help with resistor size

[Mick Bailey]

Where voltage could be a consideration is with bleed or balance resistors across PSU caps. In that case they're across the supply (or half the supply if they're balancing resistors).

[mikepukmel]

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.

I guess I can stick with these and see what blows up. Got through the data sheet a few times. There are drawings in the data sheet showing horizontal mounting. They crimp the leads so that the body of the resistor sits up off the board about a body diameter or so. Must be to let air circulate underneath. With these resistors I'm guessing that we'll want to keep them from being pushed up tight against other components as well, which is easy since the cap's are so small compared to the old huge cardboard can electrolytics.

[mikepukmel]

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.

Thanks, I didn't know they were so cheap, its been ages since I looked at cool stuff like temp measuring devices. I spent a lot of time looking at voltage ratings, power ratings, tolerances and temp induced component drift, noise, ... but I didn't even look at the physical dimensions!!! Akk.

[g1]

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.

Seems to me that larger sized resistors usually have higher voltage ratings, so another benefit of bigger 'style' resistors.
I agree with Leo about the 1watt carbon films. Thicker less flimsy leads are another bonus.

[Randall]

I hate these small 1W resistors. I bought a bunch through Newark a few years ago for some 5E3 builds, and they were so small there was no way I was going to install them, they just looked stupid on a fiber board, plus they are shorter, so they really didn't even fit. I always go up in wattage, so Fender builds and Repairs get 1 or 2 watts. I also like the carbon film big resistors, and I can see the color codes easily, not a small thing at my age!

[mikepukmel]

I hate these small 1W resistors. I bought a bunch through Newark a few years ago for some 5E3 builds, and they were so small there was no way I was going to install them, they just looked stupid on a fiber board, plus they are shorter, so they really didn't even fit. I always go up in wattage, so Fender builds and Repairs get 1 or 2 watts. I also like the carbon film big resistors, and I can see the color codes easily, not a small thing at my age!

I guess if I had more experience, I would give them a shot, but its my first build and Id like this one to work (need the confidence builder). Leo suggested to give them a shot, but I don't have the electronics confidence to try them. Jayzzzus they *are* tiny! And the wording on the data sheet is a little scary for me anyway.

So... I re-ordered more standard resistors, at least not these tiny mouse dropping size things. I still find it really hard to believe that PR01, PR02's would dissipate 1 or 2 watts, given the body size and lead diameter.

[mikepukmel]

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.

[ATTACH=CONFIG]44212[/ATTACH]

Dave Im more concerned about the comments about degradation on the data sheet, and comments about solder!

[Enzo]

Just my opinion:
People discover a spec and then their world centers around it. If you were never told of this spec, of this data sheet, you would walk into Radio Shack, buy plain old "resistors" off the rack, and build something with them. And it would work just fine.

What is your specific concern about "degradation"? Note the data sheet says "derating" which is very different. The Vishay chart you show says 100% up to 70 degrees. That is 70 degrees centigrade, not Fahrenheit. 70 degrees C is about 158 degrees F. You get them hotter than that and they cannot cool themselves sufficiently, so you cannot work them as hard. That is what derating means. But the 50% line is about 135 degrees C. That means if things heat up to 135C, then we can only "rate" the part at half its given rating. That is 275 degrees F. That is darn hot, and not likely on your basic tube circuit.

The new parts can run warmer than the old. WHat they are saying is that these resistors will dissipate 1 watt out from their little bodies and be OK, whereas older technology parts would be overheated. Older types needed to be larger to safely dissipate a full watt.

others have said it, higher wattage parts might be more convenient to use for their size. Longer wires fit the eyelet boards better and such.

You mentioned 220k and 1.5k. That looks to me like a plate and cathode resistor for a preamp. Hopefully my math is correct here. 1ma is a typical tube current. 1ma through a 220k resistor means 220v dropped across it. SO a 400v B+ would leave 180v on the plate. 1ma and 220v means dissipation is under a quarter watt. Since the same current flows through the cathode resistor, a 1ma current through 1.5k dissipates only a milliwatt or two. That is why those parts in Fender amps were 1/2 watt for decades.

[rjb]

...you would walk into Radio Shack, buy plain old "resistors" off the rack, and build something with them.

Not likely nowadays.

I live in Glen Burnie, just south of Baltimore, MD.
A few years ago, there were 3 Radio Shacks within less than 10 miles.
Just went to their website and entered my zip code in the store finder.
These were the top 2 results:

BOYD KING ELECTRONICS
436 SOLOMONS ISLAND RD N
PO BOX 408
PRINCE FREDRICK, MD 20678
42.87 MI

LA PLATA TV & APPL INC
5895 CRAIN HWY
PO BOX 428
LA PLATA, MD20646
47.39 MI

[Enzo]

Just a metaphor. Get yourself some "resistors" instead of "linear copper free iso-carbon sintered non-inductive temperature compensated resistors".

[rjb]

More Off-Topic

Just a metaphor.

Oh, I've got it. Radio Shack is a metaphor for the decline of... something. Making the metaphor even more poignant- I just Googled my closest RS locations, and found that La Plata TV and Appliance is "permanently closed".

