How to tell the value of an inductor?

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qdgjcl
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How to tell the value of an inductor?

Post by qdgjcl » Fri Jan 31, 2003 1:01 pm

I need to figure out how to tell the value of an inductor. I have three different sizes but I don’t know how to tell how many henrys they are. Can anyone out there tell me how?

Thanks,<p>Brad.

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Re: How to tell the value of an inductor?

Post by josmith » Fri Jan 31, 2003 2:28 pm

An "LCR" meter"L" being the part you need or grid dip meter with some work.

Dimbulb
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Re: How to tell the value of an inductor?

Post by Dimbulb » Fri Jan 31, 2003 4:13 pm

If you know how many turns,wire size and if it has ferrite you can get an approximation of its value.<p>A principal to concider with inductors is that a known capacitor and a coil make a 'tank circuit' something happens that is predictable and that is resonance. (it has the property of holding a certain wavelength) <p>As mentioned the grid dip oscillator can be used.
The voltage is highest in your tank circuit when the GDO is adjusted to the resonant frequency. your volt meter shows the highest voltage when you tune the GDO to the resonant frequency of the tank.<p>Now you have the resonant frequency in Hz and the value of capacitance in Farads. These numbers will give you the inductance of the coil in Henries. It is important as you work that you write down your test results.<p>A 1 Khz buzzer,headphones and bridge in early 1900s measured at low range with 1/10 of pico henry accuracy. My experience trying that was I got a headache listening to the BUZZZZZZZZZZZZZZZZZZZZZZZZZZZ!<p>[ February 01, 2003: Message edited by: dim bulb ]</p>

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Re: How to tell the value of an inductor?

Post by analogee » Fri Jan 31, 2003 8:03 pm

Do you have access to a meter that can measure AC volts and AC milliAmps? You also need some kind of sine wave generator appropriate to the value of the inductor (lower values work better with higher frequencies).<p>Without an idea of what value inductor you have, this may be an iterative process. But let's assume you have the above, and an inductor in the 1 mH - 1 H range, or so. Connect the output of the generator, through a resistor 'R', say 1 kOhm, to the inductor, and bring the other side of the inductor back to the ground, or common, output of the generator. Set the generator to, say, 1 kHz, with maybe 1 V of output (RMS typically, but it doesn't really matter, as long as all measurements use the same type of reading). Measure the voltage across the resistor, and the voltage across the inductor. Ideally these should be about the same voltage, within a factor of maybe 10. If the voltage across the inductor (call it VL) is very small relative to to the voltage across the resistor (call it VR), the inductive reactance is low relative to the resistance --> try increasing the frequency of the generator. Or, vice versa.<p>Once you have 'reasonable' voltages VR and VL, use this line of reasoning to infer the value of the inductor:<p>Current (I) through both the resistor and the inductor (since they are in series) is the same. I = VR/R, by Ohm's law. Save this result.<p>XL, the inductive reactance is then VL/I. Using the value of I above, compute and save this result.<p>The formula XL = 2*Pi*f*L relates L to XL (and you know 'f', the frequency of your generator). Solving for L => L = XL/(2*Pi*f). Plug in the value of f in Hz, and XL in Ohms to this formula and compute L. And there you have it.<p>For example, for 1 kHz from the generator, say you get VR = 0.7 V, and the resistor, R, is 1 k. Then I = 0.0007 A, or 0.7 mA. Now, say you measure 0.7 V across the inductor as well. XL = 0.7/0.0007 = 1000 (Ohms, the inductive reactance). Then, by the formula above, L = XL/(2*Pi*f) = 1000/(2*3.14*1000) = 0.159 Henry.<p>Hope that was clear. For reasons I don't feel like getting into right now, the answer will be only approximate. But of course, every measurement is approximate. And you should be able to get within 10% pretty readily.<p>Regards,
Todd
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Chris Smith
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Re: How to tell the value of an inductor?

Post by Chris Smith » Fri Jan 31, 2003 8:09 pm

In other words, LOTS of math, or a simple inductance meter.

analogee
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Re: How to tell the value of an inductor?

