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.
Switches seem to be the simplest of electrical components – just two pieces of metal that can be positioned to either touch each other or not. As such it would seem that it shouldn’t matter whether a switch is used for AC or DC. While that’s an easy and understandable assumption, it can also be a dangerous one, as this demo of AC and DC switching dramatically reveals.
Using a very simple test setup, consisting of an electric heater for a load, a variac to control the voltage, and a homemade switch, [John Ward] walks us through the details of what happens when those contacts get together. With low-voltage AC, the switch contacts exhibit very little arcing, and even with the voltage cranked up all the way, little more than a brief spark can be seen on either make or break. Then [John] built a simple DC supply with a big rectifier and a couple of capacitors to smooth things out and went through the same tests. Even at a low DC voltage, the arc across the switch contacts was dramatic, particularly upon break. With the voltage cranked up to the full 240-volts of the UK mains, [John]’s switch was essentially a miniature arc welder, with predictable results as the plastic holding the contacts melted. Don your welding helmet and check out the video below.
As dramatic as the demo is, it doesn’t mean we won’t ever be seeing DC in the home. It just means that a little extra engineering is needed to make sure that all the components are up to snuff.
You’ll no doubt recall [Mark Presling]’s remarkably polished and professional spot welder build that we featured some time ago. It’s a beauty, with a lot of thought and effort put into not only the fit and finish but the function as well. Still, [Mark] was not satisfied; he felt that the welder was a little underpowered, and the rewound microwave oven transformer was too noisy. Taking inspiration from an old industrial spot welder, he decided to rebuild the transformer by swapping the double loop of battery cable typically used as a secondary with a single loop of thick copper stock. Lacking the proper sized bar, though, he laminated multiple thin copper sheets together before forming the loop. On paper, the new secondary’s higher cross-sectional area should carry more current, but in practice, he saw no difference in the weld current or his results. It wasn’t all bad news, though — the welder is nearly silent now, and the replaced secondary windings were probably a safety issue anyway, since the cable insulation had started to melt.
Given [Mark]’s obvious attention to detail, we have no doubt he’ll be tackling this again, and that he’ll eventually solve the problem. What suggestions would you make? Where did the upgrade go wrong? Was it the use of a laminated secondary rather than solid bar stock? Or perhaps this is the best this MOT can do? Sound off in the comments section.
Current. Too little of it, and you can’t get where you’re going, too much and your hardware’s on fire. In many projects, it’s desirable to know just how much current is being drawn, and even more desirable to limit it to avoid catastrophic destruction. The humble current shunt is an excellent way to do just that.
To understand current, it’s important to understand Ohm’s Law, which defines the relationship between current, voltage, and resistance. If we know two out of the three, we can calculate the unknown. This is the underlying principle behind the current shunt. A current flows through a resistor, and the voltage drop across the resistor is measured. If the resistance also is known, the current can be calculated with the equation I=V/R.
This simple fact can be used to great effect. As an example, consider a microcontroller used to control a DC motor with a transistor controlled by a PWM output. A known resistance is placed inline with the motor and, the voltage drop across it measured with the onboard analog-to-digital converter. With a few lines of code, it’s simple for the microcontroller to calculate the current flowing to the motor. Armed with this knowledge, code can be crafted to limit the motor current draw for such purposes as avoiding overheating the motor, or to protect the drive transistors from failure.
In fact, such strategies can be used in a wide variety of applications. In microcontroller projects you can measure as many currents as you have spare ADC channels and time. Whether you’re driving high power LEDs or trying to build protection into a power supply, current shunts are key to doing this.
Just how much metaphorical juice is in a coin cell battery? It turns out that this seemingly simple question is impossible to answer — at least without a lot of additional information. The problem is that the total usable energy in a battery depends on how you try to get that energy out, and that is especially true of coin cells.
For instance, ask any manufacturer of the common 3 V lithium 2032 batteries, and they’ll tell you that it’s got 230 mAh. That figure is essentially constant across brands and across individual cells, and if you pull a constant 0.2 mA from the battery, at room temperature and pressure, you’ll get a bit more than the expected 1,150 hours before it dips below the arbitrary voltage threshold of 2.0 V. Just as it says on the tin.
What if you want to do anything else with a coin cell? Run an LED for a decade? Pull all the energy out right now and attempt to start a car? We had these sorts of extreme antics in mind when we created the Coin Cell Challenge, but even if you just want to do something mundane like run a low-power radio sensor node for more than a day, you’re going to need to learn something about the way coin cells behave in the real world. And to do that, you’re going to need to get beyond the milliamp hour rating. Let’s see how deep this rabbit hole goes.
One of the most versatile tools on anyone’s work bench, at least as far as electrical projects are concerned, is a power supply. Often we build our own, but after we’ve cobbled together some banana jacks with a computer’s PSU or dead-bug soldered a LM317 voltage regulator to a wall wart, how will that power supply perform? Since it’s not desirable to use a power supply that’ll let the smoke out of everything it powers (or itself, for that matter) a constant current sink, or load, can help determine the operating limits of the power supply.
[electrobob] built this particular current sink from parts he had lying around. The theory of a constant current sink is relatively straightforward so it’s easily possible to build one from parts out of the junk drawer, provided you can find a few transistors, fuses, an op amp, and some heat sinks. The full set of schematics that [electrobob] designed can be found on his main project page. He’s also gone a step further with this build as well, since he shorted out his first prototype and destroyed some of the transistors. But, using a few extra transistors in his design also improves the safety and performance of the load, so it’s a win-win.
This constant current load also has the added feature of being able to interface with a waveform generator (an Analog Discovery, specifically) and as a result can connect and disconnect the load quickly. If you aren’t in need of an industrial-grade constant current sink and you have some spare parts lying around, this would be a great one to have around the work bench.
Sometimes I see a component that’s bizarre enough that I buy it just to see if I can actually do something with it. That’s the case with today’s example, the ESP-14. At first glance, you’d ask yourself what AI Thinker, the maker of many of the more popular ESP8266 modules, was thinking.
The ESP-14 takes the phenomenally powerful ESP8266 chip and buries it underneath one of the cheapest microcontrollers around: the 8-bit STM8S003 “value line” chip. Almost all of the pins of the ESP chip are locked inside the RF cage’s metal tomb — only the power, bootloader, and serial TX/RX pins see the light of day, and the TX/RX pins are shared with the STM8S. The rest of the module’s pins are dedicated to the STM8S. Slaving the ESP8266 to an STM8S is like taking a Ferrari and wrapping it inside a VW Beetle.
I had never touched an STM8 chip before, and just wanted to see what I could do with this strange beast. In the end, ironically, I ended up doing something that wouldn’t be too far out of place on Alibaba, but with a few very Hackaday twists: a monitor for our washer and dryer that reports power usage over MQTT, programmed in Forth with a transparent WiFi serial bridge into the chip for interactive debugging without schlepping down into the basement. Everything’s open, tweakable, and the Forth implementation for the STM8S was even developed here on Hackaday.io.
It’s a weird project for the weirdest of ESP modules. I thought I’d walk you through it and see if it sparks you to come up with any alternative uses for the ESP8266-and-STM8S odd couple that is the ESP-14.