Get yourself some "resistors" instead of "linear copper free iso-carbon sintered non-inductive temperature compensated resistors".

Oh! That's very different. Never mind.

-rb

[mikepukmel]

Just my opinion:
People discover a spec and then their world centers around it. If you were never told of this spec, of this data sheet, you would walk into Radio Shack, buy plain old "resistors" off the rack, and build something with them. And it would work just fine.

What is your specific concern about "degradation"? Note the data sheet says "derating" which is very different. The Vishay chart you show says 100% up to 70 degrees. That is 70 degrees centigrade, not Fahrenheit. 70 degrees C is about 158 degrees F. You get them hotter than that and they cannot cool themselves sufficiently, so you cannot work them as hard. That is what derating means. But the 50% line is about 135 degrees C. That means if things heat up to 135C, then we can only "rate" the part at half its given rating. That is 275 degrees F. That is darn hot, and not likely on your basic tube circuit.

The new parts can run warmer than the old. WHat they are saying is that these resistors will dissipate 1 watt out from their little bodies and be OK, whereas older technology parts would be overheated. Older types needed to be larger to safely dissipate a full watt.

others have said it, higher wattage parts might be more convenient to use for their size. Longer wires fit the eyelet boards better and such.

You mentioned 220k and 1.5k. That looks to me like a plate and cathode resistor for a preamp. Hopefully my math is correct here. 1ma is a typical tube current. 1ma through a 220k resistor means 220v dropped across it. SO a 400v B+ would leave 180v on the plate. 1ma and 220v means dissipation is under a quarter watt. Since the same current flows through the cathode resistor, a 1ma current through 1.5k dissipates only a milliwatt or two. That is why those parts in Fender amps were 1/2 watt for decades.

Thanks Enzo! If I had more confidence in my ability to do those kinds of calculations, I wouldn't be so scared to blow something up, or at least, put some component in there that would die prematurely. My main question was: the old 1/2 watt CC resistors had pretty good voltage spike behavior. From what I've read, the new metal film, not so much, so some builders are putting in bigger (power) resistors to get better voltage spike.

Re derating, degrading, etc; Well, its my lack of experience with this stuff more than anything (which is why its great to have you pro's reply to my questions!!). All resistor data sheets have a straight line that starts at the top around 70C degrees and slopes down to the right.

Here's another interesting thing: there are two versions of the data sheet. One has more "watch out" type comments than the other one. Not sure why. Both are for the same resistor model.

[Enzo]

Watch out doesn't mean emminent danger. We should always wear a seat belt driving the car, but if you don't, the car won't blow up or refuse to start.

70C is very hot.

If you learn nothing else theoretical in electronics, learn Ohm's Law. it is not complex, the formula is simple, the three variations of that formula derive from one another. it simply states the relationship between voltage, current and resistance in a circuit. it is something you can count on, and something I use every day. It isn't some corny school book thing, it is a real practical useful bit of science. I have been soldering for 60 years plus, and I have a calculator next to me at all times to do the arithmetic, and I use Ohm's Law every single day I am in the shop.

[bob p]

Modern resistors can indeed be very small, I was surprised the first time that I received a batch of Vishay SFR16S series resistors.

This photo shows 3 different brands of resistors, all with 500mW ratings:



LEFT Vishay/BC Components, SFR16S series. Manufactured in Brazil and Thailand. They're diminutive in size but truly have a 500 mW power rating. Here's the data sheet:

http://www.vishay.com/docs/28722/sfr16s25.pdf

MIDDLE Wellwyn, made in the UK. They are about the size of what I think of when I think of a 1/2 watt metal film resistor.

RIGHT MultiComp, made in China. They are HUGE, and oversized for what I think of as a 500 mW resistors. They are noisy.

[bob p]

I hate these small 1W resistors. I bought a bunch through Newark a few years ago for some 5E3 builds, and they were so small there was no way I was going to install them, they just looked stupid on a fiber board, plus they are shorter, so they really didn't even fit. I always go up in wattage, so Fender builds and Repairs get 1 or 2 watts. I also like the carbon film big resistors, and I can see the color codes easily, not a small thing at my age!

I had my own reservations the first time that I came across the modern tiny metal film resistors:

thread: http://music-electronics-forum.com/t21277/

I ordered an abundant supply of passives from Newark several years ago, and ended up with a boatload of the even smaller SFR16S series in the DIN 0204 size case. I have no reservations about using them.

I've used them without any problems in the hottest location in a Fender amp: 1k5/500mW stretched right across the tube socket. Heat stability was a major concern, as heat is the worst enemy of the grid and screen resistors that get stretched across tube sockets. One of the nicest things about the physical package of these tiny resistors that they don't take up much real estate when you're working in a tight space. The DIN 0204 size is so small that the little MF grid resistors just disappear behind the MO screen resistors.

I agree -- such tiny resistors might look out of place on a 5E3 board, but that's more of a matter of our expectations than anything else. I don't hesitate to use them because they perform well in-circuit and that's what really matters.