help with resistor size

[mikepukmel]

Hi All,
I ordered 1 watt resistors, 220k and 1.5k. They arrived and they are tiny!

0.098" Dia x 0.256" L (2.50mm x 6.50mm)

I checked the order page, and its the part i ordered

Manufacturer
Vishay BC Components
Manufacturer Part Number
PR01000106800JR500
Description RES 680 OHM 1W 5% AXIAL

These are metal film, with a coating.

Are these OK to use in a tube amp where 1 watt resistors are required? All of the other 1 watt resistors Ive seen are huge, 4x larger or more.

Thanks,
MP

[Tom Phillips]

Modern resistors are built with materials that can safely operate at higher temperatures. Therefore, it is possible to build a 1 Watt (for instance) resistor that is physically smaller than those available in the past. A down side is that the smaller resistor will get hotter, when dissipating 1 Watt of energy, than a physically larger resistor. I.e. the same amount of energy dissipated in a smaller space equals a higher temperature within that smaller space. This is a factor to consider when the resistor is mounted near other heat sensitive items.

The power rating of a given resistor is based on how much energy it can dissipate without damage to itself. Although the 1 watt rating is specified at a certain ambient temperature, the temperature of the resistor itself is not normally part of the resistor's spec sheet. Those kinds of considerations are up to the circuit designer to deal with. What you will find is a de-rating curve that will tell you how much to back off the power rating if you plan to use the resistor in a hot environment.

[eschertron]

I recently ordered some 1R 1W resistors (likely the same MFGR, part series) and they are indeed small. No bigger than some 1/4W CF resistors I got when Radio Shack was downsizing. I haven't used them yet, partly because the small size is alarming when compared to a MO or wirewound resistor of equal wattage. I'm glad I'm not the only one to receive a cognitive 'shock' from seeing them!

[mikepukmel]

Thanks everyone. I'm remembering from physics what you mentioned, Tom, that basically the surface area of the thing is where a lot of the heat is dissipated from. Thinking that if a big honking old style 1 watt gets as hot as I remember (accidentally bumping my knuckle on one when it as hot) I can't imagine how hot this teeny thing will get. Its not a heck of a lot bigger in diameter than the lead wire.

Another bad thing for me anyway, is that I need the size cue to tell me what the wattage is! Geez if a circuit board is loaded with these things, I have no idea what the heck power rating is on any resistor. The 1/2 watt metal films I got, different series are several times the size of these.

Thinking I should put these in the parts box, save them for some other usage, and get some physically bigger units, even if its only so tat I can make sure I put higher watt capacity resistors in the right spot on the circuit. We're not building clock radios!

[mikepukmel]

Looking for the data sheet...

[Dave H]

Looking for the data sheet...

Vishay PR01

[mikepukmel]

Hmm. from the data sheet, these units drop from 100% rated power dissipation at 70 degrees C, to below 80% rated power dissipation at 100 degrees c. So 212F, that's pretty hot, but with 6 vacuum tubes, upside down radiating and conducting heat into the *closed* chassis, with no air circulation, in, say, a night club or practice room, with poor air circulation anyway. . . I bet it could get well over 150 degrees F inside the chassis no problem. Hmmm.

[mikepukmel]

Thanks Dave.

[Leo_Gnardo]

I ordered 1 watt resistors, 220k and 1.5k. They arrived and they are tiny!

Considering the functions you'll be using them for, they will hardly be generating any heat of their own. Stop worrying & start building. Plenty of commercial amps made with these & even smaller surface mount resistors are working fine day after day.

If you like big size resistors consider 1 watt carbon films. AFAIK they are still the size of a carpenter ant, plenty big.

[nosaj]

Considering the functions you'll be using them for, they will hardly be generating any heat of their own. Stop worrying & start building. Plenty of commercial amps made with these & even smaller surface mount resistors are working fine day after day.

If you like big size resistors consider 1 watt carbon films. AFAIK they are still the size of a carpenter ant, plenty big.

We're not building clock radios!

Nor are we building Rocket ships.

But here seems to be a good example for him to use Ohms law. If it's a 1 watt resistor using Ohm's law will tell you is it will be sufficent or not.

Me I always like ordering the Dale resistors look like a small brown hotdog with 2 leads because you could get them smaller with quality materials.


Of course I like to do scrap builds with reclaimed materials and I have no hint of perfectionism.

Don't know who said it but Perfectionism gets in the way of getting anything done.
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nosaj

[Justin Thomas]

But how does the supposed voltage handling spec come into all of this? I see those 1W resistors are rated for 350V? And the 2W only for 500V. I've never really paid any attention to the voltage spec on any of my resistors; and nothing has blown up, unless I pump current through it.

Is it really a big deal? I guess I'd be more worried about 1/4W, etc, but I just don't know.

Justin

PS I do either true PTP or on homemade turret boards. Is resistor voltage lime wire, where the spec is for wires touching, and against another part? I've usually got more space between resistors & chassis than is between tube socket pins.

[Enzo]

Hasn't been the last 80 years or so.

Cathode resistors don't face voltage, but plate resistors do, so let's say your 500v resistor is there. To fail that voltage test, your B+ would need to be 500v higher than the plate. I don't usually use 500v B+ in preamps, let alone 500v over the plate voltage. Same with 350v.

[Chuck H]

Hmm. from the data sheet, these units drop from 100% rated power dissipation at 70 degrees C, to below 80% rated power dissipation at 100 degrees c. So 212F, that's pretty hot, but with 6 vacuum tubes, upside down radiating and conducting heat into the *closed* chassis, with no air circulation, in, say, a night club or practice room, with poor air circulation anyway. . . I bet it could get well over 150 degrees F inside the chassis no problem. Hmmm.

The resistor will be trying to dissipate it's own heat into the already tube heated environment. The derate specs don't consider the internal temperature of any collective product, just the temperature of the resistor itself. Since, because of it's size, it will get hotter of itself than a physically larger resistor AND it will be dissipating any of it's own heat less efficiently in a tube amp environment it could well be that those tiny resistors aren't all that much more capable in power dissipation than their similarly sized 1/4 watt counterpart. I ordered those same resistors once. I never used them.

[Dave H]

The resistor will be trying to dissipate it's own heat into the already tube heated environment. The derate specs don't consider the internal temperature of any collective product

The derating graph says they are OK to dissipate 1W at up to 70c ambient temperature before derating. Ambient in this case is the internal temperature. I still think they are too small.

[mozz]

Buy one of those cheap $15 infrared handheld temp guns. They work great and will let you know the real chassis temps. Mouser lists the size and voltage rating on the resistors they sell on the product page.