Inductors PDF Print E-mail
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Written by Bryce Ringwood   
Thursday, 08 November 2012 12:01

The coils in radio sets are examples of inductors.  You will also see them referred to as inductances and (rarely) reactors. Inductors have the ability to pass DC, but impede the flow of AC. In radio sets, they form part of the tuned circuits, which provide the set with selectivity - the ability to separate stations. In radio transmitters, they fulfil a similar function, ensuring the transmitter emits on the desired frequency.

 

In this article we will also look at transformers, which consist of two or more inductances coupled together.

History of Inductors

The first experimenters were interested in electromagnetism. Michael Faraday and Joseph Henry independently discovered the principle of electromagnetic induction - the principle on which the transformer is based. 

This was followed by the invention of the induction coil. The induction coil is familiar to most of us as the ignition coil used to  make the spark in petrol engines. (Modern motor vehicles may use a high frequency transformer).

In early radio transmitters, the induction coil was attached to a spark gap, one side of which was earthed, and the other attached to an aerial. (Please don't do his at home.) The resulting transmission was on all frequencies from DC to daylight. Later tuned circuits were introduced, but the transmission was still very wide.

Coils as parts of a tuned circuit were introduced with early radio receivers, such as the crystal set, which consists of an inductor, a capacitor and a rectifier (cat's whisker).

The role of the inductor has slightly diminshed as digital techniques have replaced the traditional analog tuned circuits.

Coil Construction

For the most part, coils are exactly that - coils. I'm going to tell you what to expect - by frequency.

Low Frequency - Long Wave

Coils for frequencies 10kHz to 500kHz are often quite large having several hundred turns of "Litz Wire". Litz wire is made up from many individual strands of hair-fine wire lightly braided together to make a single piece of multistrand wire. This may be then covered in one or more layers of cotton or silk. The coils are always "wave wound" on an insulated phenolic or plastic

Inductors
From left to right: IF Transformer, RF Choke (Note the four pies) , HF Coil on Neosid Former (Now hard to find) , Electroniques HF Receiver Coil.

former. Sometimes a ribbed former is used at the higher frequencies. The whole assembly was very often baked in an oven to exclude all moisture, before being sealed in wax or pitch. Later radios would have a ferrite core at the centre of the coils, and somewhat less turns.

The idea of using Litz (Litzendracht) wire was to increase the surface area of the wire, because high frequency electric currents confine themselves to the surface of a conductor. Wave winding reduces the self-capacitance of the coil, as does the use of a silk or cotton covered wire, which spaces the turns.

All this is very daunting for the would-be home valve superhet constructor. You can make a coil winder to wave-wind coils, but I imagine setting it up would be maddening. Eddystone seemed to get round the problem by using quite small coils and rather large values of tuning capacitor in their 940 receiver IF coils. You can easily wind 100 turns in a kind of handmade wave wound pattern,and the unobtainable Litz wire is not a neccesity, but you can make it yourself from 5 to 7 strands of 46 SWG Wire.

MW Coils

These are similar in construction to the LW coils, but might not have as many turns. Coils for crystal set radios may still use Litz wire,and often have an open type of winding. The diameter of the wire van be quite large with a huge number of individual "fibres" - one might say. Most post-war sets have a ferrite core inside the coil former, which is used to trim the inductance value of the coil.

HF Coils 1 - 200 Mhz

These coils are often a few turns of a single layer of copper wire on a plastic former. Up to 30MHz, a ferrite core is often used. Coils for higher frequencies might be four or five turns of quite thick silver plated wire, wihout any former.

UHF and microwaves

The coils become unrecognizable loops, tuned lines and closed in on themselves to form cavities.

Variable Inductors

Most commonly, variable inductors are wound on a long thin former, through which a ferrite rod passes. In some military radios, you will find an arrangement whereby a roller contacts a silver plated coil, accompanied by numerous incomprehensible mechanical arrangements to provide linear tuning.

Then of course, there is the crystal-set project on these pages, which uses a coat-hanger for tuning.smiley

Transistor Sets

In some ways - there is no difference between coils for transistor radios and what we have discussed so far. The coils are often encased in ferrite material, which allows them to achieve high values of inductance for very few turns. Everything can be smaller.

Some sets dispense with coils altogether for the intermediate frequency (see superhet) and use ceramic filters instead.

Inductance Values

Inductance is measured in Henrys. Faraday had already had his name attached to capacitor values. To give you some idea, a power supply choke designed to block AC frequencies might have a value of 10 to 20 Henrys. RF coils will be in the mH range.

You can purchase inductances by the mH most often as surface-mount.