Post by analogee » Fri Jan 31, 2003 10:14 pm

"In other words, LOTS of math, or a simple inductance meter."<p>-- indeed. But the math is straight-forward algebra, and presumably one with access to an "inductance meter" wouldn't have asked the question. And a guy might even gain some insight with the indirect method.<p>Then again, I don't know whether he has a sine wave generator and a meter. But at least I had fun composing the reply.<p>Regards,
Todd
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Re: How to tell the value of an inductor?

Post by chessman » Fri Jan 31, 2003 11:07 pm

Wow, I really like that way of approximating the value. It's really interesting, gets the mind going a bit to think about it all.<p>Just curious:
I've never seen one before, what all comprises an inductance meter? How would it function? From the sounds of the math involved, the meter would consist of a sine wave generator, voltmeter, and amp-meter. It would be a cool project to write up some PIC code to handle all the measurements and give a semi-reliable indunctance reading on an LCD or something.<p>~Kyle

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Re: How to tell the value of an inductor?

Post by mikeb » Sat Feb 01, 2003 6:03 pm

i have the schematic for a simple inductance meter that u can make A very inexpensively inductance adapter for your dvm give me a email an ill send it to.

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Chris Smith
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Re: How to tell the value of an inductor?

Post by Chris Smith » Sat Feb 01, 2003 8:42 pm

TO "analogee"<p>
Every one has to do the math at least once in their hobby or career,..."analogee" hopefully? <p>And the writing accomplishments are just as great to be able to convey. <p>I was being facetious.

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Re: How to tell the value of an inductor?

Post by analogee » Sun Feb 02, 2003 11:04 pm

Not so sure anymore that everyone has to do the math at least once. My recent experience with trying to hire a couple analog designers would indicate not...<p>Regards,
Todd
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Edd
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Re: How to tell the value of an inductor?

Post by Edd » Mon Feb 03, 2003 4:11 pm

Brad:
Within some range, here are 2 DVM add on circuits for L and C that were initially in the ARRL Radio Handbook , but I see that Patrick has incorporated them into his website :
http://braincambre500.freeservers.com/indcap.htm <p>
73's de Edd
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[email protected]...(Firewalled-Spam*Cookies*Crumbs)<p>[ February 03, 2003: Message edited by: Edd Whatley ]</p>

Dimbulb
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Re: How to tell the value of an inductor?

Post by Dimbulb » Mon Feb 03, 2003 11:33 pm

Besides the arrl I like the "L/C IIB" of AADE almost all digital electronics.<p>It is a PIC sensation. It can tell you what a wire bent half way around your finger is or in the capacitance mode it will tell you if you have a 1.8 or 2.2 pico Farad. That is if the leads are short and undisturbed.<p>It does alot more. <p>The inductance meter schematic that someone was going to send you would be good for even big coils. This can also be real advantage. Far circuits is a noteworthy place to spend a little for some very excellant boards and instructions.<p>The PIC meter uses a variation of this sort of logic:<p>If a full capacitor takes N seconds to discharge using a certain resistor then what is its capacitance ?

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Re: How to tell the value of an inductor?

Post by chessman » Tue Feb 04, 2003 8:38 am

Dim bulb,<p>Do you have the PIC code and schematic for that great-sounding meter you speak of? That would be great, just as a fun project to build.<p>Thanks,
~Kyle

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Edd
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Re: How to tell the value of an inductor?

Post by Edd » Tue Feb 04, 2003 2:13 pm

Kyle:
So, if you have interest in the AADE unit,all of its info is at this site:
http://www.aade.com./lcm2binst/LC2Binst.htm By the schematic ,you can see the only special parts are the PIC and its coding and the display.
Just those 2 parts may cost almost as much as a kit if you were wanting them solely.
It covers the low spectrum well but most of my needs are in the higher ranges.<p>73's de Edd
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chessman
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Re: How to tell the value of an inductor?

Post by chessman » Tue Feb 04, 2003 8:26 pm

I see...they don't release the code to the public....oh well then, thanks for the link, it gives some inspiration for my own PIC code.

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