Inductors

[trash]

Coils and Inductors.

Another simple electronic device is a coil of wire. Often called an inductor.

As current flows in a wire, it creates a magnetic field around that wire. If we wrap the wire around on itself, we can concerntrate the magnetic field into the center of the coil. No big deal, we knew that already.
But that is not the end of the story. Coils can create a magnetic field from electricity, they can also create electricity from a magnetic field just as well.

So, we turn a coil on and it creates a magnet, but what makes it so special is what happens when we turn it off !!
Suddenly the magnetic field no longer has power holding it up and it
collapses. Magnetism is just another form of energy and as our friend Einstein informed us … you can’t just make it go away, it is converted into something else. In this case back into electricity.

Generally speaking a coil does not like the amount of current changing.
It is like a flywheel, it takes effort to make it spin, it takes effort to stop it from spinning.

Back to our coil, as you remember, we disconnected the power from the coil, so the coil now has electrical power it wants to get rid of, but has no where to discharge it ! It had current flowing in it, and it would like to keep that current flowing. To do so, the voltage across the coil rises… it will keep rising until the current can flow again, usually the result is a big spark across the coil as it discharges.

This is called ‘BACK EMF’ or ‘back voltage’. It is used in things like ignition coils in cars and switchmode power supplies and DC to DC voltage converters. (thanks ph2t for the cct diag).

Coils resist changes in current,
Capacitors resist changes in voltage.

Coils values are measured in Henrys. 1 Henry is a very big coil, so
again, millihenrys and microhenrys are common.

Coils will let DC pass, but will resist AC. The higher the frequency the more it will attenuate or stop it.
They are the complimentry of capacitors in almost every way.

If you think back to our mast head amplifier in capacitors.
The capacitor stops DC getting into the TV, there is also a coil in
the DC line where it attaches to the coax. This stop radio frequencies getting into the plug pack and lets only the DC power through.

[betty.k]

interesting. i knew nothing about inductors, not even that they exsisted! are they those copper coils i see on hi fi and radio pcb’s? are those ferrite tuning pots on rc pcb’s inductors?:8ball:

Edited by – betty.k on 18 November 2003 19:21:43

[trash]

Yep, they all sound like inductors.
The ferrite is used to change the inductance of the coil. The core of the coil effects the inductance. Air cores create smaller inductors than ones of the same size with iron.

You can change the inductance of a coil by…

changing the core
changing the number of turns
changing the diameter of the coil
changing the wire

Changing the core is easiest, changing the number of turns you can cheat by putting tap points along the coil and using only part of the whole coil.

Capacitors are just two metal plates with a gap in between. The gap or insulator is called the “dielectric”.

You can change the capacitor size by…

Changing the size of the plates
Changing the distance between the plates
Changing the type of dielectric

[micro_Amps]

Inductors also alter the balance and ‘lag’ current in a mains AC circuit.
The out of balance caused by the inductor can be compensated/corrected by a capacitor.
This corrects the power factor of the AC circuit.
And this is way beyond anything needed in micro r/c cars, so forget you even read it.
Trash you are dead right, inductors resist the rate of change of an ac circuit.
:)uA

[trash]

Can you make the power meter run backwards for me ? 🙂

[Crom]

*lol* I could, but you’d have to supply the electricity to pump back to the local electricity grid. :smiley2:

[trash]

Hmmm… lets see … for half the time the current is flowing forward, then the other half it is flowing back the same amount. I smell a rat, my meter shouldn’t move at all !!

[Author]

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