Old Wattmeter Uses Magnetics To Do the Math

Measuring power transfer through a circuit seems a simple task. Measure the current and voltage, do a little math courtesy of [Joule] and [Ohm], and you’ve got your answer. But what if you want to design an instrument that does the math automatically? And what if you had to do this strictly electromechanically?

That’s the question [Shahriar] tackles in his teardown of an old lab-grade wattmeter. The video is somewhat of a departure for him, honestly; we’re used to seeing instruments come across his bench that would punch a seven-figure hole in one’s wallet if acquired new. These wattmeters are from Weston Instruments and are beautiful examples of sturdy, mid-century industrial design, and seem to have been in service until at least 2013. The heavy bakelite cases and sturdy binding posts for current and voltage inputs make it seem like the meters could laugh off a tumble to the floor.

But as [Shahriar] discovers upon teardown of a sacrificial meter, the electromechanical movement behind the instrument is quite delicate. The wattmeter uses a moving coil meter much like any other panel meter, but replaces the permanent magnet stator with a pair of coils. The voltage binding posts are connected to the fine wire of the moving coil through a series resistance, while the current is passed through the heavier windings of the stator coils. The two magnetic fields act together, multiplying the voltage by the current, and deflect a needle against a spring preload to indicate the power. It’s quite clever, and the inner workings are a joy to behold.

We just love looking inside old electronics, and moving coil meters especially. They’re great gadgets, and fun to repurpose, too.

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Custom Circuit Makes for Better Battery Level Display

Isn’t it always the way? There’s a circuit right out of the textbooks, or even a chip designed to do exactly what you want — almost exactly. It’s 80% perfect for your application, and rather than accept that 20%, you decide to start from scratch and design your own solution.

That’s the position [Great Scott!] found himself in with this custom LED battery level indicator. As the video below unfolds we learn that he didn’t start exactly from scratch, though. His first pass was the entirely sensible use of the LM3914 10-LED bar graph driver chip, a device that’s been running VU meters and the like for the better part of four decades. With an internal ladder of comparators and 1-kilohm resistors, the chip lights up the 10 LEDs according to an input voltage relative to an upper and lower limit set by external resistors. Unfortunately, the fixed internal resistors make that a linear scale, which does not match the discharge curve of the battery pack he’s monitoring. So, taking design elements from the LM3914 datasheet, [Great Scott!] rolled his own six-LED display from LM324 quad-op amps. Rather than a fixed resistance for each stage, trimmers let him tweak the curve to match the battery, and now he knows the remaining battery life with greater confidence.

Perhaps the 18650 battery pack [Great Scott!] is building is for the e-bike he has been working on lately. If it is, we’re glad to see that he spot-welded the terminals, unlike a recent e-bike battery pack build that may have some problems down the road.

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Lightning Generator from Electric Lighter

Generating high voltages isn’t too hard. A decent transformer will easily get you into the 100s of kilovolts, provided you’re a power company and have access to millions of dollars and a substation to put it. If you want to go above that then things start getting difficult, and most tend to look in other places for high voltages such as voltage multipliers.

These devices use nothing but capacitors and diodes, as [Jay] from [Plasma Channel] shows us how to build a small desktop version of a voltage multiplier that can produce almost 70 kV. That’s enough to throw a substantial spark, powered by nothing but a rechargable battery found in an electric lighter. They can also be cheaper than transformers to a point, since they require less insulation and less copper and iron. The voltage multiplier works in stages, with each stage boosting the voltage to a critical level above the stage before it similar to a Marx generator.

Similar designs are used by laboratories to simulate lightning strikes, and can generate millions of volts. They’re a cost-effective way of generating huge voltage pulses and studying everything from the effects of lightning on various equipment to generating X-rays in fusion power tests. We’ve even seen them in use in lasers.

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Say It With Me: Root-Mean-Square

If you measure a DC voltage, and want to get some idea of how “big” it is over time, it’s pretty easy: just take a number of measurements and take the average. If you’re interested in the average power over the same timeframe, it’s likely to be pretty close (though not identical) to the same answer you’d get if you calculated the power using the average voltage instead of calculating instantaneous power and averaging. DC voltages don’t move around that much.

