1/8 vs1/4 watt resistors

[Stephen]

Notice lately all of the BOSS pedals use 1/8 watt resistors.....

Heck they are small and why not use them in your pedals...?

You can put three where one 1/4 watt will fit..

INTERESTING

 

[soggybag]

I think it depends on how much current goes through them. In some spots you will need a 1/4 watt resistor. I'm not sure how to figure this, maybe we can get an expert to give us some enlightenment?

[R.G.]

Why not indeed?

The only reason is if they would burn up, or are too small for other reasons.

The rated power of a 1/4W resistor is 250 mW, and a 1/8W is 125mW. At those powers, the resistors will have a surface temp of about 100C in most cases. It is good practice to derate the usable power by 50% down to 125mW for a 1/4W resistor, and 62mW for a 1/8W resistor. That's still a goodly amount of dissipation.

Where you have to watch 1/8W resistors is on things like LED current limiting or power supply dropping. An LED using 20ma of current from 9V has to waste 7V of that in a resistor. That's 7V times 0.02A, or 140mW. A 1/4W resistor is marginal for conservative design, and a 1/8W is being overdissipated and will not last for a long life.

Cutting the current to 8ma with a high brightness LED lets you get the dissipation down to 56mW and the 1/8W will do OK.

If you are using 1/8W resistors on a PCB, you can't realize all of the savings of space, because the short body lets you bend the leads down to 0.3" centers, and that means fewer traces will go under the resistor. It's a fine point for PCB artists, I'll admit. You can still bend the leads at 0.4" centers for the smaller resistors, but then you don't get the advantages of the smaller size.

It's a tradeoff. You have to know the numbers.

[GibsonGM]

Ya can, but you'd best know how much current is flowing thru that branch before you try it, LOL!  Wattage is power, defined by P=I * E.   Anywhere there's 9V in the circuit and you have more than 14mA, you'll fry the resistor...1/8 = .125,  .125/9 = .014, or 14mA. Depending on the value of the resistor....

    Of course, some parts of an effect (almost all) have much less than 9v, but my thought is the designer used 1/4 watt, for a reason!    If you like, you can do the math and change them out, tho.  Get the current thru the resistor (I=E/R), get the voltage, and do the math.  More than .125 kills the resistor.
I've tried it, he he...burned me fingers...