1/2 watt resistors from Mouser look like 1/4 watt?

[pjd3]

I just opened a package from Mouser with components for my high voltage FET series FX loop - I'm building a 2204 with mods.

Per usual I look through each bag for curiosity and assessment of parts and there were a couple of resistors that definately did not look any larger than typical 1/4 watts that I've been looking at for over 20 years! (all ordered were minimum 1/2 watt). Are they beginning to make resistors smaller that can handle more electrical work? Let me find the ones and I'll tell you the values, hold on:,

OK, they are both Vishay metal film.
One is a 10 ohm,
and the other is a 110K ohm. Odd value but, thats what the BOM called out for.

Now I'll get on Mouser website and plug in their manf #, just a sec.....
So yeah, the 10 ohm, 1/2 watt measures the dimensions Mouser states - L = 6.2mm, dia = 2.29. Its right about that.
And the 110K ohm is about the same, looks even a tad smaller but, I guess at least Vishay/Dale has formulated a way to get smaller resistors with the stated power,

Thanks for hanging in with me on this, I've been doing electronics at home and work and never really noticed a 1/2 watt resistor that was the same physical size as a traditional 1/4 watt. I have learned yet another thing tonight.
Thanks all and best,
Phil

[bepone]

i have also noticed , metal oxide 1w vishay is looking like classic 0.25w ressitor.. so i dont like this and started to order them with bigger power like 3w to pass visual acceptance

[thetragichero]

yup been there, done that. got schooled in "remember to check the datasheet dimensions section" by the nice lady on the phone from mouser

[roberto]

IIRC it considers a different standard.
EG if you drop the resistor in liquid nitrogen, you can declare it 100W

[FUCHSAUDIO]

I avoid these. I had 50 amps globally that had "1/2 watt" plate loads open up...these claim to be half watt, but they opened up, they didn't burn.
They are fine for grid leak or audio tone circuits etc. Anything running voltages or current through them, run away....

[Mac Dillard]

FWIW I don't use 1/2 watt resistors. Nothing less than 1 watt.

[roberto]

I agree. I’ve learned from Mike Soldano experience with a certain lot of SP77, and from Orange amps around 2010 to use 1 W resistor on plates (this applies as well on CF cathodes).

Some of them was failing because of this exact reason.

[maint_tech]

Lower voltage rating, again check data sheet?

[mhuss]

Yep, part of the characteristic that determines voltage tolerance is physical size. Always check the data sheet.

[Mac Dillard]

There have been several discussions about the ratings of resistors over the years. Most builders only look at the power (watt) rating of the resistor and not the voltage rating. They look at the drop across the resistor not the applied voltage...We get away with this because in most of the amps we look at the voltage is relative low. The power supply and the power tube screen resistors are generally the only ones that will see over 350volts.And most 1/2 watt resistors have a voltage rating of 350. Generally 1 watt resistors are rated 500 volt. Last time I was in this discussion I finally just gave up.

[pjd3]

Yeah Mac, I can see your point, wattage/power is a product of voltage and current that doesn't insinuate a value onto either.

So, in that case, knowing the characteristic of your amp sections would or could dictate decisions on the voltage and/or current specs of the resistors. Hadn't really thought about that much in the context of guitar tube amps for some nebulous reason.

I would also expect claims about materials and how they have developed over the years to be brought up just to further cloud the waters! Thats Ok.
Well, this had inspired me to take a closer look at the electrical specs of resistors. I can also just play it safe and always order 1 Watt resistors for everything (if they fit).

Thanks for coming on by,

Phil D

[R.G.]

To a resistor manufacturer, the power rating is just that - how much power can it dissipate and still have a long enough life? The other things like voltage rating and even more subtle, impulse power rating, are separate items.

IN the manufacturer's view, the power rating is that number of watts that keeps the resistor temperature below the value that degrades the interior. In the days of only carbon comp resistors, the carbon, binders and shell/leads had a very fixed temperature limit, and our concepts of what the size of a 1/4, 1/2, 1, 2 watt resistor was set by that limit. Carbon comp nearly died off, being kept in manufacture largely because CC has a large impulse power rating, bigger than other similar-rated resistors. In the pro side of the electronics biz, there was a move to MOX to get similar impulse ratings a few years ago.

CCs used to be rated for the power to raise their surface to 100C, largely on the degradation of the shell and binders above that. If you use materials that can stand 150C to 200C and get higher power in a smaller, less material, and potentially cheaper device that ...may... even last longer within the data sheet ratings. But the voltage and impulse power ratings may be lower...