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Moderm "high watt" resistors seem too small

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I recently bought a bunch of 5W wire wound resistors, ranging from 250R to 22K, and they seem way too small to dissipate that kind of wattage. In my current project I have a 5W 250R cathode bias resistor that is only dissipating about 1.8W and it's getting very hot. Its dimensions are 7.90mm x 23.80mm, which is big compared to some of the others which are 7.50mm x 18.00mm.

These seem way too small to me. Are they safe and reliable? They sure are condensed hot-spots.

-Dave
 
The physics of heat dissipation has not changed in the last 100 years. What has changed is the ability of modern materials to deal with and operate at high temperatures. Resistors used to be wirewound, or carbon comp. The old carbon comp resistors were famous for changing value with changing temp, so you couldn't run them too hot or they would change value, or just start smoking. The physical size was large so that they could dissipate power without becoming too hot.

Today we have ceramic cores and exotic metal films that can run at 250C without a problem. So when you see a tiny resistor rated for 2 or 3 watts, there is only one way for this to work. The little resistor will get HOT in NORMAL operation. This is usually OK, but remember this if you stuff a bunch of them into a small space or use PC board construction.

If you put a 200C resistor next to a sensitive component like a transistor you will have problems. If you are using these parts on a PC board don't pull them tight to the board, leave at least 1/4 inch between the resistor and the PC board. These resistors need air on all sides to meet their power rating spec, and hot parts can discolor the board over time.

In any of these cases I typically over spec resistors to keep temperatures down. Heat is the enemy of long term reliability.
 
Do you mean you're having reliability trouble with them? Comparing them against old resistors from my 1960's/70's amps these 5W are as small as the old 2W or 3W. The datasheet says they're good up to 250°C ambient, with a derating curve of course.

No with these resistors itself, my trouble is that they run so hot the burn components near to them, electrolytic caps, the PCB itself, etc.
 
tubelab, thanks for the explanation. I assumed newer materials were the reason, but I always have concerns about mfr's cutting corners and making parts seem grander than they are.

I originally had my expensive cathode cap and inexpensive resistor wired directly together (using point to point board) but as soon as I realised how hot they R was getting I separated them and mounted the R above the board.

I will be installing a chassis fan in this amp since it's a tube-down chassis with 9 tubes. I should borrow the IR camera from work and look at the R's to see the real temperature rather than using my finger thermometer. In terms of over rating, I've got a 5W part dissipating 1.8W. I would expect that 2.7 times over rating would be plenty. What do you think?
 
One good thing about resistors is that they give you a warning sign when they are stressed.

Many of the new ones don't they just go open. The resistance is a film coating on the surface of a ceramic cylinder covered with paint. The resistance film cal fail in several ways, all of which I have experienced. See below:

My brain has stored somewhere the smoke smell of an overheated resistor.

The new metal film and carbon film resistors have a new smell. They stink more than a vintage Allen Bradley, and they smell different if slowly cooked until failure (the paint burns), or grossly overloaded until glowing red (the paint is gone and the metalization is burning), or rapidy hit with a surge well in excess of normal (the metalization goes open often with minimal indication, sometimes there is an arc), or an extreme surge (the resistor explodes leaving only a black mark on the PC board).

I would expect that 2.7 times over rating would be plenty.

That should be fine for any resistor. I tend to use the cheap white ceramic sand filled 5 watt wirewound Xicons for cathode resistors. They will take 5 watts forever although they will eventually discolor. The blow point for long term use seems to be in the 8 to 10 watt range. I have fried two in that range and they lasted a long time before blowing. They do explode rather violently shooting ceramic and metal shrapnel in all directions if hit with a direct application of say 500+ watts (short on the B+ put 450 volts across 150 ohms).
 
tubelab, thanks for the explanation. I assumed newer materials were the reason, but I always have concerns about mfr's cutting corners and making parts seem grander than they are.

I originally had my expensive cathode cap and inexpensive resistor wired directly together (using point to point board) but as soon as I realised how hot they R was getting I separated them and mounted the R above the board.

I will be installing a chassis fan in this amp since it's a tube-down chassis with 9 tubes. I should borrow the IR camera from work and look at the R's to see the real temperature rather than using my finger thermometer. In terms of over rating, I've got a 5W part dissipating 1.8W. I would expect that 2.7 times over rating would be plenty. What do you think?

