Category Archives: Electronics

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

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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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Lightning-Induced Surge on Cable Wiring

Because I have had to deal with lightning in all sorts of contexts (e.g. military equipment, commercial hardware, consumer products), I have developed an enormous respect for the power of lightning (Figure 1). Because of this respect, I have worked to ensure that my own home has excellent lightning protection, including a sophisticated ground system. Even with all my precautions, last weekend lightning struck near my home and caused my garage door to open – letting rain into the garage, and my garage door opener to become unresponsive. Fortunately, I just had to cycle power again on the garage door opener and it started to work. Continue reading

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3D Plot Example Using PCB Trace Current vs Trace Area and Temperature Rise

I was asked this morning about how to create a 3D plot in Mathcad that shows a surface, two lines on the surface, and marks the point of intersection of the two lines (Figure 1). It just so happens that I have been looking at the amount of current that a PCB trace of a given area can carry for a given temperature rise above ambient. While the curve itself is a bit boring and viewing it in 3D does not add any value, this example does illustrate the procedure for generating this type of plot. Continue reading

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Extended Reach PoE Example

The popularity of Power Over Ethernet (PoE) has proven that customers find value in using a single cable for both data and power distribution. Unfortunately, copper-based Cat 5e/6 cable is limited to a 100 meters because of data transmission issues. To circumvent this limit, some equipment vendors are using composite fiber/copper cables – a single cable that contains fiber for data and large gauge copper wire for power distribution (Figure 1). Continue reading

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New Zealand Complex Phone Line Impedance

I received an email today asking me about the phone line impedance differences between New Zealand (Figure 1) and Australia. This is an easy question to answer, and I wrote up a quick Mathcad worksheet to perform the calculation. Continue reading

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Instrumentation Amplifier Gain Adjustment

I have an existing circuit for which I need to modify the front-end gain. The gain is provided by an LT1101, which is a common instrumentation amplifier. This part is normally used with one of its two fixed gain settings (10x, 100x). As commonly happens, I need to find a way to resolve an issue without making major changes to a circuit. The designers of the LT1101 provided you a way to modify the amplifiers gain by adding two resistors to the circuit. Figure 1 shows the modified circuit, with the added resistors marked with red ovals and labeled Rx. Continue reading

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