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 Post subject: Thermal Grease
PostPosted: Jun Sun 18, 2017 9:14 pm 
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Location: Spring Lake Mi
Working on a Stewart Warner A61P1 radio....had to replace a candohm resistor with a 1.5K 10W, 310 10W and 150 1W, and another resistor not in the candohm a 2k 5W...
I read several threads on this, like the idea of using thermal grease and clamping them to the chassis, very limited space to free air them.....what brand of thermal grease do you guy's use. I have some synthetic grease (Super Lube), it says it is good to 425 degrees, it is a dielectric, but I think it is silicone.

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 Post subject: Re: Thermal Grease
PostPosted: Jun Sun 18, 2017 9:40 pm 
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Use a thermal paste for computer CPU's. There is a very efficient one available for use by gamers who over-clock their boxes... It can be used for other transistor mounting too.

Chas

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 Post subject: Re: Thermal Grease
PostPosted: Jun Sun 18, 2017 11:11 pm 
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Perhaps a misunderstanding here.

This is not grease, as in being a lubricant.

It's simply a viscous compound with high thermal conductivity.

It's used because the mating surfaces are not perfectly flat and smooth.
The compound fills in the voids and improves heat conduction from one surface to the other.

- Leigh

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 Post subject: Re: Thermal Grease
PostPosted: Jun Sun 18, 2017 11:33 pm 
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+1 on Leigh

Thermal compound has a solid filler of fine granules.... usually something like Zinc Oxide. The object is to fill voids, not create a thick layer, with a material that has a better conductivity than air.

Rich

PS: this is typical. Ignore the hype. Don't pay extra for silver-filled compound.

https://www.walmart.com/ip/Arctic-Silve ... 3=&veh=sem


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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 12:25 am 
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For clarification there are such things as panel mount resistors (examples can be found on RS components Australia) I often use those to replace a Choke /Field and have put "Thermal grease" on some. I really don't consider brand the problem, its as said, not a lubricating grease, even that it may qualify to be described as a grease soap.

Often you can mount these resistors on a side panel and use it as part of the heat sinking. On a Grigsby-Grunow the tuning gang metal is so thick I put it on top of it where there is a huge amount of air flow. It might get luke warm on a hot day?

Marc


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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 12:41 am 
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As an aside, I've also read and heard about folks using "Desitin" as thermal compound. They say it has the added advantage of making the room smell like babies are hanging around to boot :D.

73,

Randy AB5NI

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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 12:42 am 
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ab5ni wrote:
...making the room smell like babies are hanging around to boot
Why would you want to boot a baby?

Or do you mean it smells like baby boots?

- Leigh

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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 1:10 am 
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The active ingredient (ZnO) is good. The inactive ingredients may result in "ooze" at higher temperatures.

https://www.desitin.com/diaper-rash-pro ... ngredients

Rich


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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 3:38 am 
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Leigh wrote:
ab5ni wrote:
...making the room smell like babies are hanging around to boot
Why would you want to boot a baby?

Or do you mean it smells like baby boots?

- Leigh


"Baby Booting" can be a fun pastime, of course! :D. What I meant to say was this: When the Desitin heats up, it smells like you have a baby hanging around your lab/shack, since that stuff is mainly used for diaper-rash and stuff :D.

Another aside: fingernail polish can also be used as Q-Dope. Since it's put on humans, it's regulated by the FDA, so it's an extremely pure form of lacquer.

73,

Randy AB5NI

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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 4:47 am 
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Here:

http://www.wakefield-vette.com/resource ... efield.pdf

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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 12:31 pm 
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Location: Spring Lake Mi
Thank you guy's for the reply's and the great humor, something we need each day..............
You gave me enough info to work with.
After I posted this I discovered that I can mount two of these resistor in free air clear from any components. The cabinet mount for the chassis is "U" shaped having most of the bottom of the chassis open.

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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 1:27 pm 
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Location: Near Brandon, Iowa
An alternative to heatsink paste is silicone sheeting such as in this link:

http://www.ebay.com/itm/QUALITY-BLUE-10 ... Sw8cNUSnEy

You can cut this material with kitchen scissors and create mounting holes in it with a paper punch.
About the same price as a small tube of heatsink compound, but less messy. Since it is essentially a non-adhesive gasket, it can be removed and reinstalled with its resistor many times.

I really have to question the need for any heatsinking material for a power resistor, however. Resistors are linear devices with respect to operating temperature right up to the point they fail. If the resistor is being operated so close to its rated power dissipation limit that the additional heat transfer afforded by heatsinking compound/gasket is necessary, it really needs to be replaced by a resistor of the proper (higher) power rating. Generally this is a power rating that is at least 1.4 times that of the calculated power dissipation in the part. Power resistors get hot- that's just what they do. As long as they are operated comfortably within their power rating parameters and they are situated where the heat that they generate doesn't damage other components, maximizing heat transfer by using a compound or gasket is a waste of time and money.


