What is a Coil

If you take a cardboard tube, any size, any length, and wind a length of wire around it, you create a very interesting device.   It goes by the name of a ‘coil’ or an ‘inductor’ or a ‘solenoid’.

This is a very interesting device with many uses.   It forms the heart of a radio receiver, it used to be the main component of telephone exchanges, and most electric motors use several of them. The reason for this is if a current is passed through the wire, the coil acts in exactly the same way as a bar magnet:

The main difference being that when the current is interrupted, the coil stops acting like a magnet, and that can be very useful indeed.   If an iron rod is placed inside the coil and the current switched on, the rod gets pushed to one side.   Many doorbells use this mechanism to produce a two-note chime.   A ‘relay’ uses this method to close an electrical switch and many circuits use this to switch heavy loads (a thyristor can also be used for this and it has no moving parts).

A coil of wire has one of the most peculiar features of almost any electronic component.   When the current through it is altered in any way, the coil opposes the change.   Remember the circuit for a light-operated switch using a relay?:

You will notice that the relay (which is mainly a coil of wire), has a diode across it.   Neither the relay nor the diode were mentioned in any great detail at that time as they were not that relevant to the circuit being described.   The diode is connected so that no current flows through it from the battery positive to the ‘ground’ line (the battery negative).   On the surface, it looks as if it has no use in this circuit.   In fact, it is a very important component which protects transistor TR3 from damage.

The relay coil carries current when transistor TR3 is on.   The emitter of transistor TR3 is up at about +10 Volts.   When TR3 switches off, it does so rapidly, pushing the relay connection from +10 Volts to 0 Volts.   The relay coil reacts in a most peculiar way when this happens, and instead of the current through the relay coil just stopping, the voltage on the end of the coil connected to the emitter of TR3 keeps moving downwards.   If there is no diode across the relay, the emitter voltage is forced to briefly overshoot the negative line of the circuit and gets dragged down many volts below the battery negative line.   The collector of TR3 is wired to +12 Volts, so if the emitter gets dragged down to, say, -30 Volts, TR3 gets 42 Volts placed across it.   If the transistor can only handle, say, 30 Volts, then it will be damaged by the 42 Volt peak.

The way in which coils operate seems weird.   But, knowing what is going to happen at the moment of switch-off, we deal with it by putting a diode across the coil of the relay.   At switch-on, and when the relay is powered, the diode has no effect, displaying a very high resistance to current flow.   At switch-off, when the relay voltage starts to plummet below the battery line, the diode effectively gets turned over into its conducting mode.   When the voltage reaches 0.7 Volts below the battery negative line, the diode starts conducting and pins the voltage to that level until the voltage spike generated by the relay coil has dissipated.   The more the coil tries to drag the voltage down, the harder the diode conducts, stifling the downward plunge.   This restricts the voltage across transistor TR3 to 0.7 Volts more than the battery voltage and so protects it.

Solenoid coils can be very useful.   Here is a design for a powerful electric motor patented by the American, Ben Teal, in June 1978 (US patent number 4,093,880).   This is a very simple design which you can build for yourself if you want.   Ben’s original motor was built from wood and almost any convenient material can be used.   This is the top view:

And this is the side view:

Ben has used eight solenoids to imitate the way that a car engine works.   There is a crankshaft and connecting rods, as in any car engine.   The connecting rods are connected to a slip-ring on the crankshaft and the solenoids are given a pulse of current at the appropriate moment to pull the crankshaft round.   The crankshaft receives four pulls on every revolution.   In the arrangement shown here, two solenoids pull at the same moment.

In the side view above, each layer has four solenoids and you can extend the crankshaft to have as many layers of four solenoids as you wish.   The engine power increases with every layer added.   Two layers should be quite adequate as it is a powerful motor with just two layers.

An interesting point is that as a solenoid pulse is terminated, its pull is briefly changed to a push due to the weird nature of coils.   If the timing of the pulses is just right on this motor, that brief push can be used to increase the power of the motor instead of opposing the motor rotation.   This feature is also used in the Adams motor described in the ‘Free-Energy’ section of this document.

The strength of the magnetic field produced by the solenoid is affected by the number of turns in the coil, the current flowing through the coil and the nature of what is inside the coil ‘former’ (the tube on which the coil is wound).   In passing, there are several fancy ways of winding coils which can also have an effect, but here we will only talk about coils where the turns are wound side by side at right angles to the former.

    1. Every turn wound on the coil, increases the magnetic field.   The thicker the wire used, the greater the current which will flow in the coil for any voltage placed across the coil.   Unfortunately, the thicker the wire, the more space each turn takes up, so the choice of wire is somewhat of a compromise.
    1. The power supplied to the coil depends on the voltage placed across it.   Watts = Volts x Amps so the greater the Volts, the greater the power supplied.   But we also know from Ohm’s Law that Ohms = Volts / Amps which can also be written as Ohms x Amps = Volts.   The Ohms in this instance is fixed by the wire chosen and the number of turns, so if we double the Voltage then we double the current.For example: Suppose the coil resistance is 1 ohm, the Voltage 1 Volt and the Current 1 Amp.   Then the power in Watts is Volts x Amps or 1 x 1 which is 1 Watt.

      Now, double the voltage to 2 Volts.   The coil resistance is still 1 ohm so the Current is now 2 Amps.   The power in Watts is Volts x Amps or 2 x 2 which is 4 Watts.   Doubling the voltage has quadrupled the power.

      If the voltage is increased to 3 Volts.   The coil resistance is still 1 ohm so the Current is now 3 Amps.   The power in Watts is Volts x Amps or 3 x 3 which is 9 Watts.   The power is Ohms x Amps squared, or Watts = Ohms x Amps x Amps.   From this we see that the voltage applied to any coil or solenoid is critical to the power developed by the coil.

  1. What the coil is wound on is also of considerable importance.   If the coil is wound on a rod of soft iron covered with a layer of paper, then the magnetic effect is increased dramatically.   If the rod ends are tapered like a flat screwdriver or filed down to a sharp point, then the magnetic lines of force cluster together when they leave the iron and the magnetic effect is increased further.

If the soft iron core is solid, some energy is lost by currents flowing round in the iron.   These currents can be minimised by using thin slivers of metal (called ‘laminations’) which are insulated from each other.   You see this most often in the construction of transformers, where you have two coils wound on a single core.   As it is convenient for mass production, transformers are usually wound as two separate coils which are then placed on a figure-of-eight laminated core.

Electronics Tutorial

  1. WOW just what I was searching for. Came here by searching for how to make
    a magnetic generator

  2. I thought I knew everything there is to know about this thing, but seems we are never to old to learn..;)

  3. I really like this subject and can’t read enough about it!

  4. Fantastic site you have over here! Just wanna say thanks for that and keep up the great work!

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