Radio Tuning The Quicksilver Way

Modern radios are often digital affairs, in which the frequency is derived from a stable crystal oscillator and varied through a microprocessor controlled frequency synthesiser. It won’t drift, and it’s exactly on the frequency dialed in. Older radios though relied on a tuned circuit, a combination of capacitor and inductor, for their frequency selection. If you were curious enough to peer inside — and we know you were — you’d have seen the moving vanes of a variable capacitor controlled by the tuning knob.

Of course, there is another way to adjust a tuned circuit: by changing the value of the inductor. Older car radios for instance moved a ferrite slug inside a coil to tune from station to station. But that method is not good enough for [David Mills]. Being in possession of some finely graduated syringes he decided to try liquid tuning by increasing the volume within the coil.

Solutions of salts made little difference, so he reached for some mercury. The result is an RF inductor wound round a syringe body, with a body of mercury whose position can be adjusted by the plunger. He measures the Q factor of the coil with air core or mercury core, and as the inductance decreases with more mercury, so does the Q.

We see home-made parts from time to time, and there’s nothing too special about permeability tuning. However, this unusual take on the matter makes this one rather special. We doubt we’ll see its like very often in the future.

Gyrators: The Fifth Element

A few years ago, there was a stir about a new fundamental component called a memristor. That wasn’t the first time a new component type was theorized though. In 1948 [Bernard Tellegen] postulated the gyrator. While you can’t buy one as a component, you can build one using other components. In fact, they are very necessary for some types of design. Put simply, a gyrator is a two-terminal device that inverts the current-voltage characteristic of an electrical component. Therefore, you can use a gyrator to convert a capacitor into an inductor or vice versa.

Keep in mind, the conversion is simply the electrical properties. Normally, current leads voltage in a capacitor and lags it in an inductor, and that’s what a gyrator changes. If you use a gyrator and a capacitor to make a virtual inductor, that inductor won’t magnetically couple to another inductor, real or simulated. There’s no magnetic field to do so. You also don’t get big voltage spikes caused by back EMF, which depending on your application could be a plus or a minus. But if you need an ungainly inductor in a circuit for its phase response, a gyrator may be just the ticket.

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Design a Coil for a Specific Inductance

YouTuber [RimstarOrg], AKA Hackaday’s own [Steven Dufresne], shows how to make a DIY inductor for a specific inductance. This is obviously a great skill to learn as sometimes your design may call for a very accurate inductance that may be otherwise hard to find.

Making your own inductor may seem daunting. You will have to answer a few questions such as: “what type of core will I use?”, “how many turns does my coil need?”, or “how do I calculate these parameters to create the specific inductance I desire?”. [RimstarOrg] goes through all of this, and even has a handy inductance calculator on his website to make it easier for you. He also provides all the formulae needed to calculate the inductance in the video below.

Using a DIY AM Radio receiver, he demonstrates in a visual way how to tune an AM Radio with a wiper on his home-built coil. Changing the inductance with a wiper changes the frequency of the radio: this is a variable inductor,

This video is great for understanding the foundations of inductors. While you may just go to a supplier and buy yours, it’s always great to know how to build your own when you can’t find a supplier, or just can’t wait.

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What Lies Within: SMT Inductor Teardown

Ever wonder what’s inside a surface-mount inductor? Wonder no more as you watch this SMT inductor teardown video.

“Teardown” isn’t really accurate here, at least by the standard of [electronupdate]’s other component teardowns, like his looks inside LED light bulbs and das blinkenlights. “Rubdown” is more like it here, because what starts out as a rather solid looking SMT component needs to be ground down bit by bit to reveal the inner ferrite and copper goodness. [electronupdate] embedded the R30 SMT inductor in epoxy and hand lapped the whole thing until the windings were visible. Of course, just peeking inside is never enough, so he set upon an analysis of the inductor’s innards. Using a little careful macro photography and some simple image analysis, he verified the component’s data sheet claims; as an aside, is anyone else surprised that a tiny SMT component can handle 30 amps?

Looking for more practical applications for decapping components? How about iPhone brain surgery?

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The Science Behind Boost Converters

[Ludic Science] shows us the basic principles that lie behind the humble boost converter. We all take them for granted, especially when you can make your own boost converter or buy one for only a few dollars, but sometimes it’s good to get back to basics and understand exactly how things work.

The circuit in question is probably as simple as it gets when it comes to a boost converter, and is not really a practical design. However it helps visualize what is going on, and exactly how a boost converter works, using just a few parts, a screw, enameled wire, diode, capacitor and a push button installed on a board.

The video goes on to show us the science behind a boost converter, starting with adding a battery from which the inductor stores a charge in the form of an electromagnetic field. When the button is released, the magnetic field collapses, and this causes a voltage in the circuit which is then fed through a diode and charges the capacitor a little bit. If you toggle the switch fast enough the capacitor will continue to charge, and its voltage will start to rise. This then creates a larger voltage on the output than the input voltage, depending on the value of the inductor. If you were to use this design in a real life application, of course you would use a transistor to do the switching rather than a push button, it’s so much faster and you won’t get a sore finger.

This is very basic stuff,  but the video gives us a great explanation of what is happening in the circuit and why. If you liked this article, we’re sure you’ll love Hackaday’s own [Jenny List] explain everything you need to know about inductors.

(updated thanks to [Unferium] – I made a mistake about the magnetic field collapsing when the button is pressed , When in reality it’s when the button is released that this happens. Apologies for confusion.)

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Yet Another Inductance Measuring Scheme

How do you measure the value of an unknown inductor? If you have an LCR bridge or meter, you are probably going to use that. If not, there are many different techniques you can use. All of them rely on the same thing my Algebra teacher Mr. Harder used to say back in the 1970’s: you have to use what you know to get what you don’t know.

[Ronald Dekker] must think the same way. He took a 50-ohm signal generator and a scope. He puts the signal output to about 20kHz and adjusts for 1V peak-to-peak on the scope. Then he puts the unknown inductor across the signal and adjusts the frequency (and only the frequency) for an output of 1/2 volt peak-to-peak.

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Making an Inductor Saturation Current 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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