Day: May 15, 2024

A Slice Of Simulation, Google Sheets Style

May 15, 2024 by 9 Comments

Have you ever tried to eat one jelly bean or one potato chip? It is nearly impossible. Some of us have the same problem with hardware projects. It all started when I wrote about the old bitslice chips people used to build computers before you could easily get a whole CPU on a chip. Bitslice is basically Lego blocks that build CPUs. I have always wanted to play with technology, so when I wrote that piece, I looked on eBay to see if I could find any leftovers from this 1970-era tech. It turns out that the chips are easy to find, but I found something even better. A mint condition AM2900 evaluation board. These aren’t easy to find, so the chances that you can try one out yourself are pretty low. But I’m going to fix that, virtually speaking.

This was just the second potato chip. Programming the board, as you can see in the video below, is tedious, with lots of binary switch-flipping. To simplify things, I took another potato chip — a Google Sheet that generates the binary from a quasi-assembly language. That should have been enough, but I had to take another chip from the bag. I extended the spreadsheet to actually emulate the system. It is a terrible hack, and Google Sheets’ performance for this sort of thing could be better. But it works.

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Introduction To MOSFET Switching Losses

May 15, 2024 by 17 Comments

Metal-oxide semiconductor field-effect transistors (MOSFETs) see common use in applications ranging from the very small (like CPU transistors) to very large (power) switching applications. Although its main advantage is its high power efficiency, MOSFETs are not ideal switches with a perfect on or off state. Understanding the three main sources of switching losses is crucial when designing with MOSFETs, with a recent All About Circuits article by [Robert Keim] providing a primer on the subject.

As it’s a primer, the subthreshold mode of MOSFET modes of operation is omitted, leaving the focus on the linear (ohmic) mode where the MOSFET’s drain-source is conducting, but with a resistance that’s determined by the gate voltage. In the saturated mode the drain-source resistance is relatively minor (though still relevant), but the turn-on time (RDS(on)) before this mode is reached is where major switching losses occur. Simply switching faster is not a solution, as driving the gate incurs its own losses, leaving the circuit designer to carefully balance the properties of the MOSFET.

For those interested in a more in-depth study of MOSFETs in e.g. power supplies, there are many articles on the subject, such as this article (PDF) from Texas Instruments.

Custom Polyurethane Belts Made Easy

May 15, 2024 by 13 Comments

If you need to make polyurethane belts in custom lengths, it’s not too hard. You just need to take lengths of flexible polyurethane filament, heat the ends, and join them together. In practice, it’s difficult to get it right by hand. That’s why [JBVCreative] built a 3D printed jig to make it easy. 

The jig consists of two printed sliders that mount on a pair of steel rods. Each slider has a screw-down clamp on top. The clamps are used to hold down each end of the polyurethane filament to be joined. Once installed in the jig, the ends of the filament can be heated with a soldering iron or other element. and then gently pushed together. The steel rods simply enable the filament to be constrained linearly so the ends don’t shift during the joining process.

The jig doesn’t produce perfect belts. There’s still a small seam at the join that is larger than the filament’s base diameter. A second jig for trimming the belt to size could be helpful in this regard. Still, it’s a super useful technique for making custom belts. This could be super useful to anyone needing to restore old cassette decks or similar mechanical hardware.

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