inductors! and,...... questions!

Discussion in 'General Electronics Chat' started by ninjaman, Nov 19, 2013.

May 18, 2013
306
1
hello
I have an arduino and a few components to make an inductance meter.

parts list was small, lm339 two resistors, two capacitors and a diode. copy and paste the code and bobs your uncle, fannies your aunt!

I tried to measure the value of the inductors I have and no luck. I tried a 330uh axial. (maybe this is where I am going wrong).
the frequency used to measure it is 17857hz and for 28u seconds. is this correct.
I know current and frequency are what can affect the inductance.
I have been looking for a way to try and figure out if this is working or not and if I have to adjust the code a little so it comes out closer to the proper value.
is there a way to calculate the value at that frequency?
I just looked on the site and it says that "The output works better with a high current and uses ~33mA at 5V".

i am using electrolytic caps, which were a no-no. i didn't realise this may effect the result in some way. does anyone know how?

cheers
all the best or 73's
simon

2. #12 Expert

Nov 30, 2010
16,665
7,310
I can't see quite how it works, but that's probably your problem, too.

The principle is that a current through an inductor will persist after a fast shut off according to Energy = 1/2 L I^2
You create a known current, the inductor pulls the input pin negative, and you measure the magnitude of the voltage...then calculate according to Energy = 1/2 CV^2

But you are still discussing the amount of current.

The LM339 is a comparator. I can't see how switching on or off will give you a magnitude. Must be something about the software knowing when to measure.

Here's a circuit I did that might point out some information to you. L + C + "known current" is enough to do the calculation. Do you have those parameters defined?

Yes, I know this is only partially helpful...if I'm lucky. Other people will also assist.

3. SgtWookie Expert

Jul 17, 2007
22,183
1,728
I simulated the circuit for you, increasing the inductance to 470uH to more clearly show the oscillations in the tank circuit:

The input signal briefly "whacks" the inductor with a blip of voltage via R1; see the green trace V(in) on the right; current begins to flow through L1. R1 limits the current (to 5v/150 Ohms ~=33.3ma).

Note the red trace, V(tank) on the right; it goes above and below ground. If you reverse the polarity across a polarized capacitor, it will act like a short circuit instead of a capacitor, and the oscillation will immediately die (go to 0v), which is why you aren't seeing an output.

Note that the blue trace, v(out), takes a millisecond or so to stabilize. Once it's stable, the microcontroller can simply time how long it takes from the start of one pulse to the start of the next pulse; the resonant frequency is the reciprocal of that time.

Note also that I increased the value of the pull-up resistor on the output of the LM339; it wasn't rated for the amount of current they were trying to sink.

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