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Old 14th March 2012, 05:19 PM   #11
LAJ is offline LAJ  United States
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Be wary of high wattage surface mount resistors!
Ikegami used 7W smt resistors in one of their commercial products.
When the chip it supplied shorted out it burned up the 7W resistor burning a hole in the circuit board.
Then they try to sell you a new board.
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Old 14th March 2012, 05:29 PM   #12
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Quote:
Originally Posted by doozerdave View Post
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.
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Old 14th March 2012, 06:41 PM   #13
Cassiel is offline Cassiel  Libya
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Old 14th March 2012, 06:52 PM   #14
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Originally Posted by doozerdave View Post
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.
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Old 14th March 2012, 07:18 PM   #15
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Originally Posted by Miles Prower View Post
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.
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Old 14th March 2012, 07:29 PM   #16
tomchr is offline tomchr  United States
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Originally Posted by doozerdave View Post
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...

Quote:
Originally Posted by doozerdave View Post
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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Last edited by tomchr; 14th March 2012 at 07:31 PM.
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Old 14th March 2012, 08:14 PM   #17
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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).
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Old 15th March 2012, 04:21 AM   #18
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I've been using these and overrate x3. Very stable.

http://www.vishay.com/docs/30204/rsns.pdf
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Old 15th March 2012, 05:03 AM   #19
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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.

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Old 15th March 2012, 06:54 AM   #20
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Smile 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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