A Cold Hard Look at FPGAs

Researchers at the Delft University of Technology wanted to use FPGAs at cryogenic temperatures down around 4 degrees Kelvin. They knew from previous research that many FPGAs that use submicron fabrication technology actually work pretty well at those temperatures. It is the other components that misbehave — in particular, capacitors and voltage regulators. They worked out an interesting strategy to get around this problem.

The common solution is to move the power supply away from the FPGA and out of the cold environment. The problem is, that means long wires and fluctuating current demands will cause a variable voltage drop at the end of the long wire. The traditional answer to that problem is to have the remote regulator sense the voltage close to the load. This works because the current going through the sense wires is a small fraction of the load current and should be relatively constant. The Delft team took a different approach because they found sensing power supplies reacted too slowly: they created an FPGA design that draws nearly the same current no matter what it is doing.

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Thermoelectric Dry Ice Generator Does Not Work (Yet)

[Pabr] is trying to make dry ice the hard way by building a thermoelectric dry ice generator. The project is a well planned round trip through thermodynamics and cryogenics with a hard landing on the icy grounds of trial and error.

[Pabr’s] four stage Peltier element on a heatsink.
While dry ice can be obtained with simpler methods, for example by venting gaseous CO2 from fire extinguishers and collecting the forming CO2 flakes, [pabr’s] method is indeed attractive as a more compact solid-state solution. The setup employs a four stage Peltier element, which uses four Peltier stages to achieve a high temperature differential. With sufficient cooling on the high-temperature side of the element, it should be well capable of achieving temperatures below -78.5 °C, the sublimation temperature of CO2. So far, [pabr] has built three different setups to expose small amounts of CO2 to the cold of the Peltier element, hoping to observe the formation of little dry ice flakes.

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Cryogenic Machining: Custom Rubber Parts

Fashioning a custom, one-off rubber part for your project isn’t usually an option, but [Ben Krasnow] has an alternative to injection molding and casting: machining frozen rubber.

As [Ben] points out, you can’t exactly pop a sheet of rubber on your mill and CNC the needed shape; the bit will push the material around rather than cut it. Freezing the rubber first, however, allows you to carve into the now-hardened material.

His initial setup consisted of a sheet of aluminum with water drizzled on top, a square of neoprene placed on the water, and a steady stream of -60 to -80C alcohol flowing directly onto the rubber. The water underneath freezes, holding the neoprene in place. This proved problematic as the ice-clamp gives way before the milling is complete. [Ben] later adds some bolts to clamp the pieces down, allowing the milling process finish as planned.

A small plastic tray sits underneath this assembly to capture the alcohol as it runs off, feeding it back with some tubing. [Ben] recommends against a submersible aquarium pump—his initial choice—because the pump stopped working after a few minutes immersed in the chilly alcohol. An external, magnetically-driven pump solved the problem although it does require manual priming.

Stick around after the jump for the video and check out some of [Ben’s] other projects, like his quest for the perfect cookie, or CT scanning a turkey.

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Homemade Liquid Nitrogen

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As far as DIY cryogenics are concerned, dry ice is easy mode. You can get frozen carbon dioxide at WalMart, or from a nozzle that screws onto a CO2 tank. It’s all very ordinary, and not really special at all. Want to know what’s cool? Making liquid nitrogen at home.

[imsmooth] is getting his nitrogen from a standard tank, sending the gas through a CO2 and H2O scrubber, compressing it, putting the compressed gas in an ice bath, and slowly diffusing the compressed, cooled gas into a vacuum reservoir. When the cold compressed gas is released into the reservoir, Boyle’s law happens and liquid nitrogen condenses in a flask.

As far as materials and equipment are concerned, [imsmooth] is using a PVC tower filled with zeolite to filter out the CO2 and H2O, a SCUBA compressor (no oil), and an almost absurd amount of stainless steel tubing for the precooler and regenerative cooling tower. Except for a few expensive valves, dewar, and the SCUBA compressor, it’s all stuff you could easily scrounge up from the usual home improvement stores.

[imsmooth] is producing about 350cc/hr of liquid nitrogen,  or more than enough for anyone who isn’t running an industrial process in their garage. Check out the video of the build below.

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