Silly newbie resistor question

[Harry Bissell]

Hi Michael (et al..)

It depends on the voltage arcoss the resistor, and the value (ohms) of the
resistor.

Lats assume an absolute worst case... with +-15 volt supplies... there could be
30V across a resistor (this is a silly case, as almost ALWAYS there will be no
more than 15V across the resistor... )

So we have to keep the power in the resistor below .125W. In real life a 1/8w
resistor
running 1/8 watt will get REAL HOT... so if you want it cool you need to derate
to about .8w max...

Power = .8
Power = Voltage Squared / Resistance.
.8 = 30^2 / x
.8 = 900 / x
x = 900 / .8
x = 1125 ohms

So any reisitor larger than 1125 Ohms is absolutely NO PROBLEM..
If you remember that most resistors will have only 1/2 the supply across them
(15V)
that safe number drops to 281 ohms.  If there are resistors in series... you
have to do a slightly longer calculation... but if the sum of the resistors is
greater than the minimum, again you are safe.

One more thing... some resistors rated "1/8 watt" that are mil-spec parts
(usually 1% metal film) are sometimes specified with a 70 Celsius temperature
rating. The clue is these
are the body size of the "standard" 1/4w part. These are 1/8w at 70C, but they
are 1/4w
at 25C (room temperature).  If they are the tiny dinky little resistors... they
are really 1/8
watt.

So the advice of the esteemed Mr Perry. (thats ok Paul you know where to send
the check :^) is good advice indeed.  I'd be on the lookout for low value
resistors (250 ohms or less). If they get hot they are too small.

H^)  harry  (the only flames should be on the list... not your PC board !!!)

Michael Moncur wrote:

> I have a pile of brand new 1/8W resistors lying around, and was planning to
> use these in a few projects, but I noticed that most of the plans I have
> around specify 1/4W.
>
> So, I know there's no simple answer to this question, but can I substitute
> 1/8W in any of these cases? Do synthesizer circuits frequently have more
> than 1/8W running through any particular resistor? (With the exception of
> power supplies and power amps?)
>
> I did try looking at the schematics and calculating wattage, but I didn't
> come up with any reasonable answers, so apparently I need to hit the books
> again.
>
> --
> michael moncur   mgm at starlingtech.com   http://www.starlingtech.com/
> "Basic research is what I am doing when I don't know what I am doing."
>                  -- Werner von Braun
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