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Voltage ratings on components

Discussion in 'Electronic Basics' started by Midnight Oil, Oct 7, 2005.

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  1. Midnight Oil

    Midnight Oil Guest

    I have a question about the meaning of voltage ratings on electronic
    components. Does the voltage rating mean it is the maximum voltage you can
    apply to a device before it melts down? Or does it have a more subtle
    meaning?

    For instance, if I have an LED with a 5V rating on the package, do the
    put that on there to say "If you apply more than 5 volts of pressure on
    this component, it will 'snap' inside and all the magic smoke will come
    out."

    Or, is there another meaning behind it that engineers use?





    The Moon is Waxing Crescent (18% of Full)
     
  2. PeteS

    PeteS Guest

    Voltage ratings are in the 'It depends' category.

    For most devices, you should read the data sheet for the specific
    statement of just what the voltage rating implies. For some parts, the
    rating is understood to mean specific things.

    The voltage rating of an electrolytic cap, for instance, is usually
    taken to mean it's rated 'Working Voltage' - i.e. the voltage it can
    withstand continuously in normal operation (although I would never use
    a 10V electrolytic in a 9V system...)

    For semiconductors, it may mean the normal (highest) rated working
    voltage, or (depending on context) it may mean the absolute max rating.


    So the answer is - find a voltage rating that you are unsure of, read
    the datasheet, and if it's still not clear, ask - someone here should
    know what it means.

    Your LED rating may refer to either it's maximum forward voltage drop
    (probable) or it's reverse voltage rating (possible but unlikely).

    Cheers

    PeteS
     
  3. For some components (Transistors, Capacitor, resistors...) the voltage
    rating is a maximum recommended operating voltage.

    For ICs, the data sheets will often list a recommended operating
    voltage and a maximum voltage beyond which the device may be damaged.

    For light bulbs and relay coils (and probably your "5 volt" LED) the
    stated voltage is the recommended operating voltage. (Normally, a bare
    LED requires a current limiting resistor in series - an LED spec'd at
    5 volts probably has an internal limiting resistor, suitable for
    operation on 5 volts.)


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  4. Tim Williams

    Tim Williams Guest

    Maximum, *recommended*? You sound like you're talking about TOOBS!
    That's more like it.. ;-)

    Yeah transistors should be lumped here.. I have yet to meet a transistor
    that doesn't explode over its limit (or at least act like a zener, i.e.
    avalanche effect).

    To the OP: for things like resistors, you have three intersecting limit
    curves, depending on wattage and value. The straight bounding limits are
    current (I don't know what a typical 1/4 or 1/2W resistor is rated for) and
    voltage, which depend on the physical size of the device as an absolute
    rating. For instance, 1/4W resistors are rated as 250V or so, while 1/2W
    resistors are good for 400V and 1Ws are good for 600V. But in almost all
    cases, the curved limit of power wins out: for example, a 100k 1/2W resistor
    is at maximum dissipation at 224V, well below the voltage limit for that
    package.

    What you get is, for resistors of yea wattage within whatever range of
    resistance, you only have to worry about power. For resistors below some
    fractional value, the current limit goes into effect. For values above some
    large value (for 1/2W, around 330k), the voltage limit is imposed before you
    can reach full power.

    For values outside of the dissipation-limited zone, you can still burn out
    the resistor by exceeding the limit, BUT, because the resistance is so low
    or so high, you can never technically "burn" it out (by which I mean, exceed
    dissipation rating).

    And all this applies moreso for semiconductors, which are often, in effect,
    variable resistors. ;-)

    Tim
     
  5. Jasen Betts

    Jasen Betts Guest

    that's pretty much what it means
    on a LED that's probably the maximum reverse voltage the manfacurer
    guarantees it for, I've seen LEDs witstand as much as 12V (in the reverse
    direction) without failing.
    it's basically the design limit of the part.
    all good parts will exceed the design limit by some extent. those that don't
    are often relabled and sold as a less robust part.

    Bye.
    Jasen
     
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