Try the same trick with an AC voltage, and you get zero, or something nearby. Why? With an AC waveform, the positive voltage excursions cancel out the negative ones. You’d get the same result if the flip were switched off. Clearly, a simple average isn’t capturing what we think of as “size” in an AC waveform; we need a new concept of “size”. Enter root-mean-square (RMS) voltage.

To calculate the RMS voltage, you take a number of voltage readings, square them, add them all together, and then divide by the number of entries in the average before taking the square root: \sqrt{\frac{1}{n} \left(v_1^2 + v_2^2 +...+ v_n^2\right)} . The rationale behind this strange averaging procedure is that the resulting number can be used in calculating average power for AC waveforms through simple multiplication as you would for DC voltages. If that answer isn’t entirely satisfying to you, read on. Hopefully we’ll help it make a little more sense.

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Building Transistors with Transistors

Since the 1940s when the first transistor was created, transistors have evolved from ornery blocks of germanium wrangled into basic amplifiers into thousands and thousands of different devices made of all kinds of material that make any number of electrical applications possible, cheap, and reliable. MOSFETs can come in at least four types: P- or N-channel, and enhancement or depletion mode. They also bear different power ratings. And some varieties are more loved than others; for instance, depletion-mode, N-channel power MOSFETs are comparatively scarce. [DeepSOIC] was trying to find one before he decided to make his own by hacking a more readily available enhancement-mode transistor.

For those not intimately familiar with semiconductor physics, the difference between these two modes is essentially the difference between a relay that is normally closed and one that’s normally open. Enhancement-mode transistors are “normally off” and are easy to obtain and (for most of us) useful for almost all applications. On the other hand, if you need a “normally on” transistor, you will need to source a depletion mode transistor. [DeepSOIC] was able to create a depletion mode transistor by “torturing” the transistor to effectively retrain the semiconductor junctions in the device.

If you’re interested in semiconductors and how transistors work on an atomic level, [DeepSOIC]’s project will keep you on the edge of your seat. On the other hand, if you’re new to the field and looking to get a more basic understanding, look no further than these DIY diodes.

Isolated Voltage Measurements Through Frequency

This one’s not a flashy hack, it’s a great piece of work and a good trick to have up your sleeve. Sometimes you’ve got a voltage difference that you’d like to measure, but either the ground potential is at a different level, or the voltages are too high for your lowly microcontroller.

There are tons of tricks with resistive voltage dividers that you can play. But if you want serious electrical isolation from the target, there’s only one way to go — an optocoupler. But optocouplers only really transmit digital signals, and [Giovanni Carrera] needed to measure an analog voltage.


Enter the voltage-to-frequency IC that does just what it says: produces a square wave with a frequency that’s proportional to the voltage applied. Pass this square wave through an optocoupler, and you can hit one side with voltages approaching lightning strikes without damaging the microcontroller on the other side. And you’re still able to measure the voltage accurately by measuring the frequency on the digital I/O pins of the microcontroller.

[Giovanni] built up and documented a nice circuit. He even tested it for linearity. If you’re ever in the position of needing to measure a voltage in a non-traditional way, you’ll thank him later.

“Who is John Galt?” Finally Answered

For those who haven’t read [Ayn Rand’s] philosophical tome Atlas Shrugged, there’s a pretty cool piece of engineering stuffed in between the 100-page-long monologues. Although fictional, a character manages to harness atmospheric static electricity and convert it into kinetic energy and (spoilers!) revolutionize the world. Harnessing atmospheric static electricity isn’t just something for fanciful works of fiction, though. It’s a real-world phenomenon and it’s actually possible to build this motor.

who-is-john-galt-thumbAs [Richard Feynman] showed, there is an exploitable electrical potential gradient in the atmosphere. By suspending a tall wire in the air, it is possible to obtain voltages in the tens of thousands of volts. In this particular demonstration, a hexacopter is used to suspend a wire with a set of needles on the end. The needles help facilitate the flow of electrons into the atmosphere, driving a current that spins the corona motor at the bottom of the wire.

There’s not much torque or power generated, but the proof of concept is very interesting to see. Of course, the higher you can go the more voltage is available to you, so maybe future devices such as this could exploit atmospheric electricity to go beyond a demonstration and do useful work. We’ve actually featured the motor that was used in this demonstration before, though, so if you’re curious as to how a corona motor works you should head over there.

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