1/8 watt rating resistors ok?

[mindwave_21]

What are 1/8 watt resistors ok for in pedals?  I'm thinking about building a tubescreamer from GGG (my first project), and I thought I would stock up on resistors, so I threw down $10 at electronics goldmine for a box of 1000.  Anyways, the 1/4 watts are in sets of 100 orsomething, and there aren't any of the values I need.  However, they gave me a crapload of 1/8 watt resistors (the tiny tiny ones) that I could use, but I'm afraid of blowing some out.  Any response would be great, as I'm planning to finish the project this week.  In addition, is it generally safer to use 1/4 watt or 1/8 watt for lower values of resistance? (24K or 2.4K or 240, etc.)
Thanks again for the help guys and girls!

[Peter Snowberg]

For 99.5%+ of pedal uses, 1/8 watt resistors are just fine. The major exception is with tube circuits where you really want the additional voltage handling capability of larger resistors as well as the higher current rating.

Take care,
-Peter

[Mark Hammer]

I agree, though for that extra margin of safety I'd probably want to use 1/4W for any current-limiting resistors coming off the battery (e.g., those 100R or 47R resistors you often see between the battery lead and the V+ terminal on a chip?) and also for Zobel networks on a miniamp.  Zobel networks are the resistor/cap combination between power-amp chip output and ground.  They run in parallel with the speaker.  Call me crazy but an 1/8W resistor on the output of a half-watt amp just seems like you're asking for trouble.....albeit on a small scale. :lol:

[zachary vex]

ohm's law tells you the answer to questions of this type.  figure things based on the worst anything can get inside your circuit... for example, consider the possibility that your active components have shorted out and become, essentially, conductors directly to ground.  any resistors feeding them from the 9V battery will be subject to the following rules:

E=IR
P=EI=E*E/R=I*I*R

E is voltage, figure worst case at 9 volts.  R is your resistor... few circuits use values less than 100 ohms for guitar pedals.  let's consider that value...

9 Volts=I*100 ohms.  nothing could get worse than this in a typical guitar pedal circuit.

I=9/470=.019
P=9*.019=.171 watts.  ok, things might get hot for a 1/8 watt resistor (limited to .125 watts average) in this situation, but not very.  and the vast majority of resistors are higher in value than this (in a guitar effect circuit) so the discussion is obviated.  and consider another thing... any 9 Volt battery that is being drained at a rate of 1/8 watt isn't going to last long enough to start an eighth-watt resistor on fire.  8^)  unless it's actually a power supply.  

which leads me to my final point.  if you are going to introduce power supplies to your circuit boards, be sure that the traces leading to and from the DC power jack connections are thick, and use a protection diode of some type... pick a Schottkey if you want to use a series polarity diode to preserve maximum voltage and pick something with a lot of current capability if you want to use a brute-force device like a crowbar to stop reverse polarity.  a hardcore approach (mean-style crowbar) is to put a rectifier from one side of the DC jack to the other which shorts out the power supply if it arrives "wrong way 'round."  this approach doesn't reduce voltage in any way on the way into your pedal.  RG Keen has of course suggested another method using multiple parts on his website, with no voltage loss as well, but i just like the idea of a crowbar.  8^)

[R.G.]

ohm's law tells you the answer to questions of this type...
P=EI=E*E/R=I*I*R
...
P=9*.019=.171 watts. ok, things might get hot for a 1/8 watt resistor (limited to .125 watts average)

Pretty engineer-ey zach 8-) but accurate for the 470 ohm resistor you calculated for.

The other useful form of that is P = V*V/R. So for the 470, P = 81/470 = .171W  The 100 ohm resistor is P = 81/100 = .81W.

Resistor ratings are based on surface temperature and the ability of the resistor body to get rid of heat. For small cylindrical epoxy coated things, the normal limits are that the resistor can dissipate its full power rating up to 75C ambient, and after that it loses 50% of dissipation ability for each 100C rise. As a power supply designer, I never spec'ed a resistor at less than twice the actual dissipation, but not many pedals go over 75C (167F) inside, so we're OK using 100% dissipation for most things.

The resistor that's 1/8W at 9V is R = V*V/P = 81/.125 = 675, so any resistor 680 ohms or over *can't* overdissipate 1/8W or more with a 9V battery unless you're doing power supply gymnastics somewhere in there.