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6.81K resistors for phantom power - 1/4 or 1/2 watt?

Discussion in 'Electronic Basics' started by tempus fugit, Aug 19, 2005.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I am building a mic preamp with phantom power. I ordered all my parts,
    including the current limiting resistors. These were supposed to be rated
    1/2 watt (it said 1/4 watt, rated to 1/2 watt in the Mouser catalog), but
    when I got them, they looked more like 1/4w than 1/2. Do these need to be
    1/2w? By my calculations, the power dissipated by them will be .33w, which
    suggests that I need 1/2w resistors. Can I get away with the ones I've got,
    or do I (argghh...) have to order some 1/2 watters?

    Also, does anyone have a clue what "rated to 1/2w" means?

  2. tempus fugit

    tempus fugit Guest

  3. Tom LeMense

    Tom LeMense Guest

    That particular resistor is a "dual-rated" type - if you apply it as you
    would a 1/4W part, it meets tighter specifications than if you apply it as
    you would a 1/2W part.

    The "Performance" chart on the last page of the datasheet gives some idea
    what this means:

    In other words, to put words into Vishay's mouth: if you're looking to
    design something with a very high MTBF/MTTF and has to meet a whole host of
    environmental tests, than you should not use the CCF55 series if the
    application will result in Pd above 1/4W. On the other hand, if the exact
    value of the resistance isn't critical and the device is serviceable, go
    ahead and use the CCF55 as you would a 1/2W unit.

    Because your application will result in 1/3W, you're somewhere in between
    these two extremes. Chances are it'll be OK.

    Finally, note that ALL resistors have a thermal derating curve, so if your
    ambient temperature is greater than 70C, you'll need to re-think using this
    part. See the "Derating" chart on the same page. Operating voltage also
    leads to derating, but this particular datasheet doesn't include a voltage
    derating chart.

    Good luck!

  4. tempus fugit

    tempus fugit Guest

    Thanks for your reply Tom. That helps clear some things up.

    I guess I also have to consider the fact that the mic itself will have some
    resistance as well; most mics only draw 3 - 10 mA of current anyway, so it's
    not like the 6.8k resistors are connected to ground or anything, although if
    there was a fault in the cable that possibility would exist.

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