Projects

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. The only major disadvantages of the specific model I got were the low depth of 245mm and the fact that there are no ventilation slots at all. Fully assembled with the parts provided, that is… ELMA offers a huge variety of variants, so if your pockets are deep and you happen to have your own business, you can pick and choose and configure it the way your project needs it to be. If it’s just a hobby, you might not be as lucky… ...

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. Resistor power rating and derating In this particular implementation, the assumption of equal power ratings of all resistors is only true for the upper five decades-the first decade uses resistors with only P00=0.125 WP_{00} = 0.125 \text{ W} instead of 0.4 W0.4 \text{ W} due to limited availability of higher rated ones. Now the equation looks like this: ...

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): nn Decade nn providing resistance values 0Ω0\Omega, 1⋅10nΩ1 \cdot 10^{n}\Omega to 2⋅10nΩ2 \cdot 10^{n}\Omega RnR_{n} Total resistance of decade nn as set by the switches R0nR_{0n} Resistance of an individual resistor of decade nn (in the picture above: R00=2ΩR_{00} = 2 \Omega) P0nP_{0n} Power rating of any individual resistor of decade nn. I0nI_{0n} Current through resistor of decade nn @ power rating Imax,nI_{max,n} Maximum allowable current for decade nn Pmax,nP_{max,n} Maximum allowable power dissipation decade nn Umax,nU_{max,n} Maximum allowable voltage for decade nn Imax,totI_{max,tot} Maximum allowable current for the programmable resistor (all decades) Pmax,totP_{max,tot} Maximum allowable power dissipation of the programmable resistor (all decades) Umax,totU_{max,tot} Maximum allowable voltage for the programmable resistor (all decades) The definitions imply that we use identical resistors throughout a decade. For this post we also assume that the power rating of all resistors are the same. In reality this is not the case. That’s one of the reasons for part 2 of this post… ...

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: Schematic of the schmitt-trigger oscillator plus output driver ...

The HP 5328 series counters are a range of 0 to 100 MHz Universal counter from the 70’s. While fairly similar on the outside, internally the updated B model differs from the A model significantly. The HP 5328B’s build quality impresses still today - even for the front panel they didn’t use plastic back then. Its specs however don’t. The direct count measurement provides a resolution of up to 8 digits/s, far less than modern but much more expensive reciprocal counters (with an interpolator) achieve. In my configuration the counter features an 8 digit LED display. In contrast to most newer counters the HP 5328B has dedicated marker outputs (trigger outputs) for each channel on the front panel. ...

In this post I want to share how I selected a suitable topology for the resistor network that will be the core of the programmable decade resistor. Although I did some online “research”, the following criteria determined my choices: Switch selection: The switches shall be operated electrically. Solid state switches are great for many applications, but tend to have undesired characteristics. In this case, electromagnetic (signal) relays are a logical choice, because they provide ...

I briefly mentioned the Siemens B2086 Decade Resistor in the previous post. In this post I want to provide additional information on this interesting “vintage” unit, which seems to be geared towards AC application, not just DC. I couldn’t find a date code on it. However, back in the day they published proper manuals including part lists and schematics (yes!). Those are from 1975 to 1981, so it could very well be 30+ years old. ...

In 2021/2022 I designed a DC electronic load that would be more capable, but also much more complex than the usual DIY solutions. However, after building a working breadboard prototype of the analog circuitry with 12 ICs including multiple precision and dual opamps, I thought that it might be better to start with a smaller project that would allow me to gain a lot of experience and write much of the non-application specific code that I could use later on for the digital part. ...