A Crash Course In Thermodynamics For Electrical Engineers

It’s a simple fact that, in this universe at least, energy is always conserved. For the typical electronic system, this means that the energy put into the system must eventually leave the system. Typically, much of this energy will leave a system as heat, and managing this properly is key to building devices that don’t melt under load. It can be a daunting subject for the uninitiated, but never fear — Adam Zeloof delivered a talk at Supercon 2019 that’s a perfect crash course for beginners in thermodynamics.

Adam’s talk begins by driving home that central rule, that energy in equals energy out. It’s good to keep in the back of one’s mind at all times when designing circuits to avoid nasty, burning surprises. But it’s only the first lesson in a series of many, which serve to give the budding engineer an intuitive understanding of the principles of heat transfer. The aim of the talk is to avoid getting deep into the heavy underlying math, and instead provide simple tools for doing quick, useful approximations.

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Liquid Cooling Keeps This Electronic Load’s MOSFETs From Burning

Problem: your electronic load works fine, except for the occasional MOSFET bursting into flames. Solution: do what [tbladykas] did, and build a water-cooled electronic load.

One can quibble that perhaps there are other ways to go about preventing your MOSFETs from burning, including changes to the electrical design. But he decided to take a page from [Kerry Wong]’s design book and go big. [Kerry]’s electronic load was air-cooled and capable of sinking 100 amps; [tbladykas] only needed 60 or 70 amps or so. Since he had an all-in-one liquid CPU cooler on hand, it was only natural to use that for cooling.

The IXYS linear MOSFET dangles off the end of the controller PCB, where the TO-247 device is soldered directly to the copper cold plate of the AiO cooler. This might seem sketchy as the solder could melt if things got out of hand, but then again drilling and tapping the cold plate could lead to leakage of the thermal coupling fluid. It hasn’t had any rigorous testing yet – his guesstimate is 300 Watts dissipation at this point – but as his primary endpoint was to stop the MOSFET fires, the exact details aren’t that important.

We’ve seen a fair number of liquid-cooled Raspberry Pis and Arduinos before, but we can’t find an example of a liquid-cooled electronic load. Perhaps [tbladykas] is onto something with this design.

Extreme Pi Overclocking With Mineral Oil

Liquid cooling is a popular way to get a bit of extra performance out of your computer. Usually this is done in desktops, where a special heat sink with copper tubing is glued to the CPU, and the copper tubes are plumbed to a radiator. If you want dive deeper into the world of liquid cooling, you can alternatively submerge your entire computer in a bath of mineral oil like [Timm] has done.

The computer in question here is a Raspberry Pi, and it’s being housed in a purpose-built laser cut acrylic case full of mineral oil. As a SoC, it’s easier to submerge the entire computer than it is to get a tiny liquid-cooled heat sink for the processor. While we’ve seen other builds like this before, [Timm] has taken a different approach to accessing the GPIO, USB, and other connectors through the oil bath. The ports are desoldered from the board and a purpose-built header is soldered on. From there, the wires can be routed out of the liquid and sealed off.

One other detail used here that  we haven’t seen in builds like this before was the practice of “rounding” the flat ribbon cable typically used for GPIO. Back in the days of IDE cables, it was common to cut the individual wires apart and re-bundle them into a cylindrical shape. Now that SATA is more popular this practice has been largely forgotten, but in this build [Timm] uses it to improve the mineral oil circulation and make the build easier to manage.

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Measuring The Cooling Effect Of Transformer Oil

Transformer oil has long served two purposes, cooling and insulating. The large, steel encased transformers we see connected to the electrical grid are filled with transformer oil which is circulated through radiator fins for dumping heat to the surrounding air. In the hacker world, we use transformer oil for cooling RF dummy loads and insulating high voltage components. [GreatScott] decided to do some tests of his own to see just how good it is for cooling circuits.

Thermal measurement resultsHe started with testing canola oil but found that it breaks down from contact with air and becomes rancid. So he purchased some transformer oil. First, testing its suitability for submerging circuits, he found that he couldn’t see any current above his meter’s 0.0 μA limit when applying 15 V no matter how close together he brought his contacts. At 1 cm he got around 2 μA with 230 VAC, likely from parasitic capacitance, for a resistance of 115 Mohm/cm.

Moving on to thermal testing, he purchased a 4.7 ohm, 100 watt, heatsink encased resistor and attached a temperature probe to it with Kapton tape. Submerging it in transformer oil and applying 25 watts through it continuously, he measured a temperature of 46.8°C after seven minutes. The same test with distilled water reached 35.3°C. Water’s heat capacity is 4187 J/kg∙K, not surprisingly much better than the transformer oil’s 2090 J/kg∙K which in turn is twice as good as air’s 1005 J/kg∙K.

He performed a few more experiments but we’ll leave those to his video below.

We’ve run across a number of tests running boards submerged in various oils before. For example, we’ve seen Raspberry Pi’s running in vegetable oil and mineral oil as well as an Arduino running in a non-conductive liquid coolant, all either overclocked or under heavy load.

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Chilling A Hot Camera

[Eric]’s camera has a problem. It overheats. While this wouldn’t be an issue if [Eric] was taking one picture at a time, this camera also has a video mode, which is supposed to take several pictures in a row, one right after the other. While a camera that overheats when it’s used is probably evidence of poor thermal engineering, the solution is extremely simple: strap a gigantic heat sink to the back. That’s exactly what [Eric] did, and the finished product looks great.

The heatsink chosen for this application is a gigantic cube of aluminum, most likely taken from an old Pentium 4 CPU cooler. Of course, there’s almost no way [Eric] would have found a sufficiently large heat sink that would precisely fit the back of his camera, which meant he had to mill down the sides of this gigantic heat sink. [Eric] actually did this in his drill press using a cross slide vice and an endmill. This is surely not the correct, sane, or safe way of doing things, but we’ll let the peanut gallery weigh in on that below.

The heatsink is held on by a technique we don’t see much around here — wire bending. [Eric] used 0.055″ (1.3 mm) piano wire, and carefully bent it to wrap around both the heatsink and the camera body. Does the heatsink cool the camera? Yes, and the little flip-up screen of the camera makes this camera a very convenient video recording device. You can check out the video of this build below.

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The Surface Area To Volume Ratio Or Why Elephants Have Big Ears

There are very few things that are so far reaching across many different disciplines, ranging from biology to engineering, as is the relation of the surface area to the volume of a body. This is not a law, as Newton’s second one, or a theory as Darwin’s evolution theory. But it has consequences in a diverse set of situations. It explains why cells are the size they are, why some animals have a strange morphology, why flour explodes while wheat grains don’t and many other phenomena that we will explore in this article.

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Pi Keeps Cool At 1.5 GHz

Hackers have a long history of overclocking CPUs ranging from desktop computers to Arduinos. [Jacken] wanted a little more oomph for his Pi Zero-Raspberry Pi-based media center, so he naturally wanted to boost the clock frequency. Like most overclocking though, the biggest limit is how much heat you can dump off the chip.

[Jacken] removed the normal heat sink and built a new one out of inexpensive copper shim, thermal compound, and super glue. The result isn’t very pretty, but it does let him run the Zero Pi at 1.5 GHz reliably. The heat sink is very low profile and doesn’t interfere with plugging other things into the board. Naturally, your results may vary on clock frequency and stability.

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