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Why seperate overvoltage and overcurrent protection?

Discussion in 'Electronic Design' started by Ultimate Buu, Sep 8, 2003.

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  1. Ultimate Buu

    Ultimate Buu Guest

    Most telecoms equipment have both fuses and thyristor like device for
    overcurrent and overvoltage protection respectively. Why is this? Most
    overvoltage situations will result in overcurrent as wel, right? Unless the
    device isn't connected, I guess, and an open circuit exists.
  2. A fuse blows relatively slowly. It helps keep a device from catching on
    fire in the event of an internal or downstream failure that causes it to
    draw excessive current. It can also help ensure that in the event of an
    internal failure, a device does not present a shock hazard. It does not
    often protect a device from getting damaged; it just keeps the damage from

    An MOV or other overvoltage protector triggers very quickly. It helps
    protect the device from getting damaged by external factors such as power
  3. A crowbar circuit is a nice feature with a linear pass regulator. If the
    pass element shorts, the full unregulated input voltage can appear across
    the output and possibly destroy expensive circuits. Crowbars are often SCRs
    which are triggered when the regulated output goes to some predetermined
    level. To summarize, a crowbar protects loads from a regulator failure and
    the overcurrent circuit protects the power supply in the case of a shorted
    load. Having both is a good idea.
  4. Tom Bruhns

    Tom Bruhns Guest

    Cuz by then it's too late?? The current may only go up slowly with
    increased voltage, depending on the device. The fuse is likely rated
    for quite a bit more than the nominal current, to allow for starting
    surges, etc. You likely would have to get the voltage to levels that
    would destroy the semiconductors (or other parts) before the current
    became great enough to blow the fuse. OTOH, if you take it that high,
    it's pretty likely to blow the fuse.... ;-)

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