Making an Inductor Saturation Current Tester

inductor saturation tester

[Kalle] tipped us about a quick project he made over a couple of evenings: an inductor saturation current tester. All the components used for it were salvaged from a beefy telecom power supply, which allows the tester to run currents up to 100A during 30us in the inductors to be characterized.

Knowing the limits of an inductor is very convenient when designing Switch Mode Power Supplies (SMPS) as an inadequate choice may result in very poor performances under high loads. [Kalle]‘s tester simply consists in a N-Mosfet switching power through a load while a shunt allows current measurements. The saturation point is then found when the current going through the inductor suddenly peaks. As you can see from the picture above, 16 4700uF electrolytic caps are used to compensate for the sudden voltage drop when the Mosfet is activated. A video of the system in action is embedded after the break.

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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?

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Making Logic With Inductors


We’ve seen NAND and NOR logic gates – the building blocks of everything digital – made out of everything from marbles to Minecraft redstone. [kos] has outdone himself this time with a logic circuit we’ve never seen before. It’s based on magnets and induction, making a NOR gate out of nothing but a ferrite core, some wire, and a diode.

The theory of operations for this magnetic NOR gate goes as follows: If two of the input windings around the core have current passing in different directions, the fields cancel out. This could either be done by positive or negative voltages, or by simply changing the phase of the winding. To keep things simple, [kos] chose the latter. The truth table for a simple two-input, one-output gate gets pretty complicated (or exceedingly cool if you’d like to build a trinary computer), so to get absolute values of 1 and 0, a separate ‘clock’ winding was also added to the core.

One thing to note about [kos]‘ gate is its innovation on techniques described in the relevant literature. Previously, these kinds of magnetic gates were built with square ferrites, while this version can work with any magnetic core.

While this isn’t a very practical approach towards building anything more complex than a memory cell, it is an exercise of what could have been in an alternate universe where tube technology and the transistor just didn’t happen.

Making a Power Inductor Checker


Back to the basics: there are three kinds of passive electronic components: Inductors, Capacitors and Resistors. An inductor can be easily built and many types of core and bobbin kits are available. However, characterizing one hypothetical coil you just made is quite tricky as its inductance will depend on the measurement frequency and DC bias current. That’s why [ChaN] designed the circuit shown above.

As you may guess, RF enthusiasts are more interested in the inductance vs frequency curve while power circuit designers prefer inductance vs load current (for a given frequency). The basic principle behind the circuit shown above is to load an inductor for repetitive short periods and visualizing the current curve with an oscilloscope connected to a sense resistor. When loading the inductor, the current curve will be composed of two consecutive slopes as at a given moment the coil’s core will be saturated. Measuring the slope coefficient then allows us to compute the corresponding inductance.

[Via Dangerous Prototypes]

Wireless water heater monitor uses whatever was lying around

[Chris] set out to build a monitoring system for his water heater. It doesn’t Tweet or send SMS messages. It simply lights up an LED when the water heater is active. The one thing that complicates the setup is that he didn’t want to pull any wire from the garage into the house. What you see above is the wireless setup he used to accomplish this goal.

This is an electric water heater, so [Chris] patched into the 230V heating element feed. When the water heater is idle this connection is cut off. He used a transformer to step the voltage down to 17V and rectified it before feeding a 7805 power regulator. The rest of the transmitter circuit consists of a 555 timer driving the coil seen on the left. It is made out of telephone wire, with each of the four conductors inside connected together to multiply the number of windings. The box of breakfast sausages hosts the receiver coil. His hardware takes the induced current from that coil and amplifies it, feeding the signal to the base of a transistor responsible for switching the status LED. This works through the 6″ thick garage wall, although he did have to use a battery on the receiving end as his wall wart was injecting way too much noise into the system to work.

EMF oscilloscope probe

[Tuomas Nylund] wanted a way to visualize the electromagnetic fields (EMF) around him. He figured the oscilloscope was the tool best suited for the task, but he needed a way to pick up the fields and feed them into one of the scope’s probes. He ended up building this EFM probe dongle to accomplish the task.

He admits that this isn’t much more than just an inductor connected to the probe and should not be used for serious measurements. But we think he’s selling himself short. It may not be what he considers precision, but the amplification circuit and filtering components he rolled into the device appear to provide very reliable input signals. We also appreciate the use of a BNC connector for easy interface. Check out the demo video after the break to see the EMF coming off of a soldering station controller, from a scanning LCD screen, and that of a switch-mode power supply.

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Metal detection using an inductor instead of a clock crystal.

This project from a few years back is an interesting take on a metal detector. Instead of building a detection circuit, [Bruno Gavand] replaced the external clock crystal with an inductor. Here you can see the inductor coil next to the PIC 12F683. You can see two components jumping from one breadboard to the other. These are smoothing capacitors on the inductor lines.

The watchdog timer for the chip is run by the internal RC oscillator. When the external crystal receives a pulse due to metal inducing a current in the coil, the value of the watchdog timer is compared to it. This data is filtered and if the proper parameters are present the green LED blinks. This is bicolor LED. If the inductor circuit is functioning properly it will blink red at power up. [Bruno] says that results will vary based on that inductor so you may need to try a few to get the calibration light to blink.

We’re thinking this would make a simple stud finder (by detecting where the nails/screws are in the wall). Check out the demo after the break, then let us know what you would use this for by leaving a comment.

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