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

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

1. ### tempus fugitGuest

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?

Thanks

3. ### Tom LeMenseGuest

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:

http://www.vishay.com/docs/31015/ccf5560.pdf

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!

TJL

4. ### tempus fugitGuest

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.

Thanks