1/4w resistors for 18v pedals?

[nognow]

is it a good idea to use 1/4w resistors for 18v pedals?
Thanks!

[Mike Burgundy]

Ohm's law.
There's no definite yes or no answer without more detailed questions. You can figure out what the voltage across any resistor is, and the calculate dissipated power.
In a 9V circuit, a 1 Ohm 1/4W resistor will fry if you stick it across the rails. So will a 10W, assuming the power supply can source that current.

[GibsonGM]

Ohm's law.
There's no definite yes or no answer without more detailed questions. You can figure out what the voltage across any resistor is, and the calculate dissipated power.
In a 9V circuit, a 1 Ohm 1/4W resistor will fry if you stick it across the rails. So will a 10W, assuming the power supply can source that current.

Exactly.  Use the formulas.  Make them second-nature.

Let's say you had a 330 ohm collector resistor on a BJT, power supply of 18V.     18V/330R = .055A, or 55mA.     

Now find out the power dissipation (Power, in Watts):  18V * .055 = .99W      Sizzle sizzle, why is there smoke in here?? 

Use I^2 * R,   E*R, etc. and determine what rating you need - awesome practice for the times when it will REALLY make a HUGE difference!   Say on a project you spent 60 hours putting together, and the wrong resistor might cause it all to go up in smoke when you power it up! 

Seems like magic until you do it a dozen times, then it becomes totally natural, trust me.

[PRR]

Put your finger in a 9V pedal while it is on. (You should disconnect your amplifier...)

Is there ANY warmth in the 1/4W and 1/8W resistors?

Usually not a darn bit of warmth. Often dissipating 0.01 Watts or less.

At 18V they will run four times warmer. Still usually not an issue.

*Usually*. Apparently Mike can put a 1 Ohm directly across the power rails. In Guitar World, where we mostly drive 50,000 Ohms and higher, there's rarely a need for such huge power. However there ARE oddballs. You SHOULD poke a few numbers and get a sense of what is going to happen.

And there's always the cave-man approach. Use 1/4W, power-up. If something smokes, replace with 1/2W, then 1W, until you find a big-enuff resistor OR prove that you done something wrong that needs to be fixed.

[nognow]

Put your finger in a 9V pedal while it is on. (You should disconnect your amplifier...)

Is there ANY warmth in the 1/4W and 1/8W resistors?

Usually not a darn bit of warmth. Often dissipating 0.01 Watts or less.

At 18V they will run four times warmer. Still usually not an issue.

*Usually*. Apparently Mike can put a 1 Ohm directly across the power rails. In Guitar World, where we mostly drive 50,000 Ohms and higher, there's rarely a need for such huge power. However there ARE oddballs. You SHOULD poke a few numbers and get a sense of what is going to happen.

And there's always the cave-man approach. Use 1/4W, power-up. If something smokes, replace with 1/2W, then 1W, until you find a big-enuff resistor OR prove that you done something wrong that needs to be fixed.

Ohm's law.
There's no definite yes or no answer without more detailed questions. You can figure out what the voltage across any resistor is, and the calculate dissipated power.
In a 9V circuit, a 1 Ohm 1/4W resistor will fry if you stick it across the rails. So will a 10W, assuming the power supply can source that current.

Exactly.  Use the formulas.  Make them second-nature.

Let's say you had a 330 ohm collector resistor on a BJT, power supply of 18V.     18V/330R = .055A, or 55mA.     

Now find out the power dissipation (Power, in Watts):  18V * .055 = .99W      Sizzle sizzle, why is there smoke in here?? 

Use I^2 * R,   E*R, etc. and determine what rating you need - awesome practice for the times when it will REALLY make a HUGE difference!   Say on a project you spent 60 hours putting together, and the wrong resistor might cause it all to go up in smoke when you power it up! 

Seems like magic until you do it a dozen times, then it becomes totally natural, trust me.

Ohm's law.
There's no definite yes or no answer without more detailed questions. You can figure out what the voltage across any resistor is, and the calculate dissipated power.
In a 9V circuit, a 1 Ohm 1/4W resistor will fry if you stick it across the rails. So will a 10W, assuming the power supply can source that current.

Thanks!

[GibsonGM]

Everyone is right....best way I've found is to try to locate the highest current going thru your circuit, off the schematic, by eye.   Do the quick math, you should have a good idea what's going on!

[Seljer]

P = U^2 / R
R = U^2 / P

so with 18V and 1/4W

(18volts)^2 / 0.25watts = 1296 ohms

anything higher than that you don't have to worry about it at all

smaller than that, then observe what it's doing in the circuit (it's most likely not being shorted directly from the positive supply to ground  ) and do the math to see if you need to worry

[Mark Hammer]

1/4W for any resistors directly connected to the power supply, but everything else is likely fine at 1/8W.  If you look at most of the SMD boards coming out of the major manufacturers, for pedals that run on 18v (either charge pump or external 18-24V supply), they will be those teeny tiny resistors that are likely less than 1/8W...except for the power-supply-connected ones.

[Johan]

If you know the power consumption of the box you're building, nothing inside that box will eat that power alone. ..say 9 volt and 10mA ( pretty typical) thats 0.09watts. 1/8=0.125watts, so 1/8 should be fine.
9 volt 25mA=0.22watt. If all you've got in there is a single resistor, a 1/4 is good to go but perhaps a little too close for comfort. .
J