# measuring inductors

Discussion in 'General Electronics Chat' started by gregm122, Jun 27, 2007.

1. ### gregm122 Thread Starter New Member

Jun 5, 2007
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Hello, if any one can help me out it would be greatly appreciated.
~How do you measure the osc frequency of an inductor/coil with a scope~
I am trying to measure the oscillating frequency of a coil which I have made for a project. I used 30 guage insulated wire and turned it about 200 or so turns around a one inch wooden dowel. I have an oscilloscope and have become familiar with its functions. I am not sure exactly how to connect the scope to the coil to measure its frequency. I know to have the scope on AC, but Should I have the coil connected to the powered circuit when I measure it or just measure the secluded coil? Plans say it should show about 1/2 to 1 volt p to p so I would guess that means measure the coil when it is hooked up to the powered circuit, correct?
when connect the positive and negative of the oscilloscope to the two leads on the coil i get no signal, a straight line.
Thank you very much, Greg

2. ### recca02 Senior Member

Apr 2, 2007
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i m a bit confused.
according to me
unless u r talking about tank circuits inductors/coils have no frequency of thier
own. it is the frequency of the applied signal that inductors oppose so in short u shud be measuring the frequency of the signal and if u want to measure with an inductor it must be powered.
m i missing something here?

3. ### gregm122 Thread Starter New Member

Jun 5, 2007
5
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hey, its a metal detector i am constructing, the circuit is known as a beat frequency oscillator. http://home.clara.net/saxons/bfo.htm OR search google "DIY metal detector" half way down you will see a paragraph titled coils, apparently i have to tune the two coils untill they are in phase, but what you are saying is that the incoming signal into the coils are what dictates its oscillating frequency?

maybe i have to measure the frequency of the oscillator as a whole? i have no idea and am in it way over my head, but thanks recca

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
The principle of the detector is that a reference coil and a detector coil are elements in oscillator circuits. Both oscillators should run at the same frequency, as determined by the coils.

In operation, the detector coil's inductance will be affected by ferrous metal objects, and that oscillator's frequency will change. The frequency shift will show up as a beat frequency, which will be amplified and be presented to the speaker or headphones.

For your purposes, it's probably more important that both oscillators run at the same frequency. If you have a frequency counter, you can check with that. Otherwise, the mismatch will make a beat, and be audible. There should be trim capacitors included to tune the oscillator circuits.

What is most critical is the construction of both coils. The construction article should specify the diameter and shape of the coils, as well as the exact number of turns of whatever gauge wire in each coil. If you follow the instructions carefully, then adjusting the frequencies should be fairly simple. The absolute frequency is less important than that both oscillators run at the same frequency.

5. ### gregm122 Thread Starter New Member

Jun 5, 2007
5
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true the directions would help but apparently if the two frequencys are way off you wont even get a sound, i dont even know if its the circuit that is not working or if its the coils phased out way too much.

a frequency counter is what you need? i only have an oscilloscope, have no idea how to check the frequency of the coils still, anyone have any ideas?

and can anyone tell me if that is a tank circuit?

thanks again! Greg

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
If your oscope has dual traces, display one oscillator signal on the a channel and the other on b. If the traces move only slightly, you're very close in frequency. If the non-triggered trace is just a blur, the freqs are way off.

Jun 25, 2007
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8. ### mrmeval Distinguished Member

Jun 30, 2006
833
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If you can view both wave forms seperately you can use a dual channel scope to determine when they match.
This is a simulation. The only part that matters is the X/Y frequencies. When the match you get an oval.
http://www.mathematik.ch/anwendungenmath/lissajou/lab/

There should be some sort of setup explanation in the scope users manual.

9. ### gregm122 Thread Starter New Member

Jun 5, 2007
5
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still I dont understand exactly where to place the positive and negative probes of my scope to the oscillator or coil to measure the frequency. If anyone has any ideas it would help a bunch, or maybe check out the circuit on the site i provided above and help me figure out how to measure these coils, thanks, greg

10. ### arthur92710 Active Member

Jun 25, 2007
307
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get a cheap RCL or LCR meter and it will find the inductance.