already agreed all round), it can also be important to fit the correct type.

    So for example, where a circuit may pull additional current in starting up, but then settle to a lower operating level, it's not unusual to find an "anti surge" fuse fitted which can cope with this type of situation. If a quick "blow fuse" were fitted by mistake, it may well blow even though there is no fault present in the circuit.
     
  11. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Yes, you are right. If you put a not anti-surge fuse into such a circuit it will blow when the current surges initially even without a fault in the circuit.
     
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