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Electronics/Resistance of a coil of wire


Hello. I am an auto technician and I was just wondering if you could answer a question  for me.all my working life i have been checking resistance of components and until today i never really thought about this. Say i have to check a coil/winding resistance.  Are those coils insulated from each other.  I have seen motor windings being made on YouTube that don't appear to be. Most coils I test for resistance and insulation.  So if i have a straight piece of wire in a car i expect less than 1 ohm but I was wondering why does a wire in a winding have more resistance.or even a bulb for example. is it because of the length of the wire in the winding. Longer the wire greater the resistance.  Or is it something to do with inductance. If the wires aren't insulated wouldn't electrons take the shortest path between the touching parts of the circuit. I have tried to find an answer online but to no avail. Perhaps you can shed some light on the subject for me.  Regards, Eddie.

There are several principles involved in your question.

ONE:  Ohms law for voltage, resistance and current.  In a circuit, simple or otherwise, it always is governed by ohms law.  Voltage = Current X Resistance.  An ohmmeter, for example, tests the resistance of an object by applying a voltage which causes a small current to pass through the resistance object under test.  The meter is calibrated in ohms, but it really is a solution for the R = V/C.

TWO:  The object under test has resistance to the flow of current. This resistance is a function of the material itself and its geometric form; such as a wire, plate, connection point, etc. For example, copper is low resistance, relatively, but silver and gold are much lower for a given testing.

The above will give you an idea about the relative resistance per foot for the different sizes or gauge of wires.

THREE:  The coils in a motor or generator are composed, usually, of many turns of copper wire.  The turns in the winding represent a resistance equal to the diameter of the wire and its total length; each winding for a given motor or generator will be different.  If two windings are in series the total resistance will be equal to twice that of a single winding.  If they are wired in parallel then the resistance would be half of on coil by itself.  Got the idea?

Now the problem is you must know when measuring coils within a motor or generator that other connected circuits are not in parallel with the winding/coil or otherwise it will lower the measurement results.  So when taking coil measurements you must be sure you are measuring only the single coil and its connections to other parts of the motor circuit are disconnected.

Light bulbs are high resistance material; tungsten for example and the satisfy ohms law.  If a 100w light bulb is connected to 120v the current is therefore sufficient to dissipate the 100w only and is determined by the bulb manufacturer.

There is much more to learn but I'll stop there and let you digest.  Send more questions if need be.  


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Graduate electrical engineer with over 40 years in electronic design, manufacturing, project organization and patent review. Experience in fields of industrial and consumer electronics (audio, video, acoustics, etc.)

IEEE (Institute of Electrical and Electronics Engineers); Senior Life member AES (Audio Engineering Society), Fellow Life member

BSEE University of North Dakota

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