Newer resistors

[Valvehead]

Ive noticed some brands of resistors in the last couple years have "shrunk in size"

as in a 1/2 watt rated , now looks like a 1/8 watt ...super tiny

I buy most from Mouser.

Is it that they are just more efficient now.. or ?

[Justin Thomas]

Probably for lower voltage, too... I'm mixed in how I feel about the size change. I don't mind the smaller body IF they're truly rated at what they say they are but I REALLY don't like the super-skinny leads that they come with. Kinda sucks for PTP, and I'm not paying for NOS CC, because they drift like mad!

Justin

[The Dude]

Yes, and these days you have to be more careful about filing them. You can't always judge by size anymore (in situations where wattage is more critical).

[R.G.]

A resistor's power rating is dependent on the materials it's made of.

I had to not only sit through a fluid mechanics and heat transfer class, I had to make a passing grade, so this stuff got forced in.

If something dissipates a quarter watt, how hot will it get?

There is no way to know. The temperature of any object depends on two things: how much heat is either coming into the object or generated there, and how fast it can get rid of it.

For instance, if you have a resistor rated at 342 ohms, and feed it 0.42W, how hot will it get? If it's a grain of rice bulb rated for 12V and 35ma, the answer is about 5000 degrees F. That's because the bulb is set up to make it hard for the heat to conduct or convect away, and all it can do is radiate, so the temperature rises until the photon radiation equals the incoming electrical heat. If something has heat fed into it or generated inside it, its temperature will rise WITHOUT LIMIT until the sum of radiation, conduction, and convection heat transfer equals the incoming/generated power.

By now I can see your eyes crossing, and thinking "What is he droning on about and what has that to do with resistors?"

If your resistors burn up when their surface gets to 100C/212F, you must make them big enough so that they can dump the rated heat without their surface being any hotter than that, minus some safety margin to keep you from going broke from warranty costs.

That's a profound statement that is a valid answer to your question. As resistor makers have found better materials than paper/phenolic tubes filled with a mixture of glue and charcoal particles (that is, carbon composition), they have been able to raise the surface temperature the user can operate the resistor at, and hence the "maximum power" rating of the resistor. A resistor that can dissipate, say, 1/2W without either burning up, being permanently damaged by the heat, or being so changed in resistance by the heat that it's out of tolerance and also lives a reasonable service life is a 1/2W resistor. There is no other good definition of a 1/2W (or any other power) resistor.

So the answer to your "shrunk in size" is: they've found better materials that let them run the resistors hotter without drifting or being damaged. This lets them put less material into the resistors and probably sell them cheaper. It's likely that they're also tougher and more reliable, since high temp materials tend to fail less often. Many resistors are a film of either carbon or a metal film or oxide on a ceramic core. Tough stuff indeed.

[g1]

Probably for lower voltage, too...

I would agree. I think there would be a more direct correlation between size and voltage rating. Something that should be taken into account with tube amps, probably doesn't matter much for SS.
I don't recall ever even hearing about voltage ratings of resistors till they started getting small, maybe around the advent of 1/8th watters?

[Tom Phillips]

...I don't recall ever even hearing about voltage ratings of resistors till they started getting small, maybe around the advent of 1/8th watters?

Turns out that the voltage rating was always there. Maybe not in the re-seller's catalogs but in the full manufacturer's data sheets.

I agree with Justin that the leads on the currently available axial components are really lacking when you want to use them for hand wired circuits.

Another factor with the small bodied devices is that, as R.G. explained, a given dissipation produces a given thermal output. Therefore, if two resistors are dissipating the same amount of power, a smaller resistor get hotter than a larger resistor. If this is taken too far then the concentrated hot spot produced by the smaller device could cause problems for surrounding components if they are mounted too close. It can even cause deterioration of PC board material and the solder connections holding it on the PC board.

[mhuss]

...the leads on the currently available axial components are really lacking when you want to use them for hand wired circuits.

I think we all can agree there. I've been using 1W CF for turret/p2p work, because they are similar in size to the old 1/2 watt ones, and the leads are a bit sturdier.