[morzh]

Did you put some thermal grease under the TIP50?

[electricroo]

Yes I did. I always keep Artic Silver 5 handy for things like this and computer projects. It's really a must, especially with the mica insulators. Here's the modified 42-221schematic. It also has a couple of labling errors fixed. They mixed up ID numbers on  the brocast and oscillator coils and also two components.

[DaleHCook]

I always keep Artic Silver 5 handy for things like this and computer projects.

That's what I would use for a CPU or GPU heatsink, but it is pretty expensive for a power semi such as your TIP50. My standard high-performance heat-sink compound is what is used by Nautel, the premiere manufacturer of AM and FM broadcast transmitters, for their power MOSFETs. It is AAVID Thermalcote 249G, which has much higher thermal conductivity than standard silicone compounds such as GC Type Z-9.

[morzh]

We had a case of the compound discarded. it is in small tubs. I saved a couple for myself. As for the cost of it, the amount needed is very small, so one tub lasts you for a long time.

[DaleHCook]

Mike -

I buy the Aavid compound in tubes because I cannot afford to buy it the way that Nautel buys it - in 5 gallon buckets.

[morzh]

That would last my company several hundrd years.

What happens with us is, we us Contract Manufacturers.
Whoever before me specified that BOM, it included a small tub (50ml? Less?) of that thermal grease. I forgot who makes it. Working from home, so can't look at it.
So, I think they bought a tub for every board.

A couple of years ago they found the strategic supply of that grease and took it to the dumpster.
I appropriated a few for my own use.

[Arran]

I have a larger pill bottle with heat sink grease that someone gave me years ago, and also a small jar that came from the estate of a TV repairman. I'm not sure what is in the jar, I will have to find it, but the pill bottle has about three tablespoons of the white grease variety. In parting out old equipment I've run into a compound that looks like blue or red bathtub caulk so I assume that was silicone based. I've heard that was the failing point in some video game consoles, they skimped on the heat sink compound and the chips ran too hot.
Regards
Arran

[DaleHCook]

I stock four different heat sink compounds, with different properties, for different purposes. I tend to stock variety in many things. For example, I stock nine polishing compounds, for everything from plastics to the hardest metals. I stock a dozen lubricants for a variety of purposes.

[electricroo]

"That's what I would use for a CPU or GPU heatsink, but it is pretty expensive for a power semi such as your TIP50."
Well I could stock others, but you only need a very small amount, as that stuff goes a long way and works great. I bought a two pack for $13 a few years back, it goes a long way!

[morzh]

A little dab will do ya!

[radiorich]

Hello Ron,
That little heatsink should do well and it looks good nice job!
Sincerely Richard

[DaleHCook]

I do not really trust Arctic Silver 5 because there are no published specifications. I find it is hard to trust a heat sink compound which claims to be great, when the manufacturer will not even tell you its specifications.

[morzh]

Dale

At least they are supposed to publish "Safety Data sheet" from which you could try to surmise about what is in the compound.
there is, of course, what I'd call "evidence based" performance, at least for personal use (for manufacturing, of course, I will not put anything without an officially published specs in any of my designs).

Our thermal compounds, what little we use, were specified before my taking over. They are alll white silicon grease.
And used in on-critical applications, so I go by "don't stir the s**t, it won't stink".

[DaleHCook]

I am a long time user of SDS, MSDS, and PSDS. They are about safety, not performance. I like to see specifications measured in accordance with standard tests, such as ASTM D5470 for thermal conductivity, or ASTM D149 for dielectric strength.

I am interested in those specifications for my work. I have seen broadcast transmitters occasionally fail because of the use of the incorrect heat sink compound. Of course, those are not as common as failures from the use of too little, or especially the use of too much, compound.

[morzh]

Dale,

Honestlt, if I use a compound (rarely), is try to design thermal path such that it will not fail should the compound be measured a trifle incorrectly. That is, I try not to design close enough to critical point.
With or without compound, I try to ensure that, while within the specs of the ambient, the device never gets too close to it, even if it is still within the safety zone.

For instance, I use Vicor power supplies (yes, I understand we are talking of a bit different power levels) that are 400W bricks, that specify 80-85% efficiency. Which is, to think of it, dismal. It means that, should i put out 400W, I will be dissipating up to 60W at the thermal plate (this is where the power transistors are mounted). Given that our device is only specified to 250W power, this would be 38W, but even this is a lot. And it is (not through my fault, the design is legacy and the rest of the team is not willing to change the mechanical design) mounted to a 3mm or so ANODIZED aluminum plate, which purpose, BTW, is not being a heatsink. It carries other stuff.
There is a fan in the device, but it is fairly weak as we do not want lots of airflow, as it is a fiber splicer and works with (can work with) fairly fine fiber which will oscillate because of that flow, and you will not be able to splice.
And the response of the mechanical engineer was "I do not want to change anything" even all that I was asking for was, let's mask the plate under Vicor during anodizing and iridite it instead. Let alone the fact that the Vicor's plate has to be electrically connected to its metal mounting plane to use it for EMI puwposes, if it is Grounded (and I also found out it was not). Iriditing would ensure botyh thermal and electrical connection.
So, eventually I was able to make them to mask that part of the plate.
But the plate, the way it is, is not a good heatsink.

So I went and measured the temperatures right at the Vicor plate, where it meets the mounting plate.
What I found out that, somehow, without reflecting it in datasheets, Vicor MAXI series is quite a bit more efficient than what they claim. Obviously, there has been some improvement over all these years (the thing is getting close to 30 years, and is very unique amongst the Telecom Bricks by it's Trim range, from 10% to 110% of the output voltage, and in reality this is even better).
So, now I know we have good margin. But I would never approve the newer designs (not that they would listen; after all the devices are precision mechanics and my role is supportive, controlling it all, the secret sauce is in the mechanics and the process) explicitly, had I not known that I had had a very good temperature margin. That said, while some compound is good (Vicor does not exactly has the smoothest thermal plate surface; BTW if it does, it is the tell-tell of counterfeits), its less than correct application will not put the brick in danger zone, even in still air.

I do know about price/reliability design compromises, but, even within the budget, I do not go by "OKI, it is STILL not in the danger zone, fine". I go by "OK, it is 50 degrees away from the danger zone even if all goes wrong."