For projects like the programmable resistor I was looking for a case, preferably made of aluminum and 19" compatible. In order to save some space on the desk I hoped to find one with a width of 42HP (i. e. half width). What I found was a beautiful case made by ELMA (2 RU, 42HP), called Stylebox 15 Standard.
For a long time I wanted to build an SCPI capable thermometer to keep track of the ambient temperature in my small lab. The list of requirements was fairly short: Reasonably high precision (Sensor with <= 0.3°C max. uncertainty) Power and SCPI communication over USB Compact enclosure with a display Design After researching temperature sensors I chose a Texas Instruments TMP117M temperature sensor with quite impressive specs. With an accuracy of ±0.1°C (max.) between 25°C to 50°C and ±0.05°C (typ.) between 0°C and 70°C it is much more than good enough for my purpose, but why not overspec the sensor a bit? Because of the tiny package (WSON6, 2mm x 2mm) I was a bit nervous. As it later turned out, soldering the IC using a hot air soldering station was a bit more time consuming than e. g. a typical SOIC package (preparation), but not that difficult.
In the last post we discussed how to calculate the power rating of the programmable decade resistor. We assumed that the decade resistor is made of resistors with the same power rating - in reality this might not be the case. Also, it might be a good thing to derate the resistors a bit. So let’s see what the implications are.
In the last post we worked on some implementation details to consolidate the bill of material while improving the power rating of the decade resistor. However, we didn’t actually analyze the design regarding power dissipation. So let’s get some definitions out of the way (goto Conclusion for tldr):
Many older HP/Agilent instrument share a similar feet design. There are at least two different types though: One with a metal part to prop up the instrument and one without it that can be used on all four corners. The feet can be detached from the instrument, e. g. for use in 19" racks. This also means they got thrown away too often, so that many used instruments come without them. For some time they were available on ebay at a reasonable price.
Almost two years ago I bought a Brymen BM789 Multimeter. And there is a lot to like about it. It has great specs, is feature-packed, feels high-quality and yet it’s well priced. But there are also some mostly minor aspects I don’t like:
The other day I noticed an issue with one of my Agilent 34401A’s: I measured a constant DC voltage and after a few minutes the measured value would start dropping and become unstable. Then I got curious and checked the two-wire resistance mode. Same story, maybe worse.
In the last post we selected a resistor network and switch topology that suits the needs of this programmable resistor. Now we will apply simple optimizations that allow an effective and cost-efficient implementation. Topology selected for this programmable decade resistor Optimizing the bill of material In order to reduce the number of different components the single 1Ω1 \Omega is replaced by two parallel 2Ω2 \Omega resistors. As a very welcome side effect this doubles the power rating for the setting 1Ω1 \Omega. This is very helpful, because the lower the selected resistance value of the decade, the fewer resistors the power dissipation is distributed over.
As many people in the community I really enjoy watching W2AEW’s YouTube channel. Almost a decade ago he posted a video showing a cheap and easy to build pulse generator for time domain reflectometry (TDR), consisting of only a simple hex inverter with schmitt-trigger inputs 74AC14 and a few passive components. The schematic looks something like this:
For engineers analog oscilloscopes are a bit like vinyls and tube amplifiers: Most don’t use them anymore, but if they see one, they think: Nice! And are happy that they don’t have to deal with them on a daily basis ;) However, I bought a Tektronix 465B with a faulty second channel, just to repair it. For this one, I made a quick YouTube video.