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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 4:16 pm 
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Silicone sheet is probably worse than no heatsink compound. Silicone is used where you need electrical insulation as well as thermal conductivity. There is a trade off in getting the electrical isolation.

https://www.onsemi.com/pub/Collateral/AN1040-D.PDF

If you check page 5, note that insulating sheets are 2 to 4 times WORSE thermal impedance than a simple metal-to-metal mounting.

Rich

PS: If you really want a good chassis mount resistor, try a Dale aluminum case with heatsink compound.

Image


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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 4:34 pm 
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Rich, W3HWJ wrote:
Silicone sheet is probably worse than no heatsink compound. Silicone is used where you need electrical insulation as well as thermal conductivity. There is a trade off in getting the electrical isolation.

https://www.onsemi.com/pub/Collateral/AN1040-D.PDF

If you check page 5, note that insulating sheets are 2 to 4 times WORSE thermal impedance than a simple metal-to-metal mounting.

Rich

PS: If you really want a good chassis mount resistor, try a Dale aluminum case with heatsink compound.

Image

Are we discussing the same thing? Take a look at the final paragraph on page 6 of your referenced document.

The document is largely irrelevant to this particular discussion anyway since the referenced document entirely pertains to semiconductor heatsinking, not power resistors. They are very different animals.
Optimized heatsinking of semiconductors is generally obligatory; power resistors, not at all for a properly designed application.


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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 4:35 pm 
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Thermal greases come in many flavors. The silver-bearing CPU thermal compounds mentioned upstream (such as Arctic Fox) are pricey, and for this application are overkill.

Basic thermal grease, using zinc oxide in a silicone base, will do fine. Any parts jobber should have it (I use either GC type Z9 or NTE type 303, depending on what I find when I need more). In addition to Arctic Fox I keep two other thermal greases on hand - GC type 44, a non-silicone grease that will not ooze or bleed at higher temperatures the way that silicone greases can, and Aavid Thermalcote 249G, a high thermal conductivity grease used for power MOSFETs in broadcast transmitter amplifier modules.

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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 6:15 pm 
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The other day I was looking into building an IRF510 amplifier for my homebrew QRP transmitter. While I was opening a new can of coffee in the kitchen, I noticed that the can was hermetically sealed with a thin, pliable layer of aluminium. Just pull on a small extension tab, and the vacuum is broken and you can access the coffee.

While playing around with this flexible aluminum sheet, I got wondering if I could use it as a cheap heatsink. That is, all I'd have to do is add some thermal compound to the stuff -- in my case Desitin :D -- clamp it onto the IRF510 with a clothespin and let her rip. If you're being cheap, you might as well go all the way :D. I have yet to try it, but I'm pretty sure it will work, and if I need more surface area for cooling, I can always add a few more and fan them out a bit. I might even add a 12 volt PC fan to the arrangement, just to be on the safe side. BTW, this amp is going to be on a breadboard, so all the extra space required will not be a big deal at all. Sure, it's going to look like crap, but I'll be more concerned with the overall functionality rather than aesthetics.

73,

Randy AB5NI

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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 7:53 pm 
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Quote:
The document is largely irrelevant to this particular discussion anyway since the referenced document entirely pertains to semiconductor heatsinking, not power resistors. They are very different animals.


All power generating devices in metal cases are pretty much the same. Use the same calculations, heatsinks, and materials. The big difference is that many power resistors are rated for over 200C ! I never run them that hot.

The best mounting is Metal-to-Metal with a thin void filling compound. Any plastic or insulating material makes heat transfer worse, not better. Adding another layer of aluminum foil, plastic, mica, etc. just adds another thermal resistance to the chain. If you don't need the electrical insulation, don't add any intermediate layer.... just a void filler.

By all means read page 6 carefully.

Quote:
Silicone rubber insulators have gained favor because
they are somewhat conformal under pressure. Their ability
to fill in most of the metal voids at the interface reduces the
need for thermal grease


"Reduces" not "eliminates."

Quote:
The conclusions to be drawn from all this data is that
some types of silicon rubber pads, mounted dry, will
outperform the commonly used mica with grease.


But if you don't need the insulation... the net result is non-optimum thermal resistance.

Silicone pads were developed for companies that didn't want to deal with messy grease or fragile mica.

Rich


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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 8:00 pm 
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The Vishay power resistor mounting notes are pretty much identical to semiconductor mounting instructions:

http://www.vishay.com/docs/50007/rps250.pdf

Rich


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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 9:21 pm 
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Great info, Rich. Thanks for all the input.

73,

Randy AB5NI

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 Post subject: Re: Thermal Grease
PostPosted: Jun Mon 19, 2017 9:34 pm 
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Heatsink compound is often necessary when mounting semiconductors with insulators on heat sinks, but it rarely does anything except create a mess under power resistors which are already insulated and good metat-to-metal contact can be obtained to the heat sink. If you are running things so hot that a dab of compound makes a difference, the heat sink is poor or the resistor is undersized.

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