Inductance Problems

In a nutshell- the wire breaks. It breaks often and for no reason. It breaks in the centre of a coil with 400 turns for no reason other than to aggravate. Sometimes it breaks at the end, but only if it can do so at a dry joint that works sometimes but not others.  These problems are all easy to measure (not neccessarily to repair) - but the human mind often overlooks the inductor. Its best to accept that they are unreliable - much worse than capacitors in my experience.

Inductors with ferrite cores can also be a nuisance. By now, the core-locking compound will have the strength and consistency of kryptonite, and there is no chance of making adjustments without ruining the core, which is irreplaceable. In any case,some previous repairer will probably have used a screwdriver and cracked the core. This means that the coil will often have to be replaced by rewinding it on a new former. It sounds terrible, but it almost always works out fine. Be sure to carefully unwind the old coil, counting the turns.

By the way, sometimes inductances fool you by developing an internal short. Everything looks OK - but the wretched set won't tune.

The Mathematics of Inductors

There are, of course, inductor mathematics. For example, the voltage across an inductor is proportional to its inductance and the change in current through the inductance.

\displaystyle V  = {\displaystyle {L \times{ di \over dt}}}   ..... (1)
  

 

NOTE that the change in current is di/dt - meaning that if you have a large inductor and change the current suddenly by removing it - the voltage across the inductor will be very large indeed. So large, in fact, that it can affect sensitive circuitry in the vicinity and burn out switch and relay contacts. Putting a .1µ and 100 Ohm resistor in series across the switch to the inductor can help. This is called a snubber.

There are formulae for calculating inductance - but they are empirical (found by experiment) and so complicated that I'm going to give you a calculator on this web-site.

Finally, we can't divorce inductors from impedance and tuned-circuit calculations, so we'll hold all the formulae over until we discuss impedance.

Chokes

These are inductances specifically designed to block AC, but allow DC to pass. They are used in power supplies (not nowadays) and in certain valve circuits to prevent unwanted oscillation. A rule of thumb for VHF chokes is to take one-third of a wavelength of fine wire, and wind it round  a high value resistor.

The fremodyne circuit uses quite a few RF chokes, but I have to say they are not commonly used. The National NC100 uses an RF choke in its anode-bend detector circuit - but this is also a quite uncommon circuit. This choke is wound as several separate pies or sections.

Nowadays, small commercially wound chokes are used in switchmode power supplies for all sorts of things, from LED and CFL lamps to battery chargers. Switchmode power supplies are things best left left to the professionals.

Transformers

In old radios, we encounter two types of transformer.

  • Power transformers (including audio output and interstage)
  • Intermediate frequency transformers

Low frequency power transformers

These have a primary winding and one or more secondary windings, usually wound on an iron core consisting of many E and I shaped laminations. An alternating current through the primary winding causes a changing magnetic field in the the iron core. This in turn induces a voltage in the transformer secondary.

Transformers are used to convert mains voltage to a different voltage for all sorts of equipment. The normal transformers you buy operate at mains frequency of 50Hz (in South Africa).

Audio transformers

These are used in valve (and some transistor) circuits to match the valve anode impedance (to be explained) to the loudspeaker - which usually has an impedance of anything from 2 to 15 Ohms. (Unless its in a Philips radio). Why do loudspeakers have a low imedance ? - well it keeps the voice coil nice and lightweight.

Audio transformers are also used in early valve amplifiers for interstage coupling and phas-splitting in push-pull amplifiers. They have also been used this way in transistor amplifiers, the Bailey design, for example.

Intermediate frequency transformers

These are used to couple the intermediate frequency stages of superheterodyne recievrs. They often have a 1:1 turns ratio in valve radios and the primary and secondary can both be tuned. In transistor radios, there is only tuning of the primary circuit - unless you get elaborate double tuned transformers for your project. I have only seen these used in military sets.

Making your own transformers

Most of the time its pointless even making your own power supply, let alone transformers. There are a number of good transformer winders in South Africa, and they will happily provide you with whatever you need - and they do understand valve circuits. They will even rewind burned out transformers for you.

IF transformers for transistor sets are readily available locally in both single and double-tuned varieties. I suspect the double tuned ones will work with valves - but perhaps not very well. That's an experiment for another day.

Unfortunately IF transformers for valve radios are almost unobtainable and are not easy to make yourself. (Not impossible - just not easy).

Summmary

  • Inductors are coils which store energy in a magnetic field when energized,
  • They are used as a component in tuned circuits and frequency filters,
  • The collapsing magnetic field in an inductor can produce high voltages when thecurrent rapidly falls ,
  • Transformers consist of two inductors coupled together via a common magnetic field.

This article serves as an introduction to tuned circuits, impedance and other topics, which we will discuss in another feature.

 

 

 

 

 

 

 

  

Last Updated on Friday, 17 May 2013 08:31
 
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