Electronics > Beginners
Two air coils next to each other - transformer or not?
petert:
Hello,
I wound two air coils next to each other (with little distance), on a paper towel roll. One with 25 turns, and the other with 90.
The signal from the primary coil couples into the secondary coil as expected.
I chose various sine waves from 10-100kHz to feed into the primary coil to test the coil coupling properties.
Interestingly, it does not matter which of the two coils I pick as primary (the one with 25 or 90 turns). On the secondary coil the signal will always be significantly weaker (but still have the proper frequency). Is that to be expected?
Shouldn't it make a difference which is the primary, and which the secondary, or does this only apply to transformers that have a magnetic core?
Cerebus:
They are going to be very weakly coupled in the magnetic domain. I *suspect* that what you're seeing is them being linked by an electric field rather than by a magnetic field. It's something that you can calculate, if you've got all the parameters but a quick test for whether it's e-field or magnetic coupling is to stick a chunk of something ferromagnetic into the core and see if it starts behaving more like you'd expect a transformer to behave.
petert:
I see a minimal change if I put something metalic in the roll or on top of the roll, such as a wrench, or a ruler.
I don't have a proper inductance meter, just a component tester (TC1). It says the 90 turn coil has an inductance of 0.23mH and the smaller of 0.04mH. The turn ratio is 90/25 = 3.6, but the inductance ratio is 0.23/0.04 = 5.75. I expected the turn and inductance ratio to match more closely.
Cerebus:
Inductance of a classic single layer solenoid scales with the square of the number of turns. However, their are assumptions about the geometry that this works on, end effects, proximity effects and the like, so one shouldn't expect to get an exact theoretical 'proportional to the square of the number of turns'. You figures seem plausible for the inductance ratio you've given.
See https://en.wikipedia.org/wiki/Inductance#Inductance_of_a_solenoid for the formulae.
The figure that you're looking for to see how well coupled the two coils should be is 'mutual inductance' but be warned, calculating it is a job for one more calculus capable than me (unless my life depends on it). See https://en.wikipedia.org/wiki/Inductance#Mutual_inductance if you're feeling brave.
TimFox:
Ordinarily, a coil pair of this geometry, with a relatively low magnetic coupling, is used in narrow band applications, with one coil (sometimes both) resonated with a parallel capacitor. Otherwise, the leakage flux (flux that does not link the two coils) reduces the efficiency of the transformer.
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