The dissipation ratings for leaded resistors is based on some ideal conditions e.g. free air circulation, that are basically impossible to meet outside of a test fixture. My own derating for leaded resistors tends to be about 3X to 5X. For one watt dissipation I will use at least a 3 watt resistor. Also for dissipation over one watt I will provide some air space; 1/4" to 1/2" thermal clearance. This is all assuming that there is some air space inside rthe chassis.

For grins one time just work out all the delta T's for a TO-220 package and typical board mount heatsink and realize that these so-called 40 watt packages can only be used at about 10W max reliably without some heroic construction.

There are products that change color at particular temperatures. Tempilac, heat indicating crayons, etc. are applied to the surface in question and can be checked later for overtemp. There are also self adhesive labels with temperature scales printed on them with temp sensing dyes.
 
I recently bought a bunch of 5W wire wound resistors, ranging from 250R to 22K, and they seem way too small to dissipate that kind of wattage.

Yeah, I know. I ordered some 1W resistors from Mouser and thought they'd gotten the order wrong, as these resistors were about the same size as 1/4W C-comps.

That may be OK for SS designs, but I question the ability of these tiny resistors to stand up to the higher voltages of HS designs without flashing over and/or leaking current. If you have to use 'em, best to wire two or three in series.

Best to keep 'em well away from circuit boards or other heat sensitive areas because they will get plenty warm, especially if used near the power rating.
 
Yeah, I know. I ordered some 1W resistors from Mouser and thought they'd gotten the order wrong, as these resistors were about the same size as 1/4W C-comps.

The exact same thing happened to me. Ordered 2 watt resistors without studying the datasheets. These 2 watt resistors are between 1/4 and 1/2 watt size. I believe they were from Vishay. My Sencore MU-140 tube tester quit working so I decided to do a complete overhaul of almost every resistor and capacitor. Hopefully these resistors will be fine with intermittent use.
 
I should borrow the IR camera from work and look at the R's to see the real temperature rather than using my finger thermometer.

YES!! Absolutely use a NO-contact thermometer such as an IR probe or IR imaging camera for measuring the temperature of the resistors. I've had resistors where the exterior coating had worn thin making it possible to get your fingers on B+ while "measuring" the temperature using the finger method. I got an IR thermometer shortly thereafter...

In terms of over rating, I've got a 5W part dissipating 1.8W. I would expect that 2.7 times over rating would be plenty. What do you think?

I don't like my resistors to be too hot. Above 85~100 C is what I consider "too hot". So I generally don't run resistors much over 1/4~1/3 of their rated power. 1/2.7 wouldn't bother me much, though, I'd place the resistor in an area where it can dissipate the heat safely.

You can always check the datasheet curves to find the resistor temperature as function of the dissipated power. If the datasheet doesn't show it directly, you can usually infer the temp vs power from the derating curve and the max operating temp.

~Tom
 
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Thanks everyone for all the great info.

My day job is electronics design (primarily digital, I do industrial network communications products) so I'm frequently doing thermal and power calculations for surface mount products. So Theta J-A types of calculations are nothing new, I'll work the numbers for the parts in question. What I've learned over the years is that even the smallest amount of air flow makes an enormous difference, no matter the type of component. I will likely have a low noise fan in every one of my amp designs as a means of significantly increasing reliability (on top of conservative part ratings).
 
A "tube down" chassis? Am I to assume you have a PCB with nine tubes 'upside down' in your chassis? If so, you've got heat rising up toward the PCB....Yeah, some forced air cooling would definately be in order.
When we build these things we are spending hundreds of dollars on TXs....what's fifty dollars or so upping the ratings...as high as we dare?...Stuffing big wirewounds should not be a spatial problem inside differing chassis'......Too often when one designs.....component available space is not given enough priority. We tend to rush to build....only when fingers get burned consistently during wire-up do we tend to realize our chassis' are too small. But by that time...it's too late, & a full chassis re-do is not "preferred" One tends to NOT want to start all over......."It'll be OK!" is the refrain.
Perhaps a rule of thumb should be implemented.....If you calculate your chassis 'should' be X size.........double it.....you won't be sorry.

___________________________________________________Rick...........
 
Hot resistor or bad math

Hi

I would do some paper and pen math, dump the CAD program

I use my meter, and my calculator

150 MA through a 250 ohm resistor = 37.5 volts = 5.625 watts

125 MA though a 250 ohm resistor = 31.25 volts = 3.9 watts

ETC

What is the volts across????

Stop guessing and pretend you are in High School!
 
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