Category Archives: Electronics

Yet Another FPGA Differential Termination Example

An engineer asked me for assistance on determining the termination circuit for a Xilinx uG476 series 7 FPGA. The circuit works is slightly different manner than those termination circuits I have developed before (here and here) because there is not termination voltage, so I thought I should document my work in detail. I will be using Mathcad 15 to determining the optimal resistor values for (1) terminating the circuit in printed circuit board's characteristic impedance (Z0), and (2) ensuring that I preserve as much of the transmit signal level as possible without exceeding the input circuit's maximum voltage level. Continue reading

 
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Temperature Sensing with a Bandgap Reference

I have decided that my next home electronics project will be a precision thermometer that I can read over the Internet. I will be mounting the sensor at my cabin in Northern Minnesota, where winter temperatures can drop to -40 °C or lower. During the summer, temperatures can rise to nearly 40 °C. My plan is to connect the unit to a Raspberry Pie that I use to provide remote monitoring and control. I decided that I going to use a Texas Instruments' LMT70 precision temperature sensor, which uses a well-known circuit called a Brokaw bandgap reference to measure the temperature of its die. Continue reading

 
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Maximum Reach Estimates for Low-Voltage DC Power Delivery

It is no secret that I prefer Mathcad for the vast bulk of my computational work, but I live in a world in which Excel is universally available. As such, I must prepare Excel workbooks for others to use. Today, I was asked to prepare an Excel worksheet that our salesman could use for estimating the maximum range over which different combinations of wire and voltage could deliver useful power. I include my Excel workbook here for those who wish to follow along. Continue reading

 
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Ensuring Stable DC Power Delivery To Switching Loads

I presented a seminar over lunch today on short-range DC power distribution, which I believe is one of the most exciting areas in electronics today. AC power distribution has dominated power engineering since the "War of the Currents" ended with Westinghouse's AC system winning a decisive victory over Edison's DC system back in the 1890s. Starting in 1930s Europe, high-voltage DC distribution has slowly gained a foothold in some long-haul, high power distribution applications, but most power distribution has continued to be dominated by AC. Continue reading

 
Posted in Electronics, Power Over Ethernet | Leave a comment

Effect of Wire Length on Surge Protector Let-Through Voltage

While reading about how these units worked, I noticed that the amount of surge voltage they let pass (called let-through voltage) is a function of the hookup wire length. The units are tested with a hookup length of 6 inches, and the user is warned that the let-through voltage increased by ~20 V per inch of additional wire. I became curious about the origin of this rule of thumb. In this post, I will show you where this rule of thumb comes from. Continue reading

 
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Quick Look at a High-Power PoE Graph

I have been sitting in a meeting on a high power version of Power over Ethernet (PoE) known as IEEE 802.3bt. It supports 90 W of output power with a guarantee of 71 W at the load. During the talk, Figure 1 was discussed (my version of the chart). When I am given some mathematical information, I like to experiment with it to see if I understand what I am being told. Continue reading

 
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Using SUMPRODUCT to Evaluate Two-Variable Polynomial

One of the most common computation tasks that my customers face is estimating battery capacity based on the battery's temperature and discharge current. Figure 1 shows a example of the capacity curves for a typical lead-acid battery. Ten years ago, I chose to implement this function with an Excel spreadsheet that used a polynomial approximation for this function. An engineer today asked me to explain how my Excel implementation works, and I felt this would be a good topic for a post. This approach is implemented using SUMPRODUCT – no helper cells were required. Continue reading

 
Posted in Batteries, Electronics | 2 Comments

Open-Drain Comparator Circuit With Settable Trigger and Output Levels

Quote of the Day If you don't stick to your values when they are being tested, they're not values; they're hobbies. — Jon Stewart Introduction I received a circuit design question from a reader who was asking how to design … Continue reading

 
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Calculating a Parameter's Worst-Case Range

Many electronic systems are required to generate an alarm when they detect their power failing – the alarm is referred to as a "dying gasp". These systems are required to generate a dying gasp alarm when their input voltage drops below a specified level. Continue reading

 
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My Personal PCB Design and Build Process

I have been designing circuits since I was a boy, and my passion for circuits has only grown over time. My home designs focus on sensor interfaces that I hook up to the Internet using Raspberry Pie and Arduino digital interfaces. These open-source HW interfaces make it possible to create incredibly powerful designs at home. Continue reading

 
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