Mains Power Supply for ATtiny Project is Probably a Bad Idea

When designing a mains power supply for a small load DC circuit, there are plenty of considerations. Small size, efficiency, and cost of materials all spring to mind. Potential lethality seems like it would be a bad thing to design in, but that didn’t stop [Great Scott!] from exploring capacitive drop power supplies. You know, for science.

The backstory here is that [Great Scott!] is working on a super-secret ATtiny project that needs to be powered off mains. Switching power supplies are practically de rigueur for such applications, but compared to the intended microcontroller circuit they are actually quite large, and they’ve just been so done before. So in order to learn a thing or two, [Scott!] designed a capacitive dropper supply, where the reactance of the cap acts like a dropping resistor to limit the current. His first try was just a capacitor in series with an LED; this didn’t end well for the LED.

To understand why, he reverse-engineered a few low-current mains devices and found that practical capacitive droppers need a few more components, chiefly a series resistance to prevent inrush current from getting out of hand, but also a bridge rectifier and a zener to clamp things down. Wiring up all that resulted in a working capacitive dropper supply, but a the cost of as much real estate as a small switcher, and with the extra bonus of being potentially lethal if the power supply is plugged in the wrong way. Side note: we thought German line cords were polarized to prevent this, but apparently not? (Ed Note: Nope!)

As always, even when [Great Scott!]’s projects don’t exactly work out, like a suboptimal 3D-printed BLDC or why not to bother building your own DC-AC inverter, we enjoy the learning that results.

Continue reading “Mains Power Supply for ATtiny Project is Probably a Bad Idea”

Sign of the Smith Chart Times

The Smith chart is a staple for analyzing complex impedance. [W2AEW] notes that a lot of inexpensive test gear like the MFJ-259B gives you complex readings, but fails to provide the sign of the imaginary part of the complex number. That makes it difficult to plot the results on a Smith chart or carry out other analysis. As you might expect, though, he has a solution for you that you can see in the video, below.

A common method is to increase the frequency slightly. In a simple case, you’d expect the imaginary part — the reactance — to go down for a capacitive impedance and up for an inductive one. Unfortunately, this doesn’t apply in many common cases, including when you are measuring through a transmission line which is probably what most people are doing with this type of test gear.

Continue reading “Sign of the Smith Chart Times”

The Beginnings of an LCR Meter

LCR Meter

The inductor is an often forgotten passive electrical elements used to design analog circuitry. [Charles’s] latest proof of concept demonstrates how to measure inductance with an oscilloscope, with the hopes of making a PIC based LCR meter.

It is not that often one needs to measure inductance, but inductors are used in switching regulators, motor circuits, wireless designs, analog audio circuitry, and many other types of projects. The principles of measuring inductance can be used to test inductors that you have made yourself, and you can even use this knowledge to measure capacitance.

[Charles] originally saw a great guide on how to measure impedance by [Alan], and decided to run with the idea. Why spend over $200 on an LCR meter when you can just build one? That’s the spirit! Be sure to watch [Alan’s] and [Charles’s] videos after the break. What kind of test equipment have you built in order to save money?

Continue reading “The Beginnings of an LCR Meter”